Download PCB Layout Design Considerations

ECE 477
Digital Systems Senior Design Project
Rev 9/12
Homework 6: Printed Circuit Board Layout Design Narrative
Team Code Name: Autonomous Rescue Vehicle
Group No. 10
Team Member Completing This Homework: Ruiyang Lin
E-mail Address of Team Member: lin132 @ purdue.edu
Evaluation:
SEC
DESCRIPTION
MAX
1.0
Introduction
5
2.0
PCB Layout Design Considerations - Overall
20
3.0
PCB Layout Design Considerations - Microcontroller
10
4.0
PCB Layout Design Considerations - Power Supply
10
5.0
Summary
5
6.0
List of References
10
App A
PCB Layout Top & Bottom Copper Screenshot
20
App B
PCB Layout To-Scale Component Side Layout
20
TOTAL
100
Comments:
SCORE
ECE 477
Digital Systems Senior Design Project
Rev 9/12
1.0 Introduction
The purpose of our team’s project is to design a prototype of an autonomous robot that can
track the GPS location of a lost person, and then navigate by itself to find the person and bring
him/her back. The PCB design for this project includes the design of the main control circuit that
will be mounted on the robot and a rescue point transceiver (RPT) which will be carried by the
person. The PCB in the RPT needs to be small enough to fit a box that can be easily carried
around by a person. The components such as the power switch, the RF antenna, and LEDs need
to be placed such that they are easily accessible by the user. The PCB on the robot needs to
accommodate multiple grounds and power levels. The components also need to be carefully
placed to accommodate communication between peripherals that will not be mounted on the
PCB, such as motors and GPS receivers. For the microcontrollers, decoupling capacitors and
bulk capacitors need to be placed as close to the microcontrollers as possible. In addition,
headers need to be installed on both PCBs so that signals coming out of the microcontrollers can
be probed for debugging. ICSP programming interface will also be installed to facilitate onboard programming.
2.0 PCB Layout Design Considerations - Overall
For both PCBs, the power and ground traces are 40 mills while signal traces are 12 mills.
Because of the size of the robot chassis, space is not a main constraint for the PCB on the robot.
Therefore, the size of the PCB for the robot was chosen to be about 4 in * 3 in, as is allowed by
the Lite version of EAGLE Layout Editor. The three connectors for ultrasonic sensors will be
mounted on the left side of the board, and three H-bridge circuits will be mounted on the bottom
side of the board while the other three will be mounted on the top. Two-pin connectors will be
installed in the vicinity of each H-bridge circuit. The connectors were arranged such that wirings
between the PCB and other devices (sensors, motors, etc.) will be as short as possible.
For the PCB in the Rescue Point Transceiver, the size of the board was determined to be
about 3 in * 2 in. A toggle switch and two LED lights will be installed on the lower left hand
corner of the board. These components were placed such that, when the PCB is placed inside the
box, they can be easily accessed from the outside by the user. The RF module will be mounted
on the upper left hand corner and placed as close to the edge of the board as possible. When the
PCB is placed in the box, the antenna connector of the RF module will remain on the outside so
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Digital Systems Senior Design Project
Rev 9/12
that the antenna can be removed when the transceiver is not in use. A 5-pin connector will be
placed at the bottom side of the board for communication between the GPS receiver and the
microcontroller.
.
3.0 PCB Layout Design Considerations - Microcontroller
According to the datasheet for PIC32 family, decoupling capacitors are required for the
microcontrollers to work properly. The decoupling capacitors need to be as close to the pins as
possible. The trace length from the pin to the capacitor need to be within 6mm in length [1].
Apart from decoupling capacitors, a bulk capacitor is also recommended to improve power
supply stability [1]. A ceramic bypass capacitor of 0.1uF has to be placed across the Vdd pin and
Vss pin. Another 0.1uF capacitor needs to be placed across the analog-to-digital converter
reference lines (AVdd and AVss). The capacitance for the bulk capacitor was chosen to be 10
uF. On the PCBs, all capacitors were placed right next to the pins they were supposed to be
connected to. In addition, connecting the analog voltage references (AVdd and AVss) directly
with digital power lines (Vdd and Vss) can severely jeopardize the performance of analog-todigital converters on the microcontroller [2]. Therefore, a 10 ohm resistor is placed between Vdd
and AVdd to filter out high-frequency noise. To reduce resistance in the power traces, 40 mill
traces were used to connect Vdd and Vss pins on the microcontrollers.
