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Automotive Compilation Volume 7 December 2010 Contents Table of Contents Configurable RF Architecture Gives Engineers Greater Design Flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Design and Security Considerations for Passive Immobilizer Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Automotive LIN Bus Driving Sensor Applications . . . . . . . . 12 Designing Next Generation Key Fobs . . . . . . . . . . . . . . . . . . 15 Capacitive Proximity Detection in the Automotive Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Extending the Range of E-vehicles with Li-ion Batteries Using Active Balancing . . . . . . . . . . . . . . . . . . . . . . 24 Precision Battery Monitors for Standard Lead Acid Batteries Ensure Engine Start-up . . . . . . . . . . . . . . . . . . . . . . 27 DC and BLDC Motor Control ICs . . . . . . . . . . . . . . . . . . . . . . . 29 EMC – Synonym for Exasperating, Magic, Confusing? . . . 36 EMC Pulse Immunity System-level Tests . . . . . . . . . . . . . . . 41 Configurable RF Architecture Gives Engineers Greater Design Flexibility Ahmad Chaudhry and Jim Goings In the automotive environment, RFenabled sub-systems continue to evolve and proliferate. Such systems include Tire Pressure Monitoring Systems (TPMS), which are mainly unidirectional on a single RF channel at relatively high data rates; Remote Start (RS) systems, which are generally bidirectional on single or multiple RF channels at relatively low data rates; Passive Keyless Entry (PKE) and Remote Keyless Entry (RKE) applications, which are unidirectional on single or multiple RF channels at moderate data rates. TPMS uses both On Off Keyed (OOK) and Frequency Shift Keyed (FSK) modulation, RS uses OOK only, and RKE uses either OOK or FSK. To accommodate such multiple systems and applications, automotive RF semiconductor devices must possess flexible as well as configurable architectures. The need for architectural flexibility, combined with customer expectations of high performance, improved range, and reliability, is driving the next generation of RF IC designs. Beginning with range and reliability, improvements here are easier to realize when the transmit path has programmable parameters. One can 1 usefully employ a Power Amplifier (PA) with reserve capacity, which can be trimmed in output power level to provide the maximum allowable output in accordance with local regulatory requirements. Further benefit can be obtained from a PA whose output impedance can be trimmed to optimize the antenna match. Range and reliability gains on the receive side can be enhanced through proper selection of sensitivity-related parameters, such as RF carrier frequency, subchannels, modulation, data rate, and IF bandwidth. and RS. The devices also support all automotive bands: 310 to 318MHz, 418 to 477MHz, and 836 to 928MHz and use one device and a single crystal frequency. These two devices are also designed for architectural flexibility. A dualLNA architecture with two separate input pins natively supports multiband applications using one single IC, PCB, and bill of materials (BOM). Additionally, dual-parallel demodulation paths support simultaneous ASK and FSK sensing capability. These features Atmel Has Introduced Next-Generation Transceiver And Receiver Devices With Configurable Options Flexibility for the design engineer is also gained through access to a receiver in which these parameters are available as a programmable option. Atmel® has introduced next-generation transceiver and receiver devices with these configurable options. For example, the Atmel ATA5830 transceiver and Atmel ATA5780 receiver can accommodate automotive applications such as RKE, PKE, TPMS, accommodate multiple polling schemes including TPMS, RS and up to three RKE channels, and can be configured to support RF protocols across multiple frequency bands, modulation schemes, and data rates. To implement the vast array of configurable content in both Atmel® devices, the desired configuration settings are stored in the built-in EEPROM and automatically applied to © 2010 / www.atmel.com (Optional) (Optional) PB3 PB4 PB5 PB6 PB7 AGND TEST _ 102 SPDT_RX RFIN _HB SPDT_ANT SPDT_RX PB3 PB4 PB5 PB6 PB7 PB2 23 24 22 23 DGND PB0 21 22 DVCC DGND 20 21 PC5 DVCC 19 20 PC4 PC5 18 19 PC3 PC4 17 18 Atmel SPDT_TX ANT_TUNE RF_OUT SPDT_TX PC2 PC1 PC0 VS AVCC XTAL2 XTAL1 VS_PA XTAL2 VS_PA RF_OUT PC3 PC2 PC1 PC0 VS AVCC TEST_ EN 10 11 12 13 14 15 16 9 24 PB1 PB2 PB0 PB1 ATA5830 Atmel ATA5830 ANT_TUNE SPDT_ANT 9 AGND TEST _ 102 32 31 30 29 28 27 26 25 TEST_ EN 8 RFIN _LB RFIN _HB RFIN _LB XTAL1 1 1 3 1 4 3 5 4 6 5 7 6 8 7 ATEST _ 101 1 ATEST _ 101 32 31 30 29 28 27 26 25 17 10 11 12 13 14 15 16 VS = 3V Elements Number Inductor Elements Capacitor 2Number (3) Matching Comments 72(9) 3Matching (5) for matching (3) 43for Blocking 7 (9) (5) for matching Crystal 1 Capacitor Inductor Crystal 1 VS = 3V Comments 14single crystal for 3 for Blocking different frequency ranges 1 single crystal for 3 different frequency ranges Figure 1. Atmel ATA5830 Transceiver Application Circuit SPDT_RX RFIN _HB SPDT_ANT SPDT_RX N.C N.C MISO PWPON LED1 NSS ATA5780 MISO PWPON LED1 24 SCK MOSI NRESET VS AVCC XTAL2 NRESET XTAL2 NPWRON3 TMDO VS AVCC 10 11 12 13 14 15 16NPWRON2 17 TEST_ EN 9 XTAL1 9 MOSI 23 24 CLK_OUT SCK 22 23 DGND CLK_OUT 21 22 DVCC DGND 20 21 NPWRON5 DVCC TRPB 19 20 NPWRON4 NPWRON5 TRPB 18 19 NPWRON3 TMDO NPWRON4 NPWRON2 17 18 TRPA NPWRON1 NPWRON1 N.C N.C N.C NSS Atmel ATA5780 Atmel N.C SPDT_ANT N.C N.C AGND AGND RX_Active RX_Active LEDO LEDO NPWRONG NPWRONG IRQ IRQ TEST _ 102 TEST _ 102 32 31 30 29 28 27 26 25 TEST_ EN 8 RFIN _LB RFIN _HB RFIN _LB XTAL1 1 1 3 1 4 3 5 4 6 5 7 6 8 7 ATEST _ 101 1 ATEST _ 101 32 31 30 29 28 27 26 25 TRPA 10 11 12 13 14 15 16 NSS MISO MOSI NSS MOSI VS = 4.5v to 5.5v Microcontroller SCK Elements Number Inductor Elements 1Number Matching Comments 41 1Matching for matching Capacitor Inductor Capacitor VS = 4.5v to 5.5v Microcontroller MISO SCK Crystal Crystal 4 1 1 Comments 31for forBlocking matching 13single crystal for 3 for Blocking different frequency ranges 1 single crystal for 3 different frequency ranges accessible firmware, making it possible to develop an entire application using just one single IC. Both the ATA5830 transceiver and the Atmel ATA5780 receiver are also highly integrated, requiring very few external components. The application circuits (see Figures 1 and 2) show a standard implementation of each device. For a typical application, the ATA5830 transceiver only requires 10 external elements and the ATA5780 only six external elements. Both devices are packaged in a 5 x 5mm, 32-pin QFN package. In conclusion, the newest generation of configurable Atmel RF semiconductor devices provides the design flexibility needed for rapidly evolving automotive RF-enabled subsystems . Figure 2. Atmel ATA5780 Receiver Application Circuit Atmel Devices Also Have Enhancements Which Simplify Design and Reduce BOM Cost the device on power-up. This enables autonomous (stand-alone) operation and polling for incoming signals from multiple RF systems with differing RF carrier frequency bands, modulation formats, and data rates. Stand-alone operation allows an external controller to sleep while the device polls, validates start of frame, and checks for proper transmitter ID data. The device only wakes the controller when a valid message is detected. This is critical to reducing Ignition Off Draw (IOD) in Automotive Compilation Vol. 7 vehicle-mounted applications and to extending battery life in handheld fob applications. Atmel devices also have some other enhancements which simplify design and reduce BOM cost. The Atmel ATA5830 transceiver device, for example, has an embedded Atmel AVR® microcontroller on the same silicon die. The microcontroller includes 6kByte of Flash and a 24kByte ROM library of user- 2 Design and Security Considerations for Passive Immobilizer Systems Jim Goings, Toby Prescott, Michael Hahnen, Karl Militzer For years, consumers have come to rely on the convenience and added security that a passive vehicle immobilizer system offers . These systems consist of a key fob, carried by the driver, and a base station, mounted in the vehicle . They work together to determine if the driver is authorized to start the vehicle . Of equal or greater importance is the system’s ability to prevent unauthorized sources from using the vehicle . While top-level functionality of a vehicle immobilizer is simple to describe, the underlying technology enabling it is intriguing and sophisticated . This article will explore both the hardware and software aspects of vehicle immobilizer systems as well as offer noteworthy comments on design and security considerations . Communication The prevailing method of communication between key fob and vehicle in passive vehicle immobilizer systems today is with a modulated magnetic field. This field is created by the vehicle’s immobilizer base station from a low frequency alternating current, typically 125kHz. The magnetic field serves three fundamental purposes: A) the power source for the key fob, hence the term “passive”, B) a carrier on which information from the base station to key fob is conveyed, e.g. “downlink”, C) a carrier on which information from the key fob to the base station is conveyed, e.g. “uplink”. authentication process, which has a direct impact on response time. The sections that follow will address these topics in greater detail. System Interfaces The system architecture of a vehicle immobilizer has several layers of abstraction, each representing different system interfaces. Figure 1 provides a visual representation of these layers. Crytographic Protocol Characteristics of magnetic fields that are of particular appeal for a vehicle immobilizer system pertain most to the key fob’s need to operate completely passively, e.g. without a battery. “Downlink” field detection and “uplink” field modulation can both be achieved using circuitry that consumes very little current. Furthermore, harnessing sufficient field energy to power these circuits in the key fob electronics can be achieved with relative ease. During the system design phase, care must be taken to carefully consider key performance parameters such as key fob energy requirements, which affect antenna coil geometries and drive levels, and the 3 Logical Physical Figure 1. Vehicle Immobilizer Interface Layers The Physical Layer A t the lowest level of a vehicle immobilizer system is the physical layer. It consists of a vehicle-mounted antenna coil capable of creating a magnetic field of sufficient magnitude to enable its detection and modulation by an antenna coil mounted in the user’s key fob. © 2010 / www.atmel.com Field Generation and Modulation Vehicle immobilizer systems can be classified in one of two different ways based on how the magnetic field is used to support the transfer data: half duplex or full duplex. In a half-duplex system, the vehicle-mounted antenna coil alternates between periods of energy transfer and data transfer. Uplink data (e.g., fob to vehicle) is modulated using Frequency Shift Keying (FSK). A graphical representation of this communication method is shown in Figure 2. Two points should be intuitively obvious from viewing Figure 2. First, the rate of data transfer suffers from a significant compromise due to the recurring need to perform the energy transfer, e.g., charge up the key fob. Second, the modulated signal is very small compared to the field present during the energy transfer period, making it more susceptible to interference from ambient noise which results in reduced range. These characteristics have caused the popularity of half-duplex systems to wane. The dominant system in use today is the full-duplex system in which the vehicle-mounted antenna coil performs energy transfer AND data transfer simultaneously. Uplink data is modulated using Amplitude Shift Keying (ASK). A graphical Magnetic Field Charges Key Fob FSK Key Fob Return Frequency ƒ¹ (red) Figure 2. Half-duplex Communication Using FSK Automotive Compilation Vol. 7 FSK Key Fob Return Frequency ƒ (blue) 0 4 Magnetic Field Charges Key Fob ASK Key Fob Return Signal - Resistively Damped ( Green ) ASK Key Fob Return Signal - Undamped ( Black ) Figure 3. Full-duplex Communication Using ASK representation of this communication method is shown in Figure 3. Clearly, the ability to simultaneously transfer data while keeping the key fob energized, or charged, provides the design engineer a significant data transfer rate advantage over half-duplex systems. Additionally, the constant carrier field tends to mask out interferences and enables robust communication during the data transfer. Furthermore, this approach can be realized using simple envelope detection circuitry. Because of the popularity of the full-duplex vehicle immobilizer system in the market place today, the rest of this document will focus on this type of system. to predetermined times; T0 for a logic “0” and T1 for a logic “1”. The advantage to this approach is that it embeds energy transfer from vehicle to key fob into the data encoding and ensures the key fob will be supplied enough energy to process the encoded data. However, a side effect of this encoding method is that the data transfer baud rate depends on the logical value of the data bit stream being sent since the transmission times for each binary state are different. See Figure 4 for a more detailed graphical depiction of this coding method. System Interfaces: the Logical Layer 1-bit value The next layer above the physical layer is the logical layer. This layer captures the characteristics and requirements for the coding and transfer of data across the magnetic field. It applies to the bi-directional data transfer that takes place from vehicle to key fob, commonly referred to as “downlink”, as well as key fob to vehicle, also known as “uplink”. ‘0’ Downlink ‘1’ Downlink information is coded using Pulse Length Modulation; typically Binary Pulse Length Modulation (BPLM) or Quad Pulse Length Modulation (QPLM). This method is based on inserting a carrier field gap, Tgap, of fixed duration and setting the gap to gap timing intervals 5 BPLM bit frame example LF-Field Tgap 12 x CLKFC LF-Field Tgap 20 x CLKFC Figure 4. BPLM Coding Method © 2010 / www.atmel.com QPLM is a variation of BPLM that is sometimes used. With this modulation, two bits are transmitted after one gap, and therefore more power is available on the transponder side. In addition, the average baud rate is higher compared to BPLM. The coding method follows the same basic implementation as BPLM, except the allowed number of states is extended from two to four and the predetermined gap to gap timing Clock Data 0 0 1 1 0 1 0 0 Manchester Bi-phase Figure 6. Manchester and Bi-phase Coding 2-bit value QPLM bits frame example LF-Field ‘00’ Tgap 18 x CLKFC ‘01’ LF-Field Tgap Both encoding techniques enable clock extraction from the encoded data stream. This is possible because all time durations in the coded bit stream are quantized to one of two values; T or 2T, where T is what is referred to as a “half bit”. Data rate is fixed by the relationship 1/(2T). Clock extraction merely requires the detection of the minimum time duration element, T, and synchronizing its phase with the coded bit stream. 