Download Very good – all requirements aptly met. Minor additions/corrections

ECE 477
Digital Systems Senior Design Project
Rev 8/09
Homework 6: Printed Circuit Board Layout Design Narrative
Team Code Name: _iDine______________________________________ Group No. __13__
Team Member Completing This Homework: _Himanshu Raghav______________________
E-mail Address of Team Member: _hraghav_ @ purdue.edu
Evaluation:
SCORE
DESCRIPTION
Excellent – among the best papers submitted for this assignment. Very few
corrections needed for version submitted in Final Report.
Very good – all requirements aptly met. Minor additions/corrections needed for
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version submitted in Final Report.
Good – all requirements considered and addressed. Several noteworthy
8
additions/corrections needed for version submitted in Final Report.
Average – all requirements basically met, but some revisions in content should
7
be made for the version submitted in the Final Report.
Marginal – all requirements met at a nominal level. Significant revisions in
6
content should be made for the version submitted in the Final Report.
Below the passing threshold – major revisions required to meet report
*
requirements at a nominal level. Revise and resubmit.
* Resubmissions are due within one week of the date of return, and will be awarded a score of
“6” provided all report requirements have been met at a nominal level.
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Comments:
Comments from the grader will be inserted here.
Grader: Malek Musleh
Overall Comments: This submission is very good. Only a couple of grammer/informality
issues in writing style. Provides sufficient justification for design considerations and
choices. Some things can be specified in greater detail, such as the recommended keep away
distance of the blue tooth model. Furthermore, while you illustrate the reasoning for your
choices, you don’t necessarily illustrate the negative consequences (if any) of some of your
decisions. E.g. Placing bypass Caps on bottom of board, or use of a ground plane on impact
of having to route everything on top-layer of PCB. These are just some technical details
that should be included for the final report. See further comments below.
Grade: 10/10
ECE 477
Digital Systems Senior Design Project
Rev 8/09
1.0 Introduction
iDine is a multi-touch based dining system aimed at facilitating the management of
multiple dining tables and improving the dining experience. The EMD (External Managing
Device) is a hand held device which contains the PCB for the project. The circuit board mainly
consists of a PIC24FJ128GA010 microcontroller, battery charging circuit, battery health
monitoring circuit, power supply design and headers for off-board components.
There are two primary design constraints in the design. Being a handheld device, the size of
the PCB for the EMD should be no more than an estimated 2” by 3” to allow enough space for
other components and still be portable. Secondly, there is a constraint on power as the device is
battery operated. The circuit board must use minimal power for operation.
2.0 PCB Layout Design Considerations - Overall
The circuit board consists of two major sections which are the microcontroller and its
peripherals, and the power supply. The power supply section consists of an analog circuit which
has a high frequency switching path. This could induce noise in the digital part of the circuit
which is why it is kept in one half of the circuit board. The two sections will have separate
ground planes which will be tied together at one point only so that the two circuits have different
impedance paths. A trace width of more than 40 mils will be used for the power trace The rest of
the circuit will utilize a trace width of 12 mils. Since we are using a ground plane to ground most
of the circuit, we would utilize space more efficiently by placing components directly connected
with ground on the same layer of the circuit board.
The digital part of the circuit consists mainly of the PIC24FJ128GA010 microcontroller, a
LTC4150 coulomb counter, single-push single throw push-buttons and header breakouts for the
graphics LCD DX-160, the Bluetooth module and the SD card slot. The microcontroller and the
SD card use a 3.3 V power supply but the Bluetooth module and the SD card slot operate at 5 V
input. This part would therefore require voltage level translator to convert the 3.3 V data
transmission line from the microcontroller to the LCD and the Bluetooth module. For the data
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ECE 477
Digital Systems Senior Design Project
Rev 8/09
receiving line of the microcontroller, either a simple voltage divider circuit consisting of resistors
will be used to convert the 5 V output from the LCD and Bluetooth to the 3.3 V input of the
PIC24FJ128GA010 microcontroller. A ground plane will be used as a heat sink to dissipate the
heat accumulated by the switching voltage regulators and the battery charger circuit (around
LTC15105). The coulomb counter requires the use of a very low resistance sense resistor with a
four point Kelvin connection to the IC. Multiple resistors will be used to create the low
resistance needed and will be placed close together. The supply bypass capacitor for the
LTC41150 will be placed right under the power connection to minimize coupling.
The headers for the single-push single-throw (SPST) push buttons will be placed close to
the pins of the microcontroller to which they would be connected. The Bluetooth module and the
LCD would utilize breakout headers for their connections. The Bluetooth will be placed at a
corner of the EMD and away from the PCB to keep EMI resulting from the radio waves minimal.
The breakout headers for all the peripherals will be placed near the edges to provide easy access
and to keep connecting wires short to these peripherals. The SD card uses the SPI mode for its
interface and would share its clock with the microcontroller SPI clock. This clock signal is
highly susceptible to any kind of noise; and therefore, the SD card breakout pins will be placed
closest to the MCU.
