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Course Introduction Purpose: This course discusses techniques that can be applied to reduce problems in embedded control systems caused by electromagnetic noise Objectives: • Gain a basic knowledge about noise that affects embedded systems • Learn approaches and design methods for minimizing the noise generated by microcontroller based embedded systems • Get details about techniques chip designers use to decrease the amount of noise a microcontroller produces • Obtain basic insights for handling noise problems during the system design and development cycle Content: 19 pages 3 questions Learning Time: 30 minutes Noise Can Cause Big Problems Noise = “Unwanted electrical signals that produce undesirable effects in the circuits of control systems in which they occur.” Two types of noise: • Electromagnetic Compatibility (EMC) issues encompass both types • Noise reduction approaches: - Techniques for reducing EMI (Electromagnetic Interference) — Cutting the noise emitted by a specific system, circuit or device that causes other devices/circuits to operate incorrectly - Techniques for decreasing EMS (Electromagnetic Susceptibility) — minimizing the effect that external noise has on the operation of a system, circuit or device • Noise reduction: a goal common to both microcontroller (MCU) designers and the system engineers who apply those devices Effect of Hidden Impedances Wiring on PCB • Many significant Ls and Cs are “hidden” impedances in the circuit • Noise is the undesirable result of unintentional interaction due to stray capacitances and inductances • Parallel signal paths allow more coupling and interaction, more noise - Capacitance is proportional to the area of electrodes and inversely proportional to the distance between the electrodes - Inductance exists even in straight wires, and mutual inductance between adjacent signal lines causes electromagnetic coupling Printed Circuit Board MCU Other Device Hidden “C” and “L” Parts mounting surface Crosssection of PCB Hidden “C” and “L” Signal A Signal B Soldering surface Impedance of Interconnects • Signal interconnections on a circuit board have resistance • Example: Impedance of a copper-foil trace 35µm thick and 0.5mm wide is about 0.01Ω/cm at DC • Microcontrollers can have fast clock speeds and signals that contain very high frequency components - The frequencies in the spectrum of a digital signal increase rapidly as rise and fall times became shorter • Example: As the rise time decreases from 10nsec/5V to 2 to 5nsec/5V, a signal will contain frequency components >100MHz • Interconnection resistance increases as signal frequency components rise • Example: Impedance at 100MHz for the trace described above is Z~ ~ 6Ω/cm [~600 times larger than at DC!] • Design PCBs to handle high frequencies - Shorten the wiring traces that connect bypass capacitors to Vcc and ground and make the traces wider, etc. 10nsec Vcc Vcc x 0.8 Vcc x 0.2 GND 2 to 5nsec Vcc x 0.8 Vcc x 0.2 GND Vcc Spectrum has frequency components at 100MHz Impedance of Capacitors • Ideal capacitor: Z = 1 wC • Example: Impedance of 10pF capacitor at 100MHz is 1 ~ Z= ~ 170Ω 6 -12 2p x 100 x 10 x 10 x 10 Application Small capacity (0.01 - 0.1µF); install between Vcc/GND of IC High capacity (0.1 - 10µF); install between Vcc/GND of PCB Extra high capacity (1 - 100µF); install between power-line terminals Typical Design Choice • Ceramic capacitor Small capacitance, but good frequency response • Tantalum capacitor Smaller capacitance than aluminum electrolytic, but better frequency response • Aluminum electrolytic Poor frequency response, but less expensive and and large capacitance 1000 Bypass capacitor working area 100 Impedance (IΩI) • Frequency characteristics of actual capacitors vary by type and capacitance value - Choices for bypass capacitors: 170Ω 10pF 0.05µF Ceramic 10 0.1µF Mylar 4.7µF Tantalum Electrolytic 1 0.1 0.01 1k 0.05µF Feedthrough 5mm Wire 100µF Aluminum Electrolytic 10k 100k 1M Ideal 0.05µF Capacitor 10M Frequency (Hz) 100M 1G Question Is the following statement true or false? Click Done when you are finished. “A ceramic capacitor is a good choice for a small-capacity bypass capacitor because it has good frequency response.” True False Done Typical Causes of Noise Signal interference Signal A Signal B Waveform distortion during signal transmission Signal A Signal A’ Signal A’’ Sudden change of power current Vcc GND Narrow ground line GND Electromagnetic induction (from power line, power devices) Signal A Techniques for Reducing EMI - 1 Review values of components in the clock oscillator circuit Optimize C1, C2 and R2 to prevent unwanted emission and allow circuit to oscillate at optimum amplitude Vcc R1 Xtal C1 GND R2 Adjust C1, C2, and R2 C2 Vcc GND Change device package type from a DIP to a QFP Adopt a package with shorter lead lengths to decrease unwanted emissions from antenna effect Internal Memory MCU Memory Use a single-chip solution instead of a MCU and external memory Choose an MCU with on-chip memory to prevent unwanted emissions caused by driving an external bus line Techniques for Reducing EMI - 2 Reduce power supply voltage If possible, reduce Vcc from 5V to 3.3V to reduce the noise level Vcc=5V Vcc=3.3V Use port mode, not bus mode MCU Depending on the type of MCU, the signal on a general-purpose port may change voltage much less frequently than the signal on the bus. If this is Typical: Bus-mode the case, use the port to reduce Activates bus line when the amount of noise generated accessing peripheral IC and MCU (with on-chip memory + I/O). Use serial damping resistors Insert a damping resistor (100 to 150 Ohms) in signal lines to reduce emissions caused by signal reflection. A resistance of about 5kΩ is OK if some signal degradation is acceptable Peripheral IC MCU Peripheral IC Better: Port-mode Activates bus line only when MCU is accessing peripheral IC. Damping Resistors MCU Peripheral IC Techniques for Reducing EMI - 3 Revise system timing, if possible Try to stagger significant