28 x CLKFC LF-Field Tgap ‘10’ 40 x CLKFC ‘11’ LF-Field Tgap 62 x CLKFC Figure 5. QPLM Coding Method intervals are expanded to cover the additional states. See Figure 5 for a visual representation of QPLM. Uplink Information communicated from the user fob to the vehicle base station is typically encoded using Manchester or Biphase. These encoding methods share several characteristics that differ from the downlink: A) the encoded bit stream always has an average duty cycle of 50%, B) the time to send encoded data is based solely on the baud rate. Automotive Compilation Vol. 7 Protocol Layer The protocol layer defines how individual data bits are grouped to enable communication between the vehicle base station and key fob. It defines how many bits and in which order they are transmitted between the reader and the transponder. As a simple analogy, this can be compared to the rules governing the formation of sentences in using words. The protocol layer would be like the sentence formed from the logical layer which would be like the words. It forms a fixed set of commands along with their allowable responses. Authentication Authentication is the term used to describe the process of deciding whether the driver is authorized to start the vehicle. The simplest form of this is called unilateral authentication. In this case, the vehicle “tests” the key fob to determine if it has been associated/learned to the vehicle. When an additional step is added to this process, in which the key attempts to “test” the vehicle to determine if it has been associated with the key fob, it becomes bilateral or mutual authentication. Clearly, this added step increases security strength but comes at the expense of longer authentication time. 6 Unilateral Authentication Protocol Key Read UID Command Car LF-Field On Detection Header (optional) Key Memory ID Memory ID 4-bit Command + 4-bit CRC Key Memory Read UID ID OK? 8-bit Header + 32-bit ID + 8-bit CRC Stop N Y Random Number Challenge 8-bit Command + N Challenge-bit + 8-bit CRC AES-128 (enc.) Response Challenge AES-128 (enc.) 8-bit Header + M Response-bits + 8-bit CRC = Response N Stop Y Valid OK, it is the right key Start Authentication Command Figure 7. Unilateral Authentication Unilateral Authentication Key Learn: Open/ Secure Typically, the unilateral authentication protocol is initiated by the vehicle and consists of the following steps: 1. Vehicle reads the key fob’s unique ID (not to be confused with the secret key) The Key Learn Protocol is the process that is used to allow the vehicle to establish a secret key and share it with the key fob. Depending on the restrictions and safeguards placed on the initiated Key Learn session by the vehicle, secret keys can be shared openly or securely. 2. Vehicle generates a random number challenge and sends it to the key fob An open Key Learn process would typically consist of the following steps, also shown in Figure 8: 3. Key fob encrypts the challenge (using the secret key) and sends this response to the vehicle 1. Vehicle generates a secret key based on a random number and “proposes” it to the key fob 4. Vehicle compares key fob’s response with its calculated response (using same key and challenge) 2. Key fob “accepts” secret key, saves to memory, and responds with an acknowledgment Note: The vehicle must posses the key fob’s secret key to enable the success of this transaction. The process of sharing the secret key is called “Key Learn” and is described in the next section. 7 3. Vehicle saves secret key to memory after successful receipt of key fob’s acknowledgement If the Key Learn Protocol can’t be protected from eavesdroppers or unauthorized access to the vehicle, it may be desirable to utilize a Secure Key Learn Process. © 2010 / www.atmel.com Open Key Learn 1/2 Key Car LF-Field On Detection Header (optional) Secret Key Random Number 8-bit Command + 128 bit Key + 8-bit CRC Secret Key Learn Secret Key (1 or 2) Cmd Key Memory = Pass N Stop Y Response ( Pass/Fail ) 8-bit Header +8 bit Status + 8-bit CRC Key Memory Figure 8. Open Key Learn Bilateral or Quasi-mutual Authentication A more complex form of an authentication process is the quasi-mutual or bilateral authentication. It is not a full mutual authentication that is implemented in the Atmel® immobilizer system because it does not use random generators on both sides of the system, the car and the key. The implemented solution uses a MAC (Message Authentication Code) to authenticate the car vis-à-vis the key. Again, the authentication protocol is initiated by the vehicle and – in case of a bilateral authentication –consists of the following steps as shown in Figure 9: 1. Vehicle reads the key fob’s unique ID 2. Vehicle generates a random number challenge and sends it to the key fob 3. Vehicle encrypts the random number and appends it to the challenge Automotive Compilation Vol. 7 4. Key fob encrypts the challenge (using secret key 1) and compares it with the received encrypted challenge (MAC) 5. If the result matches, the key fob encrypts it (using secret key 2) and sends this response to the vehicle 6. Vehicle compares key fob’s response with its calculated response (using same key and challenge) Cryptographic Layer The cryptographic layer provides the highest level of encryption. It contains the mathematical function that transforms a plain text message into a secret message. Ideally, this function should have two properties: 1. Unique: For every plain text input, there must be a unique secret text output 2. Unpredictable: It must not be possible to predict a plain text to secret text pair, even if a large sample of known good plain text to secret text pairs was available for analysis 8 Bilateral Authentication Protocol Key Detection Header (optional) ID memory ID 4-bit Command + 4-bit CRC 8-bit Header + 32-bit ID + 8-bit CRC Random Number 128-bit Key 1 AES -128 encryption Stop Ok, is it the right car, continue N = Y Read UID CMD Read UID ID OK? Stop N Y Start Authenitcation Command Random Number 8-bit Command + N bit RandN + M bit (RandN) + 8-bit CRC Hidden challenge (HCH) expanded to 128 bits AES -128 encryption 128-bit Key 2 Car LF-Field On 128-bit Key 1 Hidden challenge (HCH) expanded to 128 bits AES-128 encryption 128-bit Key 2 AES-128 encryption 8-bit Header + M bit {Resp}AES + 8-bit CRC = Y N Stop Ok, is it the right key, car match Valid Figure 9. Bilateral Authentication Protocol Public vs . Private For many years, private cryptographic algorithms were commonplace. However, private algorithms have drawbacks: A) uncertainty of algorithm’s strength, B) lack of being subjected to critical peer review, C) potential wide-scale security compromise should the algorithm be leaked to the public. In recent years, several high-profile examples can be cited that illustrate these weaknesses. Perhaps even more compelling is the lack of interoperability in systems that share the same physical and logical layers. This interferes with basic competitive market forces and in many cases drives higher system costs. In an effort to address these concerns, public opinion has shifted toward the acceptance of a public domain encryption algorithm – the Advanced Encryption Standard (or AES, as it is more commonly referred to). Its origin comes from the 1997 initiative at the National Institute of Standards and Technology (NIST) to select a public-domain encryption algorithm. Within a year, fifteen candidate algorithms were identified and subjected to critical review by the cryptographic research community. This analysis included an 9 assessment of security and efficiency characteristics for each algorithm. After trimming the list of candidates from fifteen to four, NIST subjected them to a second round of public review before finally selecting the AES algorithm in 2000. AES, as we know it today, is a symmetrical block cipher that combines a 128-bit plain text input with a 128-bit secret key to create a 128-bit encrypted output. Due to its symmetrical characteristics, AES can also be used in reverse to combine the encrypted output with the secret key to find and extract the original plain text input. System Security Considerations – Attacks and Countermeasures A common misconception held today is that the security of a vehicle immobilizer system is established by the strength of the encryption algorithm. While encryption algorithm strength is important, it alone does not define the overall system’s resistance to attack. Each of the interface layers in the immobilizer system, algorithmic, protocol, logical and physical, contributes to the system’s overall security and must be studied and fortified against attack. © 2010 / www.atmel.com Algorithmic Security and Countermeasures Physical/Logical Security and Countermeasures As noted earlier, it is imperative that the encryption algorithm possess unique and unpredictable characteristics. In the case of AES, the details of how the algorithm operates is freely available to the public and as a result, it has been subject to critical review by the research community. This, by far, is the best countermeasure available. To date, scientific studies have confirmed the algorithm’s strength as it has withstood the test of time (over 10 years). However, in the case of private algorithms, scientific analysis by the research community was not possible, leaving the strength of these algorithms in question. In fact, many have failed to withstand the test of time and in recent years their weaknesses have been exposed. In recent years, attack methods have grown more sophisticated. “Side-channel” attacks such as Simple Power Analysis (SPA) and Differential Power Analysis (DPA) as well as other “invasive” attacks have been successfully applied to extract secret keys from key fobs. These socalled side-channel attacks measure and evaluate the power consumption of a cryptographic device and combine it with knowledge of the plain text or cipher text in order to extract an otherwise secret key. The theory underlying these methods is quite sophisticated and beyond the scope of this document. The strongest defense against the side-channel attacks noted above are: Protocol Security and Countermeasures In systems using unilateral authentication, attacks on the protocol layer are typically accomplished using “scan” or “dictionary” methods. In a “scan” attack, the attacker receives a challenge from the vehicle and returns random values in response. If the protocol consisted of a 56-bit response, then the bit security is 256 , i.e., it takes 256 trials to get one valid challenge-response pair. To resist this type of attack, the following measures can be considered: • Increasing the response bit length to add complexity • Having the vehicle embed exponentially growing timeouts between consecutive unsuccessful trials • Having the vehicle block trials after a fixed number of consecutive unsuccessful trials are attempted In a “dictionary” attack, the attacker collects valid challenge (from the attacker) response (from the key fob) pairs by communicating directly with the transponder. These pairs are placed in a look-up table or “dictionary” for future reference. Equipped with this dictionary, the attacker then sequentially triggers the vehicle for a challenge, which can be checked in the dictionary for a valid response. If the protocol included a 100-bit response, one would need 251 trials to get one valid challenge-response pair. The “birthday paradox” states that after 2n/2 logged challenge-response pairs and 2n/2 trials, the probability of a valid result is 0.5. Using this, it can be shown that the overall complexity of this attack is 2n/2+1 = 251. Countermeasures to consider in this case are: • Increasing the challenge bit length to add complexity • Implementing a bilateral authentication protocol Automotive Compilation Vol. 7 • Randomization of clock frequency and operation • Interleaving of digital control and the encryption operation “Invasive” attacks dwell on the physical implementation of the encryption-related circuitry on the silicon die itself. The best countermeasures are fairly simple to implement as long as they are considered early in the design process. The following are examples of steps that could be considered: • Metal shielding of memory blocks • Using non-standard synthesis libraries • Scrambling the location of critical digital elements used during encryption • Restricting memory access and automatic chip-erase function if attempted System Performance Considerations Current Consumption System performance has different aspects. One is the power consumption of the key fob. This parameter relates directly to the achievable communication distance between key fob and vehicle base station. Car manufacturers and Tier1 suppliers tend to emphasize the importance of the coupling factor as a critical system parameter. However, it mainly represents the relationship of the mechanical dimensions between the key fob’s antenna and the vehicle base station’s antenna. This parameter is only valid for a given system configuration and depends on antenna inductance, Q-factor, driver current, reader sensitivity, and ignition lock cylinder material. Because of this, use of this parameter alone to compare different systems is inadequate. Of equal if not more importance than the coupling factor is current consumption, especially given that the key side current consumption is a limiting factor in a passive, batteryless environment where 10 the energy is harvested from a magnetic field and stored in a small capacitor. By selecting system components designed for extremely low power consumption and microcontrollers capable of being programmed with well-balanced software (putting the controller in sleep mode whenever possible), the engineer is able to overcome earlier system disadvantages requiring high coupling factors to compensate for high current consumption in the key fob. Authentication Response Time Another important factor in immobilizer systems is the time it takes from turning the key fob inserted inside the lock until the engine starts. This time should be short enough to avoid the driver’s perception of a delay. Depending on mechanical and electrical system design and how quick a person can introduce and turn the key, an overall timing budget in the range of 300ms to 500ms is typically available. A significant part of this budget is consumed through mechanics and overhead in the body control module. What remains is between 100ms and 200ms for the authentication process. A good compromise in terms of speed and security seems to be a bilateral authentication with a challenge length of 100 bits and a response length of 56 bits. In most systems this results in a response time of under 100ms. Error Handling In case an authentication failed for whatever reason, today’s systems require the complete authentication cycle to restart from the beginning and allowing a maximum of three retries within a reasonable time. The retry strategy from Atmel® looks a bit different and enables the system to recover from communication errors more quickly. All commands and optionally the data are protected by a Cyclic Redundancy Check (CRC). Both the key fob and the base station can make use of the CRC to detect errors and signal these conditions to their respective communication partner. This enables the base station to be selective about the amount of repeated info, the last action, the last response, or the last command. This feature enables quicker communication recovery and more attempts at communication recovery in the same amount of time (five-seven retries instead of three). 