The power supply section consists of a LT1510 battery charger, two DE-SW0XX switching
voltage regulators, one with a 5V output and one with a 3.3V output. This section also contains
an input from a DC 9V wall adaptor to recharge the battery. The power socket will be placed
near the bottom edge of the PCB to keep it close to the edge of the EMD. The power supply
layout will be discussed in more detail in section 4.0
3.0 PCB Layout Design Considerations - Microcontroller
The PIC24FJ128GA010 microcontroller uses four decoupling capacitors for four power
supply inputs. These would all be placed on the reverse side of the circuit board to save space.
Usually the data sheets do not recommend such a placement; however, many of the header pins
are being placed closed to the microcontrollers. As a result, placing bypass capacitor on the
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ECE 477
Digital Systems Senior Design Project
Rev 8/09
reverse side is the optimal choice because they also need to be placed very close to the
microcontroller. Along with these, the microcontroller also requires the use of two capacitors to
specify the operating voltage of its core. These need to be placed close to the microcontroller as
well. A ground plane will be used to provide easy access to ground signals for all these
capacitors. A trace/size size of 12 mils will be used for the microcontroller wherever possible.
For pin connections that are too close to each other, a trace of 10 mils will be used. Such traces
will be predominant near the microcontroller and the battery gauge (LTC4150 coulomb counter).
A header will be placed on the circuit board close to the microcontroller for debugging and for in
circuit programming.
All peripherals have been laid out in the schematics so that they are closest to their
respective pins on the microcontroller to avoid crossing of signal lines. The microcontroller will
be placed on a corner of the circuit board to keep it close the off chip components. A few header
pins will be provided for general testing of the circuit. These can be placed anywhere on the
circuit board as they are not very critical for the operation of the circuit.
4.0 PCB Layout Design Considerations - Power Supply
The power supply for the EMD will be provided using a 7.2V rechargeable lithium-ion
battery and also a 9V DC input from a wall adapter to both charge the battery and power the rest
of the circuit. We will use a 100 mil trace for the 9V input and use a bulk decoupling capacitor
near the power socket to remove most of the noise before it enters the circuit board. The LT1510
charger consists of a high frequency circulating ground path which needs to be kept very short
according to its datasheet. A ground plane will be used beneath the charger to keep traces to
ground minimal. However, the high frequency circuit also consists of an inductor which should
not have a ground plane beneath it. The plane will be cutout below the inductor. The circuit also
utilizes two Schottkey diodes. These are not surface mounted and have a variable thermal
resistance depending on the way they are placed. These will be placed in a standing position with
the cylindrical base placed on the circuit board. This configuration has the minimum thermal
resistance.
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ECE 477
Digital Systems Senior Design Project
Rev 8/09
The power supply circuit also contains two DE-SW0XX switching voltage regulators.
These too will be kept away from the rest of the circuit as they might cause RF noise interference
according to the datasheet. All these components require a heat sink as they have very high
thermal resistances. The ground plane would be used for heat sinking. If later, the temperature
increase is still high, an external heat sink will be added to keep the circuit board to dissipate the
heat. (The optimal temperature should not exceed 40º C.)
5.0 Summary
The primary constraint of size will be overcome by utilizing both sides of the circuit board.
The board will be kept cool and also noise free by the use of ground planes wherever possible.
Also, keeping the analog and digital part of the circuit separate with separate grounds will reduce
any problems due to EMI. Placement of all components based on their use on the overall
packaged device will ensure easy routing of the traces with minimal crossing and will also help
in keeping the size of the EMD small enough for it to be portable.
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ECE 477
Digital Systems Senior Design Project
Rev 8/09
List of References
[1]
Microchip, “PIC24FJ128GA010” Microchip. [Online]. Available:
http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en024805. [Accessed:
Feb 26, 2010].
[2]
Spark Fun, “Bluetooth Modem” Spark Fun. [Online]. Available:
http://www.rovingnetworks.com/documents/RN-41.pdf. [Accessed: Feb 26, 2010].
[3]
Spark Fun, “Breakout Board for SD-MMC Cards” Spark Fun. [Online]. Available:
http://www.sparkfun.com/datasheets/Prototyping/SD-Socket-PP-14446.pdf. [Accessed:
Feb 26, 2010].
[4]
Spark Fun, “DX-160 Serial Graphics LCD” Spark Fun. [Online]. Available:
http://www.sparkfun.com/datasheets/LCD/DX160.pdf. [Accessed: Feb 26, 2010].
[5]
Linear Technology, “LT1510 - Constant-Voltage/Constant-Current Battery Charger”
Linear Technology. [Online]. Available: http://cds.linear.com/docs/Datasheet/1510fc.pdf.
[Accessed: Feb 26, 2010].
[6]
Linear Technology, “LT4150 - LTC4150 - Coulomb Counter/Battery Gas Gauge” Linear
Technology. [Online]. Available: http://cds.linear.com/docs/Datasheet/4150fc.pdf.
[Accessed: Feb 26, 2010].
[7]
Dimensions Engineering, “DE-SW0XX – Switching Voltage Regulators” Dimensions
Engineering. [Online]. Available: www.dimensionengineering.com/datasheets/DESW0XX.pdf. [Accessed: Feb 26, 2010].
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