signal-driving events, even just slightly, to diffuse the noise generated by switching Insert L-C filters on each power and ground line The filters help prevent switching noise from entering the Vcc and GND lines, from which they might otherwise be radiated Use a multi-layer circuit board and apply best-practice layout methods Develop effective sheet patterns for the Vcc and GND planes; make interconnect wiring as short as possible; interleave noisy signal lines with GND and Vcc layers to shield the line; etc. Event A Event B Event C Event A Event B Event C Time Ferrite beads Vcc MCU GND Through hole GND Vcc Techniques for Reducing EMI - 4 Use a half-area pattern on the circuit board when wholearea pattern cannot be used To the greatest extent possible, make symmetrical patterns for Vcc and GND on the facing layers of a double-sided PCB Poor design GND Generated electromagnetic field Good design - + - + - - + - - + - - + - + - + - - + + + GND + + + Vcc + Select a microcontroller that produces low levels of EMI Consider the MCUs in the M16C series, devices that were designed from the outset to minimize noise problems in embedded systems M16C series MCUs Noisy! Quiet! Vcc Question Which statements about reducing EMI in embedded systems are correct? Select all that apply and then click Done. Choose a microcontroller with on-chip memory to prevent unwanted emissions caused by driving an external bus line. To reduce emissions caused by signal reflections, insert a serial matching resistor of 100 to 150 Ohms in signal lines. Use bus mode, not port mode, to reduce the number of voltage changes and, therefore, to reduce the unwanted noise. If you have a two-sided circuit board, always try to make symmetrical patterns for Vcc and GND on the facing layers. Done Designing Low-EMI MCUs - 1 Use an optimum output buffer conversion speed (through rate) Don’t set the speed any higher than necessary Change the timing of the output buffers Eliminate the simultaneous switching of all channels, if possible A Start oscillation B “L” A (High drive) B Steady-state oscillation (Low drive) Vcc GND Output terminal “H” Change the driving capability of the clock oscillation circuit Use high-drive mode to get oscillator started reliably; lowdrive mode to sustain oscillation t1 Synchronized switching t1 t2 Time-shared switching Designing Low-EMI MCUs - 2 Eliminate pass-through current in output buffer Change the timing of the N-channel and P-channel transistors to eliminate a source of high-frequency noise Optimize output impedance of output buffer Minimize ringing by matching the buffer’s output impedance to the impedance of the signal wiring, which is about 100 to 150 Ohms Vcc Control Circuit N-ch GND Internal P-ch N-ch GND External Vcc Vcc P-ch Control Circuit Reduce output amplitude of output buffer Design circuit to produce a lower amplitude and slower rise time Vcc P-ch N-ch GND P-ch N-ch GND MCU Buffer Amp Characteristic Impedance of PCB ~100-150Ω (when connected to surface-mounted components) Designing Low-EMI MCUs - 3 Use an innovative layout, enhanced with added internal capacitance Add capacitance to reduce the total impedance of the power supply distribution system that’s internal to the microcontroller IN OUT P+ N+ Vcc GND N+ P+ N N+ P+ P PVcc Vcc IN OUT IN OUT GND Optimize the driving capability of the internal buffer transistors Design very small transistors with large drive capability; adjust bus wiring, too Arrange terminals for easy mounting of bypass capacitors Noise across Vcc and GND terminals propagation Also, use parallel arrangement of to outside IC chip signal lines inside package and on the chip to achieve extra filtering GND Vcc Vcc IN OUT IN GND OUT GND Vcc Noise absorption as common mode Vcc Pin layout simplifies putting bypass capacitor at best position GND Parasitic capacitor GND Wiring inductance Pattern for Vcc, GND, Oscillator Bypass capacitor (~0.01µF) VCC VSS MCUs in the M16C family have an innovative pin layout Arrangement of pins eases the design of Vcc and GND wiring on the circuit board and aids the placement of bypass capacitors M16C Microcontroller Bypass capacitor (~0.01µF) Sheet pattern of VCC AVSS Sheet pattern of GND NMI / VCC XIN VSS XOUT XCOUT XCIN CNVSS BYTE AVCC /RESET VREF Oscillation capacitors (symetrically placed) Bypass capacitor (~0.01µF) Tantalum capacitor Oscillation capacitors (symetrically placed) Reset IC Capacitor for RESET (1000pF) GND VCC Basic Design Insights on Noise It’s very important to implement noise measures at initial stage of design work Investigations of noise problems take time and are costly If a problem is discovered in the later stages of the system development process, the noise measures required to solve the problem will end up being far more costly than expected Because it’s difficult to simulate a noise-oriented malfunction, a lot of work will be necessary to identify the root cause of the problem Noise Noise problems sometimes require a fundamental solution, such as a re-design Unless noise measures are taken at the initial design stage, a malfunction that occurs later might be extremely difficult to eliminate using superficial correction methods; a re-design may be necessary to correct the problem Attempts to eliminate all possible EMI/EMS problems typically lead to unnecessarily high costs The optimum design approach is to take pinpoint measures in key areas that require solutions Questions Match each item to the most appropriate MCU design advice by dragging the letters on the left to the correct locations on the right. Click Done when you are finished. A Through rate D Adjust locations to make it easy to install a bypass capacitor B Pass-through current A Set no higher than necessary C Signal from buffer C Reduce amplitude and slow down the rise time D Vcc and GND terminals B Eliminate by changing the timing of the N- and P-channel transistors Done Reset Show Solution Course Summary Noise problems EMI and EMS System-level EMI reduction measures Techniques for reducing EMI from microcontrollers Design insights