11 Summary By selecting system components that meet the security and performance expectations of the automotive market place, and support a highly configurable and open-source immobilizer software stack, the task of developing a robust vehicle immobilizer system can be greatly simplified. As a leader in automotive vehicle access solutions, Atmel offers such a complete system solution consisting of both hardware and software. Key fob designs can be realized with the Atmel ATA5580 and the Atmel ATA5795. These devices boast an LF front end, an AES hardware block to perform fast and efficient encryption calculations, coupled with an Atmel AVR® microcontroller that has been optimized for extremely low current operation. Both include programmable flash memory that can be used to run the Atmel open immobilizer protocol or other customerspecific software and are capable of completely passive immobilizer operation. A base station can be realized with the Atmel ATA5272. This device integrates the LF base-station function with an AVR microcontroller with 8K of programmable flash memory. As a final complement to these devices, the open immobilizer protocol software from Atmel is available to users at no cost. It provides an unprecedented level of configurability including many user selectable features enabling the dynamic evaluation of system parametric tradeoffs and accelerates the development and optimization process: 1. A logical layer with uplink and downlink baud rate, bit encoding, and modulation depth 2. A protocol layer with challenge and response bit lengths, unilateral or bilateral authentication, data field CRC, two secret keys, secure or open Key Learn 3. A cryptographic layer with AES crypto clock speed from 125 kHz to 4 MHz “on the fly” © 2010 / www.atmel.com Automotive LIN Bus Driving Sensor Applications Dr. Stephan Hartmann Today’s cars contain, on average, more than 50 different sensors to monitor various physical variables . This number is growing, driven mainly by the proliferation of actuators, which require sensors to deliver the relevant input values . In addition, requirements for signal systems are now increasing, and analog data transfer techniques are showing their limitations . The engineer now faces the challenge of transforming the sensor area into an efficient, high-performance digital subsystem . LIN bus driving sensor technology enables efficient management of digital data, combining the benefits of existing voltage modulation and current modulation approaches . Atmel® provides all necessary products independent of the integration level of LIN applications . Automotive Compilation Vol. 7 12 Sensor systems differ in many respects from other electronic components of the car. The most important difference is that sensors are mostly located outside the vehicle in harsh environments where they are subject to changes in humidity, temperature or pressure. In most cases, sensors also have to be mounted in areas with very limited space and are connected with a 2- or 3-wire harness. pull-up resistor. For the connection to the sensor, a 2-wire connection is used. Disadvantages include thermal power dissipation in the sensor module, as well as a limited data rate dominated by the pull-up resistor. Other current-based transfer methods, such as Manchester-coded protocols, require dedicated transceiver ICs, driving system costs up. The Applications for Sensors are as Diverse as the Application Areas Themselves: • In the comfort area: - Temperature sensors - Solar altitude sensors - Light sensors - Humidity sensors - Dew point sensors • In the powertrain area: - Position sensors - Speed sensors - Pressure sensors - Knock sensors • In the body control area: - Pressure sensors - Gyro sensors A typical sensor node contains the sensing element itself, a microcontroller for signal conditioning, and a transceiver for signal transmission. As the length of the data line is often more than 1m, the data transfer is dominated by analog signal conditioning which has a portion of about 90%. Analog signal conditioning does have some advantages. It is compatible with previously existing mechanical or electromechanical detection systems, and is also easy to use and to plug in. Analog data can be provided in a voltage range of, for example, 0 to 5V and the sensor can then be monitored by an ADC port on a microcontroller which converts the data into the digital domain. Generally, however, requirements for sensor systems are increasing, making analog signal conditioning less attractive. With ADC resolutions up to 10 Bit, and the ability to indicate two different types of failure modes by clamping the signal voltage either to the lowest or to the highest voltage levels, analog techniques have already reached the limits of their performance and will be replaced by different types of digital data transfer. Digital data transfer can be managed either by voltage modulation or current modulation. Both types have advantages and disadvantages. Simple current modulation allows a very cost-efficient design of the ECU as well as of the wiring harness. Inside the ECU, the different current levels can be transferred into voltage levels using a single 13 Figure 1. Basic Current Interface Set-up Voltage modulation has the advantage of allowing a variety of protocols, beginning with simple PWM and moving to more complex versions like SENT, which have higher data rates than current modulation. Additionally, the ECU input can be designed as a capture compare unit on a timer basis. The main drawback of voltage modulation is that a wiring harness with a 3-wire connection is mandatory. Further issues may arise at EMC testing because most PWM drivers do not include a slope rate control. Additionally, ESD protection is low. Figure 2. Basic Voltage Interface Set-up The LIN bus protocol combines all advantages described above. As a two-wire interface, LIN helps save cost in the wiring harness. LIN’s slew rate control in the transceiver ICs supports excellent EMC performance, while LIN’s ESD protection features allow robust system designs suitable for harsh environments. Finally, the high production volumes © 2010 / www.atmel.com for LIN bus transceivers lead to very cost-efficient designs compared to other protocols requiring transceiver ICs (e.g., current-based transfer protocols). and a microcontroller for signal conditioning and protocol handling. Atmel® serves all integration levels with LIN transceivers, LIN system basis chips (SBCs), and LIN system-in-package (SiP) devices. With SiP, the customer benefits from the ultra low-power designs of Atmel AVR® microcontrollers with Atmel picoPower® technology. As a second step, the designs can be converted into single-chip, multi-die SiP designs. This saves PCB space and allows the engineer to include all electronics in the connector of the sensor element. Finally, by integrating the sensor element into the chip, and by implementing an intelligent state machine, the engineer takes a further step toward advanced, single-chip LIN sensor node designs. Figure 3. LIN Interface Set-up Modified to Support the Current Supply of the LIN Sensor Slave UC The concept for the system design specifies a pull-up resistor at the ECU level. This is required to control the current supply for the LIN slave sensor through the data line. At the LIN slave sensor, only a buffer capacitor is required. Depending on the capacitor value, the data rate can be chosen up to 100 kbit/s. Supply voltage reduction provides an additional option for influencing the data transmission speed. As the dominant and recessive levels are referenced to the supply voltage, a reduction of the supply voltage directly reduces the gap between bus low and high level (the delta) to reach the correspondent levels. A 2V supply voltage reduction leads to an increase in the data transmission rate of roughly 15%. Unfortunately, the supply voltage cannot be lowered in all cases. The time portion of the bus-dominant level must also be considered, as this state discharges the buffer capacity of the LIN slave sensor Sensor system design using LIN can be viewed as a threestep process. First approaches to a discrete slave-node design include a sensing element, a LIN system basis chip, Voltage 99% 98% 95% 86% 63% t 1τ 2τ 3τ 4τ 5τ 1τ 2τ 3τ 4τ 5τ Figure 5. Capacity Discharge Depending on the Bus-dominant State Time In summary, LIN not only enhances the driving of cars from the in-vehicle networking point of view, it also allows the rumble change of the sensor area to a cost-efficient and high-performing digital sub-system. Atmel offers all necessary products regardless of the integration level of LIN applications. To support low-power designs, the AVR microcontroller with picoPower technology is key. In addition, engineers can design the most robust and EMV-tolerant systems with the leading-edge EMC and ESD performance of LIN transceivers and SBCs from Atmel. 20 18 16 Special thanks to Daniel Yordanov and Keith Nicholson for supporting me during the writing of this article. 14 12 10 8 VCC 6 LINH 4 2 LINL 0 Figure 4. Bus Level States Bus Level States Depending on the Supply Voltage Automotive Compilation Vol. 7 14 Designing Next-Generation Key Fobs Paul Lepek Key Fobs Today 915MHz). However, for a PE system the LF downlink is used by the key fob to compute a Received Signal Strength Indicator (RSSI) value and thus the fob’s physical coordinates in relation to the vehicle while the RF link is used to execute the authentication protocol with the vehicle. RKE and PE system fobs are designed to be powered by a small coin battery intended to last for the life of the vehicle. Today’s key fobs can be generally subdivided into two different functional categories. The first includes Remote Keyless Entry (RKE) devices which require some sort of human intervention or a physical interface of the user to the key fob (e.g., key push) in order for the fob to produce the desired function effect such as unlocking a door or opening a sunroof. The second group of devices provides similar functionality but also features an Atmel has Introduced Next-Generation Transceiver added level of comfort by performing the same and Receiver Devices with Configurable Options function without physical intervention by the user. Instead of the push button or touch sensor interface, a Passive Entry (PE) identifies the user (and the Moreover, all key fobs support engine immobilizer system key fob) as a legitimate entity and automatically triggers authentication. To prevent theft every automobile uses an authentication or issues a request (e.g., passive door unlock, immobilizer system which authenticates engine starts. In trunk release, etc.). this case the key fob acts as a passive authentication tag Both systems are based on a preprogrammed key fob device similar to the RFID tag but with a larger feature set. Most ID and authentication protocols which include an encryption automotive key fobs use Near Field Communication (NFC) stage for authorizing the issue of key fob commands to the transponders which communicate with the engine controller. vehicle. In this way the key can be identified by the vehicle The transponder is integrated into the key and is a passive and vice versa before any action is executed. All RKE-based device. It does not need a battery for operation, but instead systems require key fobs to support RF links which fall into uses a magnetic field generated by the LF vehicle coil. It also Industrial, Scientific and Medical (ISM) frequency bands transmits the device ID and executes a special immobilizer (i.e., 0 - 135kHz, 13.56MHz, 315/433MHz, 869MHz, and 15 © 2010 / www.atmel.com Table 1 . Key Fob Features Today Class Immobilizer1 Physical Interface 3 (Button, Touch, etc.) Batttery/ Recharge2 Remote Start Remote Entry Accessories Passive (Entry/Go) Accessories Personalization Settings Time/ Data Logging in Fob4 x Basic x - - - - - x RKE (std.) x x/ x - - - x RKE (ext.) x x/x x - x - x PE (std.) x x/x x x x x/x x PE (ext.) x x/x x x x x/x x x x Immobilizer support includes secure fob and vehicle authentication via the LF field using an integrated LF transponder. 1 The fob includes a built-in battery with the option of recharging the battery via the LF field. 2 Remote Start and Remote Entry are controlled via RF uni- or bi-directional link. Control of accessories can either be done using RF or IrDA links. 3 4 Data can be logged such as time stamp data, last device ID, last vehicle service date, and much more either via LF or RF links. protocol for its authentication but all communication takes place via an LF field generated by the vehicle. Expanding Fob Applications Originally key fobs were designed for only one purpose: to unlock the door and start the engine with the metal key. Later, RKE devices were used to remotely (HF field) unlock the door. The integrated contactless passive transponder (LF field) then unlocked the steering column and enabled the engine start. Only more recently fobs began to penetrate convenience, general utilities, secure communications, and secure access ID applications (even extending to payment systems and e-ticketing). These functions were not developed before due to a lack of hardware and software resources, primarily because of the fob’s physical size and power consumption. Recently, however, it has become possible to overcome these shortcomings by incorporating much larger user and program memories, and the use of faster, more compact, and ultra-low power processors without increasing the cost of production. Additionally, the integration of flexible, reconfigurable, and secure authentication peripherals can be made feasible. These types of peripherals include crypto units, secure key management features, and integration of smart cards useful for payment, user ID, and cipherbased authentication systems. An automotive key fob can therefore be used not only to interact with the vehicle but Automotive Compilation Vol. 7 also to gain entry to a park garage, ski lift or to purchase train tickets. This can deliver considerable benefits when goods are purchased using one of the major credit card networks. The secure user memory can also be used to store personal and secure information as personal data ID and provide transit information for e-ticketing. System-in-fob Hardware Resources • Ultra-low-power 8-bit microcontroller • Large Flash program and EEPROM data memory (memory segmentation with locks) • RF communication interfaces - Infrared (IrDA) IF - Immobilizer IF at 125kHz - Passive entry IF at 125kHz (RX only) - Smart card IF at 13.5MHz - RKE IF at 315, 413, 868, 915MHz (frequency hopping) • Power management (optional battery charge) • Hardware cryptological unit (AES-128) • Integrated proximity coupling smart card (ISO 14443) • Cyclic Redundancy Check (CRC) block • Serial interfaces (SPI, SSI) • ISP/debug (dW) • Analog comparator • Flexible GP timers and WDT • Oscillators: RTC, INTRC (125kHz, 4MHz) 16 The heart of a modern key is an ultra-low power microcontroller with sufficient program and data memories. Typical program memory can range from 8KB to 16KB and beyond with its data memory ranging from 1KB to 2KB depending on application requirements. Because