Download PCB Layout Design Considerations - Overall

ECE 477
Digital Systems Senior Design Project
Rev 9/12
Homework 6: Printed Circuit Board Layout Design Narrative
Team Code Name: Sports Telemetry Device
Group No. 5
Team Member Completing This Homework: Kayode Adeniji________________________
E-mail Address of Team Member: oadeniji@ 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:
Comments from the grader will be inserted here.
SCORE
ECE 477
Digital Systems Senior Design Project
Rev 9/12
1.0 Introduction
The main goal of this project is to build a wireless telemetry system that measures angular and
translational accelerations experienced by athletes and store the data on an external device. In
addition, there will be have wireless capabilities for the device to be able to transmit the stored
data from the monitoring device to a base station on the side line. The main purpose of the
project is to detect potential brain injury in athletes as quickly as possible, to ensure that the
likelihood of permanent damage is minimized.
The project contains four printed circuit boards in total, three for the monitoring device and one
for the base station. The monitoring device has two PCBs positioned above the ear of each player
and houses a 2-axis gyroscope and an accelerometer with a connector to the main PCB that is
located behind the head. The main PCB houses a microcontroller, an accelerometer, a charging
circuit; two linear voltage regulators, a capacitive proximity sensor, a radio frequency transceiver
and NAND flash memory. Connected to the linear voltage regulators will be a lithium ion battery
but it will not be housed on the board. It will be housed in a compartment close to the board and
be connected by wires. The base station PCB has similar components to the one located behind
the head excluding some components. It houses a microcontroller, an RF transceiver, NAND
flash memory and a USB controller. The base station PCB also has connection to a lithium ion
battery that will not be housed on the board itself.
A major constraint for the PCB design is the size, the monitoring device is expected to be placed
on the head of the players and should also be able to easily and comfortably fit inside a helmet
(Football) if necessary. The two PCBs that are supposed to go on the side of the head are pretty
small because they only contain a single gyroscope and accelerometer; which are also very small
components. Currently, the side PCBs are 30mm x 45mm and the PCB at the back of the head is
80mm x 50mm. These seem like good dimensions at the moment but may be subject to revision
after a closer look at packaging feasibility.
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ECE 477
Digital Systems Senior Design Project
Rev 9/12
2.0 PCB Layout Design Considerations - Overall
Due to the fact that a small PCB size is desired, the component placement is crucial, namely for
the PCB that is going at the back of the head. The microcontroller is centralized at the top of the
board because it is connected to all the other components. The capacitive sensor is located near
the microcontroller at the bottom left hand side and the accelerometer is located at the top right
hand side. The RF transceiver is isolated at the bottom right hand side to ensure that it is not
covered and is in a location that will maximize its functionality. The power supply circuit, which
includes the two linear voltage regulators, the battery connection and the charging circuit, are
isolated at the top left hand side of the board. The ground will be routed underneath the power
supply and then have the bypassing components connected to it to reduce its interference with
rest of the components on the board [1]. The oscillator circuit for the microcontroller is located at
the bottom of the board, under the micro because it is noisy and thus it is best to isolate it from
the other connections. The NAND flash memory is also located at the bottom of the board
simply for space management and the USB connection is located on the edge of the board for
easy access. The PCB has two 7-input connectors on either side of the board that will connect the
relevant signals to each of the two side PCBs. The side PCBs do not require much component
mapping because they each only have two components (the gyroscope and the accelerometer).
3.3 volts will power all of the components on the board; this comes from the 3.7-volt lithium ion
battery that is connected to two linear voltage regulators, which brings the voltage down to the
desired 3.3 volts. Since all the components can operate at the same input voltage, the main
consideration that needs to be taken, placement-wise, is isolating the power supply circuit from
the rest of the board to reduce noise. Another noise reducing strategy implemented was to place
the decoupling capacitors close to the input pins, which is done for all the components.