To make programming and debugging easier in the future, a programming header and a
reset button will be installed to both PCBs. For both PCBs, pins PGEC2 and PGED2 were
reserved for ICSP debugging and programming. A six-pin header will be placed on the PCB for
communication between ICD3 debugger and the microcontroller. A reset button will also be
installed for hard-reset for debugging purpose. The pushbutton was connected to a 9 Kohm pullup resistor such that when pressed, an active low signal will be sent to the microcontroller. In
addition, pins that are crucial to the project, such as power pins and I/O pins, were routed to
headers for easy access.
4.0 PCB Layout Design Considerations - Power Supply
The power supplies for the robot include 3.3V for the microcontroller and other digital
applications such ultrasonic sensors and GPS transmitters, -3.3V for an op-amp circuit to
monitor the battery voltage level, and 7.2V for the motors. In addition, the motor ground and
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ECE 477
Digital Systems Senior Design Project
Rev 9/12
digital ground have to be isolated to prevent noise in digital circuits created by fast switching
motor currents. Although ground planes will be ideal to reduce resistance and noise, in our case
it was deemed impractical. Designing two separate ground planes can be very time-consuming
and costly. In addition, using ground planes makes it difficult to track the flow of currents, which
might become troublesome because noisy motor currents could interfere with digital pins on the
PCB and compromise the stability of signal level. Instead of using ground planes, we decided to
route all ground signals with 40 mill traces and separate digital and analog grounds by routing
traces in different directions around the board. In addition, different voltages levels (3.3V, 7.2V,
etc.) will be routed in parallel with their respective grounds on the opposite layer to improve
power line stability.
To regulate the voltages that supply the microcontroller and other digital applications,
two regulator circuits are applied in the project. Each regulator circuit consists of a linear voltage
regulator, a filter capacitor, and a noise suppression capacitor. Both capacitors were placed close
to the regulator and connected with 40 mill traces.
For the PCB on the robot, there is an op-amp circuit to monitor the battery level of the
motor power supply. The op-amp takes +3.3V and -3.3V as power supplies and both voltages
needs to be connected to bypass capacitors. The bypass capacitors were placed close to the opamp to the best decoupling effect.
5.0 Summary
This report focused on the preliminary layout for the robot and the transceiver. The
components were carefully laid out so that wiring and debugging will be easy in the future.
Decoupling capacitors and bulk capacitors were carefully placed to ensure optimal performance
from the microcontrollers. Programming interface and headers were also installed for debugging
in the future. The power supply traces were routed to ensure stability and reduce noise. Ground
planes were not applied in the PCB layout because high motor currents might interfere with
digital voltage levels, resulting in instability of digital applications. Instead, different voltage
levels and their respective grounds were routed in parallel with each other on separate layers.
Each regulator circuit includes a filter capacitor and a noise suppression capacitor and these
components were placed in the vicinity of each other for better power supply stability. Last but
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ECE 477
Digital Systems Senior Design Project
Rev 9/12
not least, additional bypass capacitors were installed to improve the performance of the op-amp
circuit.
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Digital Systems Senior Design Project
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6.0 List of References
[1]
Microchip. “PIC32MX120F032B Datasheet”. Online. Available:
http://ww1.microchip.com/downloads/en/DeviceDoc/61168E.pdf
[2]
Mark Glenewinkel. System Design and Layout Techniques for Noise Reduction in MCU-
Based Systems. Motorola, Inc., 1995, Available:
https://docs.google.com/viewer?url=https://engineering.purdue.edu/ece477/Homework/Common
Refs/AN1259.pdf
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Digital Systems Senior Design Project
Appendix A: PCB Layout Top & Bottom Copper
Figure 1
Top Copper for the transceiver
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Digital Systems Senior Design Project
Figure 2 Bottom Copper for the transceiver
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Digital Systems Senior Design Project
Figure 3 Top copper for the robot
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Digital Systems Senior Design Project
Figure 4 Bottom copper for the robot
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ECE 477
Digital Systems Senior Design Project
Appendix B: PCB Layout To-Scale Component Side Layout
Figure 5 Component Layout of Transceiver
Figure 6 Component Layout of robot
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