of secure application support, the key fob’s program and data memories must have provisions for memory segmentation and locks in both memory blocks. For example, the application firmware resident in program memory can be divided into separate memory sectors (e.g., application and immobilizer sections). Also data memory may have its own partitioning which can allow for soft and hard memory locks when it comes to releasing user-sensitive device-stored data (e.g., authorization password or secure key). While the microcontroller core executes application firmware, the secure user and key data is stored in the on-chip nonvolatile memories (EEPROM). The core uses various wireless communication peripheral interfaces to communicate with the infrared transceiver (IrDA), the LF transponder (125kHz), the 3D LF receiver (125kHz), the smart card (13.5MHz) and the RKE transceiver (315, 413, 868, 915MHz). Flexible serial interfaces can be shared such as SPI or Serial Synchronous Interface (SSI) to enable data exchange with every communication peripheral. A hardware data integrity check module, based on a Cyclic Redundancy Check (CRC) checksum algorithm, supports validation of received data. The fob must also contain analog peripherals such as internal oscillators where F = 125 kHz and 4 MHz to generate its internal clock signals used for the transponder front end and the microcontroller core, respectively, with low frequency deviation across VDD and temperature. The supplied analog comparator can facilitate detection of VDD drops and prevent data corruption during nonvolatile data memory writes in passive mode. Power Management SPI FRC -Oscillator Contactless Interface/ Transponder EEPROM/ 2KB RTC 3D LF Receiver Volt. Monitor ECIN Clock Management & Monitoring Contactless Interface/ SmartCard SRC Oscillator Smart Card Atmel AVR - Core Watchdog Timer IR - Driver Timer Block Crypto Unit PM – ROM 2kB SRAM 512B Data Mem. Secure EEPROM . CRC POR / BOD & RESET PM - Flash 14kB debug WIRE RF TX/TRX Module Serial IF IO-Ports Figure 1. Fob Floor Plan A unique feature of the immobilizer transponder interface is that it is closely bound to the power management unit which is used to provide power supply voltage, VDD, while exchanging LF data with the immobilizer in passive mode. In this mode all other communication with the key fob is disabled to support batteryless operation. Some key fobs may also support the battery charge feature which is integrated into the power management module for recharging the battery with the engine running. Wireless Data Communication Interfaces An entry/immobilizer system consists of at least two communication partners where one side is on the vehicle and the other on the key fob. Depending on the link type, there are several possible communication interfaces available, including: 1. IrDA link for convenience and comfort applications While in secure smart card mode the device can also operate in passive mode and exchange as well as encrypt and decrypt proprietary data using its own crypto module with the reader at FC = 13.5MHz. It can use its own device memory or the fob’s internal nonvolatile memory. Integrated crypto modules can support many different cipher algorithms—the most popular being the 256-bit block cipher known as the Advanced Encryption Standard (AES) based on the Rijndael algorithm which can be used with a 128-bit, 196-bit or 256-bit secret key. 17 2. PE/PEG link to enter and start engine, including LF downlinks and UHF uplinks 3. Immobilizer link to start engine and emergency vehicle entry, including LF down- and uplinks 4. Chip card as user ID, authorization, authentication at pay stations, including HF down- and uplinks 5. RKE as entry authentication, including UHF down- and uplinks © 2010 / www.atmel.com Table 2 . Modern Key Fob Communication Links – Overview Application Standard Link Type FC Modulation Anticollision Data Encoding BR [Baud] Range Average Current PE/PEG Custom 3D LF downlink 0130kHz ASK x PIE 3.9k 3 - 5m 2 - 20μA UHF uplink 315, 433, 868, 915MHz ASK/FSK - Manchester, Biphase Up to 80k 30 - 120m 8 - 20mA LF downlink 0130kHz ASK - PIE 3.1 -8.9k 2 - 10cm 40 - 260μA Manchester 4.4k PIE, Miller, NRZ Up to 20k 5 - 20cm 20 - 120μa Up to 80k 30 - 120m 8 - 20mA Immobilizer/ Emergency Entry ISO14223 /Custom LF uplink Chip Card ISO14443 ISO15693 ISO18000 HF downlink ASK 13.56MHz HF uplink RKE Custom UHF downlink ASK/PSK x ASK/BPSK 315, 433, 868, 915MHz ASK/FSK/ PSK Manchester, NRZ x Manchester, Biphase UHF uplink VBAT=1 & VFLD=0 (Start-up) The summary of the communication channels and a brief overview is shown in table 2. Enter RKE Mode RKE Secure and Reconfigurable Firmware Application firmware which supports the complete functionality and feature set is the fundamental building block of the key fob. It may consist of many different modules and must encompass all functional and likely operating scenarios, including battery failure which comprises emergency or passive operation mode. To improve reliability it is a common practice to keep both application and immobilizer programs separate and in two distinct program spaces. While the immobilizer firmware supports distinct engine starts, the application software controls all other fob functionalities including RKE, convenience or user-ID applications. The immobilizer/emergency functionality is required to take priority over any other function, which is the equivalent of an override which suspends any function currently in progress when the LF field is detected at the transponder LF coil. Figure 2 depicts a flow diagram and interaction between application and immobilizer firmware. Automotive Compilation Vol. 7 APPLICATION SW MODULE VFLD? Yes No VBAT=0 & VFLD=1 (Start-up) Enter IMMO. Mode IMMOBILIZER APPLICATIONSW MODULE VFLD? No Yes Figure 2. Immobilizer and Application Firmware Interaction Flow Chart All data communication is fully supported in the fob’s firmware. Various communication protocols including unilateral, bilateral for immobilizer, PE/PEG and RKE systems can be fully configured by the application software. Based on 18 A major advantage of a next-generation key fob is its in-field programmability, which can be very helpful in the event of a firmware or user data upgrade or programming. The fob can be initially configured using its dedicated general-purpose software via the LF field while the final test is performed at the factory. The user data can be added later by the Tier1 or OEM without modifying the original configuration. Even while in the field the fobs can be reprogrammed with new Coupling Factors for Immobilizer SystemSystem Coupling Factors for Immobilizer 9.00% 8.00% 7.00% Coupling factor [%] protocol topology, the application software controls dedicated peripherals by enabling them, and reading data during RX phases, and writing data during TX phases of the protocol as soft triggers (e.g., immobilizer and PE applications) or hard triggers (e.g., RKE or IrDA applications using a push button interface). 6.00% 5.00% 4.00% 3.00% 2.00% 1.00% 0.00% 0 0.02 Distance d [m] Abstraction Level Software Model Figure 4. Immobilizer Coupling Application Layer Protocol/Session Layer API, Configuration Data I/O Transport/Link Layer Physical Drivers Resources Figure 3. Software Partitioning as a Bottom-up Approach application and user data via one of the communication interfaces at a later date. Of course, in this case only a single functionality can be enabled while using memory locks to provide security. This is especially useful when used as a pay token in e-commerce or e-ticketing environments. 0.04 0.06 0.08 Lr=Rehfeld, Lt=2.45mH Lr=Rehfeld, Lt=5.1mH Lr=738uH, Lt=2.45mH Lr=738uH, Lt=5.1mH Figure 5 shows one complete challenge-response authentication protocol which could be used in a passive automotive immobilizer application. The LF field voltage (green) is enabled for 160ms. The field is damped (2.2V) during RX state and then switched to the undamped level (6V) during TX state. The charge storage capacitor voltage (yellow) which provides VDD to the transponder is immediately charged to 2.2V during the RX data stage. The transponder encrypts (AES-128) received plain text data (128-bit challenge) and transmits the response. In many immobilizer systems the system authentication time is a major concern. To minimize authentication time, the number of bits transmitted can be reduced without compromising system security. It is common for authentication time TAUT < 130ms at BR = ~3.9kBaud. Transponder LF Field Coupling Transponder-to-base-station coupling still remains the most challenging aspect for key fobs. The proper transponder coupling can be achieved when sufficient energy is transferred from the base station to the transponder for the transponder to communicate with the base station. During design, the L-C tank must be carefully selected for optimum energy and communication performance. Figure 4 shows a typical transponder coupling at 125kHz vs. operating distance from the base station coil5. Figure 5. Immobilizer Protocol Execution Scope Shot/Power Analysis 5 19 Assumes the fob coil is placed in the center of the base station coil where the coupling is best © 2010 / www.atmel.com Figure 6 shows field voltage and VCC traces as the key fob is energized by the field and begins to receive a BPLM data stream. Field gaps are visible which separate field ‘On’ interval which is decoded by the fob using a dedicated hardware peripheral. The fob’s microcontroller is in sleep mode 95% of the time to save power consumption. Table 3 . RF Transceiver Parameters Sensitivity Antenna Gain POUT Average Current Receiver -109dBm -6dB - ~6 - 8mA Transmitter - -18dB 10dBm ~9 - 10mA Table 3 shows some typical RF transceiver parameters. A snapshot of the transmitter spectrum taken at 433MHz during transmission of an RKE message to the vehicle is shown in Figure 7. The transmitter carrier frequency, the span, power output, and device setting are configured using the serial interface by shifting configuration data (in this case the 32-bit configuration word) into the RF transmitter via the MCU when the user presses the open-door button. Figure 6. Transponder at Power-up RF Communication Links Both RKE and PE/PEG systems utilize HF communications links. In comparison to LF links, HF links are superior in their operating range (up to several hundred meters) and baud rate (up to 80kBaud can be achieved). RF transceivers currently available on the market use N-fractional PLL frequency tuning techniques where the carrier frequency can be selected in firmware by the MCU. Some devices allow large tuning frequency variations, permitting more design flexibility. The transceiver’s operating range remains a key performance parameter. To extend operating range, it is common for the transmitter power to be as high as 12.5dBm and the receiver sensitivity to be less than -100dBm. Antenna design is also a determining factor providing additional performance gain. Although whip antennas add additional performance gain, small loop antennas printed on the PCB are usually chosen for use in automotive key fobs. Power consumption is another critical factor on the receiver as well as on the transmitter side. Power consumption can be reduced by selecting higher data baud rates. Choosing ASK modulation tends to lower operating current since the power amplifier is momentarily disabled during modulation. Automotive Compilation Vol. 7 Figure 7. RF TX Spectrum at 433MHz References: 1. ISO 14223 – Advanced Transponders Standard 2. ISO 10536 – Close Coupling Smart Cards Standard 3. ISO 14443 – Proximity Coupling Smart Cards Standard 4. ISO 15693 – Vicinity Coupling Smart Cards Standard 5. ISO 18000 – Item Management Standard 20 Capacitive Proximity Detection in the Automotive Industry Luben Hristov There has been a steady rise in demand for proximity detection sensors in automotive applications which reliably detect the presence of objects near the sensor surface without physical contact and the number of possible proximity detection applications is countless: • Door entry control: detecting a hand approaching the door handle to initiate the car unlocking process • Illuminating and waking up the touchscreen when a hand approaches the screen surface • Switching interior car lights on/off when the hand is near the sensor • Detection of simple spatial gestures to switch devices on/off • Sensing the presence of large objects around the car during parking Many different proximity detection methods exist, for example, capacitive, infrared, ultrasonic, optical, etc. For the 5mm to 300mm proximity detection range, capacitive 21 sensing has many advantages compared to other methods: excellent reliability, simple mechanical design, low-power consumption, and low cost base. This article describes capacitive proximity detection technology from Atmel®. Atmel is a leading touch solutions manufacturer with many years of experience in this field. The company’s capacitive sensors are based on charge-transfer technology—a method pioneered by Atmel where voltage is generated on the sampling capacitor during the repetition of a specific control sequence applied over the I/O pins. Atmel currently holds multiple patents in the area of charge-transfer technology for self-capacitance sensors (Atmel QTouch®) and mutualcapacitance sensors (Atmel® QMatrix). The Atmel chargetransfer technology delivers key benefits to the user and offers advantages compared to other capacitive measuring methods: increased flexibility, very high sensitivity, excellent moisture resistance, and noise immunity. QTouch and QMatrix technologies have been implemented in multiple touch controllers supporting touch buttons, sliders, wheels as well as touchscreens. Proximity detection support is also available with some of the standard products. Atmel is now developing and manufacturing new proximity algorithms © 2010 / www.atmel.com conductive surface as shown in Figure 1. Please refer to the Atmel application note about using active shields at http://www.atmel.com/dyn/resources/prod_documents/ankd02_103-touch_secrets.pdf, page 7. A further advantage of active shields is their neutralizing effect on water films. • Moisture resistivity: Moisture-induced changes in the measured signals can be more significant than changes from approaching objects. Water film on the surface is one of the biggest problems for capacitive solutions. Water films are more or less conductive and create a change of the measured signals that is similar to normal touch events. There are mainly two ways to handle effects caused by water films: 1. Use of active shields (described above) Figure 1. Active Shielding of Proximity Sensor to increase sensitivity to support finger or hand detection ranges of up to 200mm and more. The release of standard products and software library modules is being planned for 2011 and 2012. Capacitive proximity sensors measure the capacitance change between the single electrode and ground (selfcapacitance sensors) or between two electrodes (mutual capacitance sensors) as objects approach electrodes. While constant capacitance is between 10pF to 300pF, the capacitance changes are typically extremely small, ranging from a few fF to several pF. Since the electrical field lines around the self-capacitance sensors spread far away from the sensing electrode, self capacitance is the preferred