3.0 PCB Layout Design Considerations - Microcontroller
The microcontroller is centered as much as possible on the PCB. The model being used is an
ATxmega256A3BU that has 64 pins. During the initial routing, all the pins were describing
clearance errors but a simple change of the DRC to a minimum clearance of 6 mil resolved this
issue. The external oscillator that is used is located under the microcontroller to reduce the noise
and it operates at 33kHz, which is sufficient for this project. The input and output capacitors are
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ECE 477
Digital Systems Senior Design Project
Rev 9/12
placed as close to the pins of the linear voltage regulators as possible as advised on the datasheet
[2]. The trace width size of the traces to the microcontroller pads are 10 mil, which is the same
width used around the board except for the power and ground.
4.0 PCB Layout Design Considerations - Power Supply
The power supply takes input from a USB input and a 3.7-volt lithium ion rechargeable battery,
which is the main source of power for this project, which will be located in a separate
compartment close to the PCB. The connector for this is on the bottom side of the top left hand
corner of the board. A Maxim 1555 charging circuit is used and is located close to the power
supply takes input from the USB and the output charges the battery. All these power supply
components are placed close to each other and away from the rest of the components on the
board to minimize noise.
The board uses two regulated power supplies that are both 3.3V. These supplies are linear
voltage regulators that are designed according to the correct specifications on the datasheet [2].
The main design consideration that was derived from the datasheet is placing the bypass
capacitors close to the input pins to reduce the supply ripple and also to provide instantaneous
current. For thermal dissipation considerations, a ground plane will be used, which will be
connected to the linear voltage regulators by vias that are placed underneath them.
The maximum current that is estimated on the board is 200mA and a trace width size of 12 mil
will be used for the ground and power traces. The 3.3V and the GND from the power supply will
be traced to the other side of the board so that all the components that require them will easily be
able to access them.
5.0 Summary
In conclusion, the components on the back PCB will be efficiently placed through the use of
intelligent board mapping/planning and manual trace insertion. The two, side PCBs are not really
a problem because the components are few and very small so space isn’t much of a constraint.
The PCB for the base station is similar in that there are no size limitations for it as its purpose is
simply to remain on the sideline and collect information, so this board can be significantly bigger
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ECE 477
Digital Systems Senior Design Project
Rev 9/12
than the other three PCBs. The recommended trace width size from an online trace width
calculator for the power is 3.5mil and for the other signals is about 2.3mil but the chosen trace
width sizes of 12mil and 10mil respectively should be satisfactory [3]. All that is left to finish is
the PCB for the base station, adding the ground plane and fixing a few errors. Some of these
errors include vias that are touching, a few right angles on the traces, some overlapping and
general routing changes to improve the visual appeal of the board. Generally, single point power
and ground layouts will be used, as recommended in the Motorola in their semiconductor
application note, as well as some other noise reduction techniques outlined here [4].
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ECE 477
Digital Systems Senior Design Project
Rev 9/12
6.0 List of References
[1] Texas Instruments, “PCB Design Guidelines for Reduced EMI.” [Online] Available:
http://www.ti.com/lit/an/szza009/szza009.pdf
[2] Texas Instruments, “200mA Low-Iq Low Dropout Regulator for portable devices.”
[Online] Available: http://pdf1.alldatasheet.com/datasheet-pdf/view/355086/TI/TLV70033Q1.html
[3] “ANSI IPC-2221A PCB Trace Width Calculator.” [Online] Available:
http://www.desmith.net/NMdS/Electronics/TraceWidth.html
[4] Motorola, “System Design and Layout Techniques for Noise Reduction in MCU Based
Systems.” [Online] Available:
https://engineering.purdue.edu/ece477/Homework/CommonRefs/AN1259.pdf
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ECE 477
Digital Systems Senior Design Project
Appendix A: PCB Layout Top & Bottom Copper (Back Board)
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ECE 477
Digital Systems Senior Design Project
PCB Layout Top & Bottom Copper (Side Board)
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ECE 477
Digital Systems Senior Design Project
Appendix B: PCB Layout To-Scale Component Side Layout
Back PCB
Side PCB
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