proximity detection method over mutual capacitance where field lines are largely concentrated in the area between the transmitting and receiving electrodes. Characteristics of Capacitive Proximity Sensors For Automotive Applications: • High sensitivity: Detecting small changes in the measured capacitance requires increased and stable sensitivity. Special measures should be taken to reduce negative effects on sensitivity caused by capacitive loading, especially if the sensing electrode is placed on a conductive surface (metal plane, car body, etc.). An active shield layer is used to reduce the negative effect of capacitive loading between the electrode and the Automotive Compilation Vol. 7 2. Shorter charge transfer time – the water film could be utilized as a distributed RC circuit (as shown in Figure 2). Reduced charge transfer pulses will prevent full charging of the distributed capacitors C and hence reduce the impact of the water films. Best results can be obtained if the charge transfer time is in the range of 100ns to 250ns. A proper mechanical design of the sense area and the use of the appropriate materials prevent the emergence of thick water films on the sensing area. Figure 2. A Water Film Acting as a Distributed RC Circuit • Temperature stability: In automotive applications extreme and rapid temperature changes may occur at any time. Special care should be taken with regards to a stable mechanical design – even the smallest gap changes near the conductive surfaces may cause false detection. • Noise immunity: Due to the high sensitivity, noise interference could compromise normal operation of the proximity sensor. The electrical and mechanical design of the PCB should be carried out to avoid noise interference caused by adjacent cables or conductive surfaces. • Fast response time: The expected response time is usually between 10ms and 100ms 22 The following sections provide more detailed scenarios of automotive capacitive proximity detection. Door Entry System One example of capacitive proximity detection is in car door entry systems (see Figure 3). The proximity sensor that detects hand approaches is located within the car door handle (1). Once object proximity has been detected, the main unit (2) sends a wake-up signal via the LF antenna (3) which activates the car key transmitter (4). The car key transmitter then exchanges information with the RFID receiver (5) and – if the code matches the main control unit (2) – the door is unlocked. The entire process of proximity detection and ID recognition takes a fraction of a second. This means when the hand pulls the door handle, the door is already unlocked. causes the light to switch on, a wave of the hand in the opposite direction switches it off. The system is able to analyze the signals from the proximity sensors and to decide whether to switch the lamp on or off. There are many different options available for designing sensing electrodes inside a light – from using thin copper wires to conductive polymers that can be applied directly over the plastic. The advantage of using proximity detection rather than touch detection in door entry systems is the extended time to identify a person. As a result, the door lock state will always be resolved before the door handle is pulled. Figure 4. Spatial Gestures for Switching Devices On/Off Inside the Car Conclusions • Implementation of capacitive proximity sensors in automotive applications paves the way to a broad range of comfort applications Spatial Gestures to Switch Devices On/Off The simultaneous use of two or more capacitive proximity sensors enables simple spatial gestures such as hand waving in front of the device to be detected. Figure 4 shows a simple example of such a system to switch lights on/off inside the car – a wave of the hand in front of the light in one direction 23 Signal Figure 3. Door Entry System with Proximity Detection • Moisture and rapid temperature changes are the main challenges for capacitive proximity sensors used in automotive systems. These challenges can be overcome by utilizing the most recent innovations in capacitive sensing technology from Atmel. Capacitive ProximitySensor L L R R © 2010 / www.atmel.com Extending the Range of E-vehicles with Li-ion Batteries Using Active Balancing Claus Mochel Global warming has now become the main talking point of our era, and electric or hybrid vehicles provide an excellent way of saving fuel and reducing CO2 emissions. However, electric and full hybrid vehicles have one particular Achilles heel, which is the capacity of the battery and therefore the limited range of these vehicles. The maximum battery size that can be installed in a vehicle is limited because of its volume and weight, making it all the more important to make optimum use of the battery capacity that is available. In order to provide the several hundred volts that are needed in modern high-performance batteries for electric vehicles, several individual battery cells are connected in series. Each cell in a battery pack such as this differs from the others, e.g. in terms of cell capacity, self-discharge rate, different temperature characteristics and cell impedance – differences that are intensified by the ageing effect of the battery. When the cells are being charged, this leads to a situation whereby some cells have not yet received their maximum possible charge, but other cells are already fully charged. Unless additional measures are taken the charging procedure has to be aborted, since individual cells would otherwise be overcharged, which would damage or possibly even destroy the battery cells. The situation is similar with regard to discharging. Whereas individual cells are already discharged, others still contain enough energy to continue powering the vehicle (in theory). However, the vehicle must not continue to be driven, since the weaker cells would otherwise be overdischarged, which would lead to the destruction of these cells. In order to avoid all of this, active balancing between the individual battery cells is required. Automotive Compilation Vol. 7 Passive Methods Convert Valuable Energy Into Heat Loss The method that is most widely used at present is passive balancing, in which cells that are already fully charged are discharged again using a resistor so that the other cells can continue to be charged. The disadvantages of this method are obvious: • Cells can only be discharged for the purpose of balancing • The discharge current causes power loss in the bypass resistor • Valuable energy is converted into heat and is no longer available for powering the vehicle • The range of the vehicle is reduced While the passive balancing method only converts the energy that is stored in the cells into heat loss, active balancing enables to transfer the charge from one cell to another. There are several ways of achieving this charge transfer, e.g. using switched capacitors or inductors. When the capacitive method is used, a capacitor is connected in parallel with the cell with the higher voltage. Once this has been charged, it is connected in parallel with the cell with the lower voltage and can charge it up. This procedure is repeated until the same voltage is present at both cells. The method that uses capacitors is extremely cost-effective, but has the disadvantage that the average balancing current is limited to less than 50mA. This limitation does not exist 24 MBAT7 MBATn+2 DISCH6 MBAT6 DISCH5 RIN PMOS Zelle n+1 MBAT4 CSHUFFLE MBATn+2 CIN Zelle n+1 DISCHn-+1 MBAT5 DISCH4 RIN PMOS CIN NMOS RPD DISCHn-+1 CSHUFFLE NMOS RPD MBATn+1 MBATn+1 DISCH3 MBAT3 Zelle n DISCH2 DISCHn Zelle n DISCHn MBAT2 MBATn DISCH1 MBAT1 Figure 1. Passive Balancing Using Resistors Figure 2. Active Balancing Capacitors with the inductive method, and in this case it is easily possible to have balancing currents of 1A or more. Active Balancing Method for Rapid and Almost Loss-free Charge Transfer Active balancing is realized by connecting an inductive resistor in parallel with the cell from which the charge is to be taken. This results in a constant increase in the current flow in the coil. Once the coil has been decoupled from the cell that is being discharged via a transistor, the energy stored in the cell can charge the neighboring cell via a diode. It is therefore possible to move charges to and fro between the individual cells extremely efficiently and almost loss-free. This method has some decisive advantages: • Balancing currents of 1A and more are possible • Balancing is essentially loss-free • Balancing is extremely rapid • The efficiency and the capacity of the battery are increased • The range of the vehicle is increased Compared to the other methods that have been mentioned, implementing an active balancing method using inductive resistors is not exactly low-cost because of the relatively high cost of the inductive resistor component. However, this 25 MBATn is not really a problem. A modern high-performance battery currently costs approximately USD $10,000 Even if only gaining an extra 10% of capacity using the inductive balancing method, this represents a value of USD $1,000 – an amount for which a considerable number of inductive resistors can be purchased. 1 = Discharging a cell 2 = Charging a neighboring cell Cell n Cell n-1 1 2 Figure 3. Active Balancing Using Inductors Individual cell monitoring is required for Li-ion batteries for safety reasons, since these cells burn when they are overloaded, and can even explode in extreme cases. As well as the overvoltage, undervoltage and temperature monitoring that is required, additional functions such as precise charge condition determination are needed. A component is now available on the semiconductor market with which both these functions and also the different balancing methods can be realized. With the Atmel® ATA6870, each cell has individual electronic monitoring in order to provide functions such as state-of-charge determination, active/passive balancing or overvoltage, undervoltage and temperature monitoring. © 2010 / www.atmel.com Measurement Data Recording and Active Balancing for Li-ion Battery Systems is the world’s only device with the possibility of achieving both passive balancing and active balancing between the individual cells using capacitors or inductive resistors in high-performance batteries with up to 300 cells or more connected in series. In order to prevent the balancing from taking an extremely long time because of the large number of cells, balancing can be carried out simultaneously on any number of cells with the ATA6870. With the aid of this circuit it is now possible to develop low-cost, efficient and safe battery management systems that can squeeze the last Coulomb out of a battery due to extremely precise recording of the charge status of the individual cells and the efficiency of the balancing method that is used, and make it available for driving operation. This is another step that increases the range of electric vehicles and plug-in hybrid vehicles and therefore contributes to the triumphant march of these vehicles. Cell 1 Reference ADC Charge Transfer Digital Level Converter The heart of the Atmel® ATA6870 consists of six highprecision 12-bit AD converters. Each cell is monitored by a separate AD converter, which has several advantages. On the one hand, all cells can be measured simultaneously, and on the other hand the cell voltage does not have to be transferred analog to ground, which would reduce the accuracy of the system. As well as providing efficient balancing, another prerequisite for making optimum use of the cell capacity is precise recording of the cell voltage because of the extremely flat characteristic curve of Li-ion battery cells. The ATA6870 therefore transfers the digital voltage value to ground without loss of accuracy after analog/digital conversion. As well as the six high-precision AD converters for measuring cell voltage, the ATA6870 Standby 3.3V Voltage Regulator Logic Cell 6 Reference ADC Charge Transfer Digital Level Converter 3.3V Internal Voltage Regulator Current Supply Cell Temperature Measurement Digital Level Converter Communications Bus Microcontroller Interface Microcontroller Figure 4. Block Diagram of the Atmel ATA6870 Li-ion Battery Management IC Automotive Compilation Vol. 7 26 Precision Battery Monitors for Standard Lead Acid Batteries Ensure Engine Start-up Claus Mochel Today, one in three car breakdowns is related to the vehicle’s electronic equipment. Today, high-end cars incorporate up to 65 electronic control units (ECUs), compared to only 10 ECUs in 1995. Battery monitoring is one of the most important current ECU applications, since 60% of breakdowns caused by electronic devices are caused by defective or empty batteries. Batteries now face many challenges for energy management. Formerly, the battery’s main task was to provide high-power output for a short time to enable engine start-up. This has changed dramatically, Today, the battery must provide continuous power to the large number of electronic devices inside the car. This function can be problematic in winter, when battery capability is reduced through cold temperatures and high energy consumption due to loads such as electrical seat heating. In addition, driving in slow traffic means that the car generator provides limited energy to charge the lead acid battery. 27 27 Concerns about global warming, along with resulting stringent legal provisions for reduction of vehicle CO2 emissions, are driving an additional challenge for energy management: micro-hybrids. These cars save fuel by switching off the combustion engine when stopping (for example, at a traffic light) and then automatically restarting the motor when the driver wants to continue. The primary issue with such an engine is that the vehicle’s starting capability must be guaranteed under all circumstances. Otherwise, traffic chaos is bound to develop, given the growing number of vehicles of this kind on city streets. With the advent of micro-hybrids, reliable battery diagnosis and modern vehicle energy management become all the more important. The core of such an energy management system is an intelligent battery sensor (IBS) to constantly monitor the charge and discharge currents, the battery voltage, and the temperature. The resulting data is used to determine the battery’s load and state-of-charge and for distributing power within the car. Priority is given to critical functions such as engine start-up. Furthermore, the sensor calculates the battery’s state-of-health (SoH) and alerts the driver when battery lifetime is expiring in order to avoid car malfunction or breakdown. With IBS information, engineers can also reduce fuel consumption and resulting CO2 emissions. This is done by tailoring the charging cycles depending on the required current for the various loads and the energy available from the generator. In cars without an IBS energy management system, the generator must provide a continuous electrical charge for the battery while the motor is running; with such a system, the generator can be disconnected from the battery when the battery charge is sufficient and the vehicle’s engine is under heavy load. © 2010 / www.atmel.com Atmel® has provided initial models of a microcontroller perfectly matching the requirements of an advanced IBS system. The device is based on the well-known Atmel AVR® microcontroller core in Atmel products, with additional mathematical extensions adapted to the requirements of battery management systems. The circuit has two 16-bit Σ-Δ analog-to-digital converters (ADCs) with synchronous sampling; the accuracy of these two ADCs is due to a very precise 5ppm/K reference voltage with special second-order temperature compensation. The first ADC is dedicated to current measurement and has a large dynamic range. Using a 100μOhm shunt resistor, currents from 1mA up to 1500A can be measured due to a programmable gain amplifier (PGA) at the input of the ADC. The second ADC is dedicated to system temperature determination and monitoring of the voltage of the lead acid battery. The temperature can be measured using either the IC’s in-built temperature sensor or an external sensor. The measurement of battery voltage measurement is particularly interesting. This measurement requires an extremely precise resistor divider, which is integrated into the device. As a special feature, the divider can be connected directly to the battery pole to enable precise voltage measurement without failures caused by a reverse protection diode or other protection devices. This is very important because systems require reliable battery data regarding state-of-charge and state-of-health. Reverse polarity protection at this pin also poses no problems because the device is based on SOI (silicon-on-insulator) technology. SOI enables the design of circuits operating at voltage levels lower than substrate voltage, since SOI devices are isolated by an oxide instead of P-N junctions, which avoids issues with parasitic diodes and transistors. The microcontroller device also has some additional features. First, due to its integrated voltage regulator, it is able to directly use the vehicle’s power net for operation. Second, it is available with 32kB and 64kB flash memory. Third, in addition to the in-built power supply, it incorporates a LIN transceiver based on the Atmel LIN transceiver IP with excellent electromagnetic compatibility (EMC) characteristics. Atmel products based on this LIN IP have received approval from nearly every OEM worldwide, ensuring smooth integration into a vehicle network. Fourth, Atmel provides a version of this microcontroller without a voltage regulator or LIN transceiver for systems that handle these functionalities externally. Finally, due to the restricted space and harsh environment of IBS applications, Atmel provides the ICs in a robust QFN 7 x 7mm package. This enables customers to design intelligent battery sensors the size of a postage stamp, which fit into the pole niche of almost any standard lead acid battery. In conclusion, the new microcontroller device from Atmel provides an advanced intelligent battery sensor system to ensure starting capability in today’s vehicles, including microhybrids. 16-bit S D ADC Voltage Regulator Oscillators Temperature Automotive Compilation Vol. 7 - Shunt Timer/Counter Reference Low-power Atmel AVR CPU + Watchdog 16-bit S D ADC LIN Transceiver to LIN Bus Supervision and Diagnostics 28 DC and BLDC Motor Control ICs Klaus Schweizer Atmel® has more than 25 years of in-depth automotive design expertise. The automotive specialist supplies drivers, predrivers, microcontrollers, and software for DC and brushless DC (BLDC) motor control solutions. This article describes Atmel system basis predriver ICs and emphasizes its most recently developed BLDC motor driver device. One of the key fields Atmel specializes in is application development for controlling brushed DC motors. There are a large number of suppliers offering a wide variety of automotive-qualified DC motors. Automotive applications continue to see a high demand for brushed DC motors. In most cases, these durable motors, based on proven technology, meet the customers’ application requirements and failure rates are low. But brushless DC motor applications are now emerging, and market analysts have assigned them the highest growth rate. The extended functionalities of BLDC motors are becoming more attractive. 29 Regardless of whether DC-brush or brushless DC motors are used, the challenge in electronic motor control is to design for new functionality while maintaining reliability and high-performance. Atmel smart drivers and bridges enable a myriad of electronic applications. Manufactured using 0.8μm BCD-on-SOI process technology, they support operation up to 40V and can be optimized for motor control applications in harsh environments such as the engine compartment. The Atmel systems basis predriver ICs are of particular interest in managing the shift to brushless motors. To enable new, additional BLDC motor applications, a next generation of advanced system basis B6 predrivers has been developed. Together with Atmel microcontrollers and software, these predrivers will support successful design of future BLDC motor control ICs. © 2010 / www.atmel.com Brushed DC Motor Advantages Brushed DC motors are a proven technology offering several advantages. In addition to low initial cost, brushed DC motors also stand out for their reliability, the high volume of production possible with this technology, as well as the ease with which brushed DC motor speed can be controlled. The low initial cost involved is perhaps the most important benefit. Customers still prefer brushed motor control in pricesensitive applications, which comprise almost all convenience electronics in small and mid-sized vehicles. In addition, many functions in this environment still run without semiconductor driver ICs, making them simpler and cheaper. Brushed DC Motor Disadvantages On the other hand, this technology has its drawbacks. One disadvantage is that carbon brush wear can reduce the motor’s service life. Also, brush fire of the DC motor may cause EMI. In addition, as indicated above, the low effort and cost of electronic control with brushed DC motors is associated with the use of relays, and a trend is underway to replace relays with transistors. While relays have sufficient reliability in most cases, particularly when motors are rarely switched on, they do have disadvantages. Transistors are increasingly preferred when the following variables are considered: • Mechanical stress, vibration • Switching frequency • High coil current • Speed control • Size, space • Clicking noise In summary, relay control is still the simplest method of controlling brushed DC motors, but perhaps performs better for certain applications with unidirectional operation, i.e., radiator fans, as well as for those with reversed operation, such as power windows. DC Motor Control in H-bridge Typically, half-bridge drivers are employed for brushed DC motor control, so it is important to utilize the right combination of microcontroller and driver. In reversed DC motor applications, for example, the DC motor is typically in H-bridge configuration with four power MOSFETs forming the bridge. The ATA6836, a fully protected hex half-bridge driver designed in the Atmel Smart Power SOI technology, can be used by a microcontroller to control up to six different loads. Up to a current limit of about 650mA, the Atmel® ATA6836 can be used to drive up to 5DC motors directly in H-bridge configuration. This applies, for example, to flaps in air conditioning systems and side mirrors. In the case of DC motors with higher wattage (about 10W to 800W), Atmel recommends using an integrated gate driver (Atmel ATA6823 or Atmel ATA6824), an Atmel AVR® microcontroller (e.g., Atmel ATmega32M1), and four discrete N-channel power MOSFET to be selected according to the DC motor’s wattage. The H-bridge driver ICs currently available are often simple predrivers that activate and deactivate the gates of discrete power MOSFETs. The approach taken by Atmel, however, is more complex. Based on its extensive experience with stand-alone watchdog ICs and system basis chips – including LIN transceivers, low drop voltage regulators, and window VBAT Voltage Regulator Atmel ATmega88 or Atmel ATtiny45 Watchdog Timer LIN Transceiver 2 High-side Drivers Atmel ATA6823 M 2 Low-side Drivers LIN Figure 1. Atmel ATA6823 Block Diagram Automotive Compilation Vol. 7 30 VBAT Battery VINT 5V Regulator VCC Regulator Bandgap VBG 12v Regulator OTP 12bit H2 H1 HS1 Driver EN1 Logic Control DIR LS1 Driver PWM S1 PWM mode L1 L2 WD timer CC timer VBAT LIN OV UV OT LS2 Driver Supervisor GND DG2 DG3 DG1 CC RWD EN2 LIN VCC Figure 2 31 CPLO HS2 Driver WD TX When designing brushed DC motor control systems, high-temperature is a second consideration. Taking advantage of Atmel SOI technology’s high-temperature capability, the company has designed the Atmel ATA6824. Specifically for the high demands of engine compartment applications where electronic control units are specified for ambient temperatures of 150°C or higher. However, a system basis predriver with an integrated 100mA, 3.3/5V linear regulator (designed for such an environment) needs to be qualified for a junction temperature of up to 200°C. CP VRES /RESET • Low leakage currents • High-temperature and highvoltage capability • Excellent radiation hardness • Improved latch-up immunity • High switching frequency High-Temperature Applications Charge Pump S2 Oscillator RX In summary, smart H-bridge drivers from Atmel offer a more robust and flexible approach to controlling multiple brushed DC motors. CPHI VG VINT PBAT VBAT VCC VMODE Micro Controller watchdogs in different partitioning – Atmel has combined these IPs with push-pull drivers. The drivers are designed using a very robust SOI technology (Atmel® SMART-I.S.™). The numerous advantages of this technology have been described in great detail in several previous issues of Automotive Compilation and there is no need to detail them here. They include: VBATSW CP PGND 1. Operating Current 2. Free wheeling current LIN solutions. With such as shift, the user expects that manufacturers guarantee full protection. Discrete power MOSFETs, for example, need to be protected against overcurrent conditions. This is typically done by monitoring the drain source voltage that is fed to a comparator integrated in the predriver. A high short-circuit current will flow if a high-side and a low-side power MOSFET are activated simultaneously within the same branch. As a countermeasure, Atmel system basis gate drivers feature implemented shoot-through protection. The dead time can be adjusted individually by choosing an R/C combination at the CC pin of the cross conduction timer. This allows the engineer to flexibly adapt the dead time to the switching characteristic of the power MOSFETs used. Protection Smart 2-pin Motion Control Protection is also an important consideration, particularly when customers are hesitant to adopt a new technology, as with migrating from relays to semiconductor In choosing a motor driver, the user is also looking for good speed and direction control. Characteristically, the architecture of SBCs involves the integration of all necessary peripheral functions into the driver IC. In this specific case, speed and direction control are made extremely easy. The microcontroller needs only two command lines to set the speed and direction of the DC motor: pins DIR (cw or ccw operation) and pin PWM. The moment the PWM signal is low, the system basis predriver activates both high-side drivers so that the free-wheeling current can flow without any additional microcontroller command. The Atmel ATA6823/ ATA6824’s extensive control logic supports the microcontroller by taking over several tasks. Shift to Brushless Motors As stated above, BLDC motors are only now emerging in automotive applications, although they have been popular in disk drives, industrial applications and hobby electronics for several years. Automotive industry studies reveal that about 80% of DC motor applications is still equipped with brushed DC motors. Market analysis conclusively shows, however, that brushless DC motor applications are growing at the fastest rate. This © 2010 / www.atmel.com VBAT Voltage Regulator Atmel ATmega32M1 or Atmel ATtinyx61 Watchdog Timer LIN Transceiver 3 High-side Drivers M Atmel ATA6833/34 3 Low-side Drivers LIN Figure 3. Atmel ATA6833/34 Block Diagram means that BLDC motors will soon take the lion’s share of new motor control electronics R&D. The advantages of BLDC motors over brushed motors are obvious: • Improved speed vs. torque characteristics • High dynamic response • High efficiency • Noiseless and interference-free operation • Extended speed ranges • Long operation life Maintenance-free operation is a very important advantage for all systems that operate continuously while the engine is running, such as fuel pumps or variable vanes in turbo chargers. Also, the smaller size and reduced weight are plus factors for BLDC motors. On the other hand, the increased effort and cost of electronic control (including both hardware and software) could slow the replacement of proven DC motors with BLDC technology. Customer resistance to Automotive Compilation Vol. 7 BLDC can be overcome, however, if the motor control ECU cost is effectively managed by using a BLDC motor control system approach comprising a microcontroller, a system basis gate driver, and the necessary software. The B6 Predrivers Currently, vehicle applications mainly incorporate highly efficient 3-phase brushless DC motors. Such motors typically need a B6 bridge to control three high-side and three lowside power MOSFETs. As with the system basis H-bridge predrivers Atmel® ATA6823/24, the B6 bridge predrivers Atmel ATA6833/34 include all the elements needed to form a complete system. The system consists of a pin-programmable linear voltage regulator (100mA, 3.3/5V); a LIN transceiver; and a window watchdog, in combination with six push-pull stages. The stages are needed to control the six discrete N channel power MOSFETs which operate 3-phase brushless DC motors. These predrivers also offer outstanding space saving on the PC board, helping the designer minimize board size. And because the system basis predrivers are assembled in QFN packages with exposed pad, the board design can easily be optimized for perfect heat transfer. The QFN48 7x7 mm package features a thermal resistance of Rthjc = 5K/W, and thus a thermal resistance of Rthja = 20K/W can be achieved with an elaborate board layout. Inputs/Outputs for all Kind of Commutations Flexibility is also a must for motion control of power MOSFETs in 3-phase brushless DC motors. In a B6predriver, it makes no sense to use a 2-pin motion control (as with the Atmel ATA6823 or Atmel ATA6824) because flexibility in the control of the power MOSFETs is mandatory for different kinds of commutation. Therefore, power MOSFETs should be controlled from the μC separately via the three high-side and three lowside inputs of the Atmel predrivers ATA6833/34. 32 incorporate a broad range of functionalities, some functions should be intentionally assigned to the microcontroller. U Nominal t Figure 4. Crank pulse = 12V; drop: -7V Operation During Crank Pulses In order to enable operation of BLDC motor systems during the crank pulse (as defined by ISO7637, see Figure 4), the minimum supply voltage must be as low as 5V and the predriver must be able to activate the high-side and low-side N-channel power MOSFETs. Hence the charge pump needs to be powerful enough to operate both the high-side and the low-side FETs via the internal voltage regulator. Because of this, the Atmel® ATA6833/34 devices are ideal for systems which need to be in operation during engine start, e.g., fuel pumps. B6 Driver Versions As with as the H-bridge predrivers ATA6823/24, which are both available for the standard and the extended maximum temperature range, there are also two hightemperature B6 bridge driver types available for 3-phase BLDC motor control. The ATA6833 is rated for a maximum junction temperature of 150°C. For all convenience applications, and also some powertrain applications, this temperature is sufficient. 33 The ATA6834 targets hightemperature applications with an ambient temperature of 150°C or more. The device is designed and qualified for a continuous maximum junction temperature of 200°C. Power dissipation calculations can be found in the application note “Estimated junction temperature rise due to power dissipation during operation” (see http://www.atmel.com/dyn/ resources/prod_documents/ doc9181.pdf) With temperature warning functionality and switch-off thresholds adapted to high junction temperature, and also with a special bond pad coating, the ATA6834 is perfectly suited for ECUs attached to coolant pumps, turbo chargers with variable geometry, or exhaust gas recirculation systems (EGR). Universal Use Because BLDC motor applications involve multiple types of signal conditioning, universal use of system basis gate drivers is recommended. These drivers do not handle Hall sensors and BEMF signals, for example, but can be used universally to make stock-keeping easier. And although system basis devices In case of motor current measurement to detect overcurrent, the designer should feed the voltage drop via a shunt resistor to the analog comparators of one of the microcontrollers (e.g., the Atmel ATmega32M1). These comparators can be used to condition either the Hall sensor signals of a sensor-type motor or the BEMF signals of a sensorless motor. New Generation: Atmel ATA6843, Atmel ATA6844 To enable new, additional BLDC motor applications that take advantage of extended functionalities, a next generation of advanced system basis B6 predrivers has been developed. Like their predecessors, they consist of six push-pull stages combined with a 100mA, 3.3/5V pin-programmable linear voltage regulator; a window watchdog with a separate time base that is independent of the microcontroller; and a LIN transceiver. Since this LIN transceiver is based on the stand-alone LIN transceiver IP, Atmel ATA6663, the system basis gate driver’s LIN section meets the high ESD protection requirements at the 8kV LIN pins and complies with all relevant conformance testing requirements. The increased functionalities of the next-generation devices Atmel ATA6843/44 include: • Expanded overvoltage detection threshold (up to 30V). This is an important feature for all applications requiring full functionality during jump-start © 2010 / www.atmel.com can therefore be equipped with BLDC motors: Atmel ATmega32M1 PSC PSCIN0 AMP1+;AMP1- PSCOUT0A PSCOUT0B PSCOUT0C PSCOUT1A PSCOUT1B PSCOUT1C Power Bridge + Shunt Resistor • Electronic throttle control • Exhaust gas recirculation • Turbo charger with variable geometry • Dual clutch • Automated manual transmission • Fuel pump • Coolant pump • Radiator fan • Variable manifold • Synchronous rectifier • Electric power steering PH_A PH_B Motor PH_C Over Current Current HallA HallB HallC ACMP0 ACMP1 ACMP2 PSC : Power Stage Controller ACMPi : Analog Comparator Positive Input (i = 0,1,2) AMPi+/- : Analog Differential Amplified Channel Positive/Negative Inputs (i = 0,1,2) Figure 5. conditions, such as fuel pumps. In this case, the supply voltage is in the range of 5V to 30V. • Adjustable and very low shortcircuit-detection threshold for increased compatibility with low-impedance power MOSFETs. The drain source monitoring can be adjusted by feeding a voltage in the range of 0.5V to 3.3V to pin SCREF. An internal voltage divider sets the detection threshold to 2.5V if the pin is left open. Note that the devices Atmel® ATA6833/34 do not provide this pin. Unlike the standard-temperature version Atmel ATA6843, the warning threshold of the ATA6844 is set to 150°C, and its excess temperature switch-off threshold is set to 200°C. This makes this IC able to cope with the environment of electronic devices attached to parts of the engine, such as coolant pumps, EGR, or turbo chargers. Using NiAu plating on the Aluminum pads, the Atmel hightemperature ICs are able to withstand the challenges of high ambient temperatures over a product’s complete life cycle. The plating avoids dangerous gold-aluminum corrosion (“purple plaque”). providing freedom to control discrete power MOSFETs, e.g., for sinusoidal commutation. The new predrivers also allow simple and cost-effective control of MOSFETs via the microcontroller’s three command lines: the gate driver ICs’ high-side and low-side inputs are designed with opposite input logic (ILx and /IHx). High-temperature ICs for Hot Applications These automotive applications are located within hot environments and VBAT The new predrivers include 6-pin control (as with the ATA6833/34), Automotive Compilation Vol. 7 VMODE VBAT VCC DG1 Atmel ATmega32M1 or Atmel ATtiny157 DG2 DG3 COAST 3.3/5V VCC Regulator Supervisor: short circuit openload over temperature under voltage VINT VG 13V Regulator CP 13V Regulator VBG Logic Control High Side Driver 2 Osc. Low Side Driver 1 Clear Hall A Hall B Hall C High Side Driver 1 Low Side Driver 1 IL1-3 RX PBAT High Side Driver 3 Atmel ATA6843/44 /RESET WD IH1-3 CPHI CPLO VRES WD timer LIN CC timer Low Side Driver 3 TX LIN ENx RWD CC SCREF GND H3 H2 H1 S1 S2 S3 L1 L2 L3 M Hall A Hall B Hall C • A digital input at pin COAST connected to the logic control. If this input is activated, all power MOSFETs are switched off, allowing the motor to coast. In some situations, the motor is asked to coast until it stops. Also, the coast function may be used in the case of overvoltage (e.g., load dump) or to reduce speed before reversing the BLDC motor. Note that this pin is not available in ATA6833/34. PGND Figure 6. Atmel AT6843/44 Block Diagram 34 Table 1 . System Basis B6 Predrivers Overview Tj Operating Range Drain Source Monitoring Coast Function 3-pin Control ATA6833 150°C 5-20V 4V No No ATA6834 200°C 5-20V 4V No No ATA6843 150°C 5-30V 0.5-3.3V Yes Yes ATA6844 200°C 5-30V 0.5-3.3V Yes Yes Atmel AVR Microcontollers intelligence and control, which can be optimized by the use of small, powerful microcontrollers. To support the growing demand for BLDC motors, Atmel® supplies not only systembasis gate drivers, but microcontrollers with software support as well. Taking advantage of its unsurpassed experience in embedded Flash memory microcontrollers, with a large number of devices of Atmel AVR® devices from 8- to 32- bit microcontrollers. Atmel brings innovative solutions, whether for sensor or actuator control or more sophisticated networking applications. These microcontrollers are fullyengineered to fulfill OEMs’ quality The automotive market for electronics is growing rapidly as the demand for comfort, safety and reduced fuel consumption increases. All of these new functions require local requirements towards zero defects. Several AVR microcontrollers are qualified for operation up to + 150˚C ambient temperature (AEC-Q100 Grade0). Designers can distribute intelligence and control functions directly into or near gearboxes, transfer cases, engine sensors actuators, turbo chargers and exhaust systems. Links 35 • Overview of Atmel Automotive Microcontrollers http://www.atmel. com/products/automcu/default. asp?source=overview_automotive • System basis B6 predrivers: Atmel ATA6833 Data Sheet: http://www.atmel.com/dyn/resources/ prod_documents/doc9122.pdf • Application note, “AVR194,” featuring BLDC motor basics, hardware implementation, and code example. http://atmel.com/dyn/resources/prod_ documents/doc8138.pdf • Atmel ATA6834 Data Sheet: http://www.atmel.com/dyn/resources/ prod_documents/doc9122.pdf • Application note: http://www.atmel. com/dyn/resources/prod_documents/ doc9143.pdf © 2010 / www.atmel.com EMC – Synonym for Exasperating, Magic, Confusing? Juergen Strohal The acronym EMC stands for “electromagnetic compatibility” and means the ability of an electronic device (or a module, printed circuit board or integrated circuit) to operate in an electromagnetically distorted environment while keeping its own distortions below certain thresholds so that other devices do not suffer any serious adverse effects. For many people this field of expertise seems more like a form of black magic: No matter what work is done related to EMC, not only will something completely unexpected and unpredictable happen, worse than that it can be assumed that things will always take a turn in the wrong direction. It will become apparent through this article whether this viewpoint is accurate and what engineers can do to gain more facility in this field. Those involved in the field of electronics, especially in the automotive sector, will certainly have been confronted by EMC-related issues more than once. The phenomenon of radio interference is nearly as old as the invention of radio itself and at an early stage led to the definition of guidelines for noise suppression. The other part of EMC, the immunity against distortions, only began attracting attention around fifty years ago. While it is merely a nuisance to experience radio interference, it is definitely a serious matter if an ABS or airbag may also be affected by interference, e.g., when passing a TV tower. Not only is the number of electronic control units in cars on the rise, so too is the number of electronic devices frequently used inside the cars such as cellphones, portable navigation devices, wireless headsets which may also cause interference. Making matters even worse is that more and more of devices of this kind operating at higher and higher frequencies are constantly being introduced to the market. Higher frequencies imply that Automotive Compilation Vol. 7 smaller structures can behave like an antenna and crosscoupling needs to be considered even for relatively small coupling capacitances. It is therefore only natural that there has been a growing need to define certain rules of the game over the past decades. Today all car manufacturers are aware that EMC testing is an important part of car electronics development and understand that EMC issues become costlier the later they are discovered. That is the reason why they do not just rely on a final test inside the car but insist on tests of the electronic control unit (ECU) and even on test results of the integrated circuits used in the design before deployment in vehicles. All around the world a wide variety of test methods have been developed for both unwanted electromagnetic emissions as well as the susceptibility for electromagnetic distortions. In the meantime, all integration levels are covered and over the past 10 years the various standardization committees have devoted their time to the IC level. As a semiconductor manufacturer, Atmel is confronted primarily with IC-level and ECU-level tests. Unfortunately, not only have there been quite a large number of different standards established (perhaps a bane to testing specialists), at the same time, many OEMs apply these standards in slightly different ways. Two different types of measurements can be distinguished in the case of emission as well as susceptibility tests: • The radiated measurements, involving an antenna, a coupling clamp, a magnetic or electric probe, a stripline or a TEM cell 36 • Measurements carried out using galvanic coupling to certain ports to measure or inject RF signals Semiconductor companies have do deal mostly with testing at the IC level, however certain applications such as automotive networking systems also require the proof that some module level tests are passed by those ICs that include integrated bus line transceivers. The “classic” EMC tests at the IC level measure the emissions in varying frequency bands at defined frequency steps, dwell times, measurement bandwidths and detector types as well as measuring immunity by applying an unmodulated or AM-modulated RF signal of defined amplitude, again while varying the frequency at defined steps. These tests are specified in the international standards IEC 61967 for emission and IEC 62132 for immunity. Currently these standards are supplemented by new test standards for pulse measurements. Most ports at the IC level are designed to be connected internally on the PCB only while just a few ports, such as supply pins, bus lines or antenna pins, are connected externally. While EMC requirements are less stringent for local (internal), the global (external) ports are far more critical because the cable lengths involved increase the cross-coupling between different lines or may behave as an undesired antenna for RF signals. Certainly a few ports such as those for bus lines are external ports by definition, but for most ports it is the application that determines whether they need to be treated as local or global ports. The good news is that designing circuits and board layouts which are robust will not only ensure an assembly works better and is more dependable but in most cases also help to achieve the required performance regarding both electromagnetic emissions and immunity. Having said this, how can circuit design be achieved which is that robust? Of course there are a few general rules which help to improve the EMC behavior of the circuit: • Careful consideration should be given to what clock frequency is really needed for the development application. The lowest clock frequency possible should be chosen because this is the first measure which will reduce electromagnetic emissions. • High-impedant ports are susceptible to RF distortions; therefore, impedance should be kept as low as acceptable or a low-impedance path to GND for the RF disturbance should be provided. If an integrated 37 circuit indicates that some GND pins are related to certain VCC pins or ports, where decoupling caps are placed should take this knowledge into account. Ports that are connected on the outside of the ECU require special attention – plan for decoupling capacitors to GND and series resistors if possible – 10 to 100Ω are often acceptable, whereas higher values will form a more efficient filter but also cause a higher voltage drop for DC signals. If emission of a particular port is the problem, one end of the resistor is connected to the port while the capacitor is on the other side. To protect a port against RF distortions the components are arranged inversely. For frequencies >10MHz ferrite beads may be more efficient than only utilizing small resistors and as a further benefit their DC resistance is negligible, meaning the circuit will not suffer from a drop in voltage. • For higher frequencies a capacitor functions not only as a capacitor but also has some inherent, built-in parasitic components such as series inductance and resistance – known as equivalent series resistance (ESR) – to name only the most important components with adverse effects. Because the correct choice and placement of decoupling capacitors is of vital importance, this will be discussed in more detail below. • In addition, resistors need to be regarded as a more complex component (highly depending on the type of construction and also on the resistance value). Fortunately, for a typical low-ohmic thin film resistor as they are used for building EMC filters, the contribution of its parasitic components is largely negligible up to 1GHz. • When developing the PCB layout, the different circuit blocks should be arranged in such way that sufficient space around sensitive inputs is provided towards switched signals of high amplitude and/or frequency because of the possible interferences these signals may cause. Having parallel tracks on a board provides good coupling between the signals on these tracks – if this kind of coupling is not desired, insert some GND area between the tracks; should these tracks cross each other at different layers, have the tracks cross at right angles to minimize the coupling area. Keep tracks as short as possible, especially those carrying RF or switched signals with fast slopes. Critical parts of the circuit such as tuners may require shielding. The highest frequency the circuit produces or is exposed to should be considered and the track length of critical connections kept below 1/10 of the wavelength. Two considerations must be kept in mind here: First, the © 2010 / www.atmel.com wavelength λ on the PCB is shortened due to εr of the board material, for FR4 this is typically around 4.5; however the effective εr will be somewhat lower because part of the electric field of a micro strip line is in free space. For a frequency of 3GHz the formula Capacitor ESR Parasitic Inductance Cp Rp reveals that a track length of about 50mm already equals λ/10. Secondly, the highest frequency in the circuit is Capacitor ESR – soParasitic Inductance determined by the fastest slopes if some parts of the design operate at 1MHz but with slopes of 1ns, there will be frequencies of at least 500MHz on the Cp PCB. • Differential signals must be routed close to each other with the same track length for both lines. Avoid generating large loops and keep theRppath for the return current in mind. The larger the area of a loop, the higher the susceptibility and the lower the frequencies which may impact the circuit. Analogously, this is valid for emission, too – any tracks forming a loop with an RF current flow can behave like a loop antenna. • The ground plane should be designed as solidly as possible, preferably using a multilayer PCB with dedicated layers for GND and power planes. Typically, the signal layers will be on the top and bottom side of the PCB and the GND/power planes on the inner layers. It is advantageous to keep the distance between signal layer and adjacent GND/power plane to a minimum. Doing so helps achieve relatively low track impedance even for fairly thin connections. Slots in the GND plane should be avoided to prevent creating unwanted slot antennas. In addition, small “islands” should be avoided, different GND areas need to be connected using a sufficient number of vias (one via every 3 to 5mm is sufficient for most designs). • Hopping between the layers should be kept to a minimum during board development. Every via, especially “long” ones from top to bottom layer, involve some inductance; as a rule of thumb this is in the range of 0.5 to 1nH. Particular care needs to be taken about GND connections of decoupling capacitors. Atmel® highly recommends placing several vias in parallel close to the respective capacitor. Automotive Compilation Vol. 7 Figure 1. Equivalent Circuit of a Capacitor Resistor Parasitic Inductance Cp Shunt Capacitance Figure 2. Equivalent Circuit of a Resistor Equivalent Circuit Model of the Capacitor In the capacitor’s equivalent circuit model, the simplest model comprises just a serial connection of the nominal capacitor, an equivalent series resistance and a parasitic series inductance. The ESR determines the lowest impedance reached at the capacitor’s series resonance. Above this series resonance the capacitor’s impedance will increase with frequency, thus behaving like an inductor. A more sophisticated model would also include the components Cp and Rp, connected in gray in Figure 1. Modified equivalent circuits are also found in literature which show Cp and Rp in parallel to the whole serial connection of (desired) capacitor, ESR and parasitic inductance; it is merely a question of transforming the values of the respective inherent components. The parasitic inductance together with Cp leads to a parallel resonance that is frequently neglected, because such parallel resonance of typical SMD ceramic capacitors will only appear at several GHz. The series resonance of the capacitor is determined by its type (electrolytic, foil, ceramic), mechanical dimensions (axial, radial, SMD, size) and of course its value. The higher the capacitance of a certain capacitor type, the lower the series resonance frequency. Therefore, it is advisable not just to place a single capacitor for decoupling purpose, but combine two or several caps to achieve broadband decoupling. For example, it is often recommended to pair a 38 Figure 3. Attenuation of two Ceramic Capacitors of 10nF and 100pF in Parallel in a 50Ω System 10nF capacitor for lower frequencies with a 100pF cap for higher frequencies. The following will explore whether this is advisable. A very basic linear RF simulation tool is sufficient for demonstrating this; there are even freeware tools available for this purpose. Many manufacturers of ceramic capacitors supply S-Parameter files for their products and it is advisable to use them. Figure 3 shows the attenuation of the above two capacitors when placed in parallel from a 50Ω track to GND. This looks quite acceptable. Attenuation of at least 30dB was achieved for frequencies between 20MHz and well above 1GHz. If a higher reference impedance than 50Ω had been used, it would look even better. In a rather ideal world, it would be possible to stop here. But has something been overlooked? In reality, it is not possible to connect the capacitors perfectly to GND or to the track or pad which needs to be decoupled. Every track on the PCB above behaves like a transmission line and its impedance is determined primarily by track width, the thickness of the PCB or, in case that a multilayer PCB is used, the distance between signal and GND layer, the distance to adjacent GND areas and the dielectric constant εr of the PCB material. Again, there are special books and free calculation tools available for guidance on this issue. With track widths of 0.2mm, GND area >0.5mm away from the track and εr = 4.7, impedance of well 39 above 100Ω for 2-layer boards (1.6mm standard thickness) will result and close to 50Ω will result for a multilayer board with 150μm distance between signal layer and GND plane. When considering the red curve in Figure 4, it is obvious it looks quite different from the previous graph. Figure 4 shows how decoupling performance changes if the board layout is not done with care. The assumptions for the red curve were as follows: standard 2-layer PCB, distance between caps and to their GND vias: 10mm with just one GND via per cap. Now, quite unexpectedly, there is a highly undesirable resonance around 130MHz with attenuation of only 6dB. The green graph in Figure 4 shows the performance for an improved board layout. Now a multilayer board is used, the two caps are closer to each other and each one has two GND vias only 1mm away from the respective cap. The resulting decoupling performance is significantly improved, but there still seems to be room for improvement. The lesson learned from this example is, first, the decoupling caps need to be as close as possible to each other and to the component which needs to be decoupled. Secondly, using a multilayer PCB with a GND plane just below the signal layer is beneficial too. And, finally, it appears to be a good idea to do some simulation with “real” capacitors rather than just © 2010 / www.atmel.com Red: Distance between caps and their GND vias: 10mm, one GND via per cap, track width: 0.2mm, board thickness top to GND: 1.6mm. Green: Optimized, distance between caps: 5mm, between caps and their GND vias: 1mm, two GND vias per cap, track width: 0.2mm, board thickness top to inner GND plane: 0.15mm Figure 4. Attenuation of two Ceramic Capacitors of 10nF and 100pF in Parallel in a 50Ω System with Realistic Parasitic Components in Connecting Tracks selecting them based on instinct. Innovative layout designs even place only a single centralized group of capacitors to decouple a larger area – but that is something which should not be attempted without careful simulation. As mentioned previously in this article, car manufacturers are aware that EMC issues generally become costlier the later they are discovered. Engineers may benefit from this insight within their own development work. Giving some thought to EMC behavior already when designing a circuit definitely helps avoid unpleasant surprises during EMC approval testing. Having said that much – what needs to be done if a design fails EMC testing despite the care exercised prior to testing? Or, just in case it was not possible to include sufficient time, budget or experience appropriate for EMC assessment in a project, what can be done to improve matters? The truth is that no standard procedure exists. If emission is the problem, a probe across the circuit can be attempted with a field probe to detect any potential “hot spots.” Or, if fast enough, it is possible to re-perform the particular failed emission measurement while connecting a short isolated wire to some “suspicious” spots on the PCB. If a critical one shows up, the number of spurs will increase; Automotive Compilation Vol. 7 this becomes immediately noticeable on the connected receiving instrument. If the design performs weakly in terms of susceptibility, consideration must be given to what parts of the circuit are affected (this can often be deduced from the malfunction occurring during the immunity measurement), and the coupling path must be located. Once the critical parts of the circuit have been pinpointed, the techniques described above can be used to improve EMC performance. A key consideration in this regard is that effective decoupling requires a solid GND area. If this is lacking, it may be easier to redesign the board first; alternatively some copper foil could be added to facilitate further optimization measures in the lab. Hopefully it has become apparent that there is nothing magical at all about EMC and that it is simply applied physics. Naturally, our knowledge about coupling mechanisms and particularly their parameters tends to be inaccurate and sometimes incomplete, even if highly sophisticated electromagnetic simulation tools are used. So there is a trace of magic (or uncertainty) left in the process after all. But these imponderables are also what challenge us daily in this specialized discipline. 40 EMC Pulse Immunity System-level Tests Stephan Gerlach System-level tests for EMC pulse immunity have been established in order to test the immunity of an electronic device or module against strong electromagnetic fields (e.g., the near field of an AM or FM transmitter station or the short but intense shock from a flash that can damage or at least disable vehicle electronics). Many car manufacturers and Tier1 suppliers have tried to simplify test procedures and to standardize both the test set-up as well as workbenches. By using mature but still valid test cases they aim to reduce the use of wiring harness, interacting modules and the car’s body to only the basic functional minimum. Unfortunately, this led to a large number of experimental set-ups that were similar but incompatible for comparing technical parameters. Two examples developed by car OEMs will be explained in this article. Figure 2 shows an example that is comparable to the set-up of Figure 1 regarding functionality, but leads to differing results. Connector Pin Connector + Wire Communitcation Bus or Remot I/O Harness BATTERY Default Connection for Bleed-Off Resistor DUT Test Fixture Insulator Insulator Table with Ground Plane ESD Simulator - Bleed Off Resistor ESO SImulator GND Chamber of Facility Floor Figure 1. Principle of Car Wiring Harness Pulse Immunity Test Using an ESD Simulator (“ESD Pulse Gun”) 41 © 2010 / www.atmel.com EST GR OU ND 50mm Battery AN PL 50mm E Electrostatic Discharge Generator Actuators Power Supply Sensors Figure 2. Workbench of a Major French Car Manufacturer 1 2 8 4 7 6 + - 9 10 100mm >=200 mm 80mm 3 5 10mm 1500mm >=100mm 11 Figure 3. EMC Pulse Immunity System-level Test According to ISO 10605 Annex F (1 = metal ground plane, 2 = ESD resistors, 3 = reference ground, 4 = optional ground line, 5 = DUT1 (master), 6 = wiring harness, 7 = car battery, 8 = DUT2 (slave), 9 = artificial mains network, 10 = battery ground, 11 = ESD pulse generator with gun and ground cable) Automotive Compilation Vol. 7 42 To overcome this obstacle, a major German car manufacturer started an initiative that was accepted by all major car OEMs. The proposed test set-up simulates a wiring harness that is exposed to ESD events. Since 2007 this setup has been part of the international standard ISO 10605 as Annex F. Functionality of EMC Pulse Immunity System-level Testing By using an ESD gun, different pulses with positive and negative polarity in the range of 1 to 30kV are applied to three test pads. The so-called conducted discharge is utilized to guarantee the highest possible repeatability which is a vitally important characteristic of any test procedure. Nevertheless, it cannot be ruled out that a single pulse delivers the same reaction on the device under test. The standard procedure therefore prescribes a repetition rate of 10 ESD pulses for each polarity. To guarantee 100% test reliability, this procedure has to be repeated on the test fixture for all three pads. Figure 4. Stimulated Transient Signal Measured with Current Probe (Clamp-on Ammeter) Figure 5. Workbench System-level EMC Pulse Immunity Test in Application Lab The reason for this is quite simple: To a certain extent, the shape and amplitude of the stimulated waveform within the wiring harness is arbitrary. Moreover, because the ESD pulses are applied to a “living system” (i.e., ongoing LIN communication between master and slave) they have a different impact on the DUT’s failure behavior until final destruction. Figure 4 shows that current spikes of up to 37A are created with a burst frequency of several hundred MHz. As explained above, neither the pulse shapes nor the effects are identical. A DUT can withstand five or seven pulses, but the 8th pulse will cause final destruction. Figure 6. Workbench System-level EMC Pulse Immunity Test – Pulse Measurement 43 © 2010 / www.atmel.com Atmel Corporation Atmel Asia Limited Atmel Munich GmbH Atmel Japan 2325 Orchard Parkway Unit 01-5 & 16, 19F Business Campus 9F, Tonetsu Shinkawa Bldg. San Jose, CA 95131 BEA Tower, Millennium City 5 Parkring 4 1-24-8 Shinkawa USA 418 Kwun Tong Road D-85748 Garching b. Munich Chuo-ku, Tokyo 104-0033 Tel: (+1)(408) 441-0311 Kwun Tong, Kowloon GERMANY JAPAN Fax: (+1)(408) 487-2600 HONG KONG Tel: (+49) 89-31970-0 Tel: (+81)(3) 3523-3551 www.atmel.com Tel: (+852) 2245-6100 Fax: (+49) 89-3194621 Fax: (+81)(3) 3523-7581 Fax: (+852) 2722-1369 © 2010 Atmel Corporation. All Rights Reserved. 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