ECE 477 Digital Systems Senior Design Project 2/13

Homework 6: Printed Circuit Board Layout Design Narrative

Team Code Name: The Hex Me Baby Team Group No. 3

Team Member Completing This Homework: Robert Harris

E-mail Address of Team Member: harris89 @ purdue.edu

Evaluation:

SEC DESCRIPTION MAX SCORE

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:

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1.0 Introduction

“Hackers of Catron” is an electronically enhanced version of the popular Settlers of

Catan board game. Game setup will be automated, placement of physical pieces on the board

will be tracked, and resource trading will be handled through handheld devices. These

enhancements to the game will make the game easier to set up and play, resulting in a much

improved game play experience.

While the economy is handled via a web server, most of the game takes place on a

physical game board. Catan is composed of nineteen hexagons which fit together to make up the

board. Rather than making the players setup the board themselves, simply turning on the game

can randomize the playing surface. To accomplish this, each hexagon will use LEDs to

designate its resource type. The LEDs are mounted to the surface of the PCB, and a sheet of

frosted acrylic is placed above to disperse the light. Dividers will be used to represent the

borders and separate the light between the hexagons. Hall Effect sensors will be mounted to the

surface of the PCB for the purpose of detecting the magnetic game pieces. It is important that

the distance between the PCB and frosted acrylic be optimized such that the pieces detected

without interfering with adjacent sensors.

The PCB will be approximately 15”x17” in size to accommodate the overwhelming

number of sensors and LEDs. This large size brings along with it a large price tag, so a second

board run in the case of errors is not a viable option. This report will discuss the design

considerations we will make so that our circuit will work correctly and Electromagnetic

interference (EMI) will not be a problem. The next section (Section 2.0) is about the overall

layout design considerations; it will discuss the overall layout including component placement

and signal routing. Section 3.0 is about layout considerations involving the microcontroller.

Section 4.0 is about the layout considerations revolving around the power supply circuitry.

2.0 PCB Layout Design Considerations - Overall

There are many special considerations that need to be taken into account for the PCB layout

of the project. First, due to the massive number of Hall Effect sensors, 7-segment LEDs, and

RGB LEDs on the board (145, 38, and 38, respectively) careful planning before routing signals

will be critical. Through hole components (headers and 7-segment displays) add to the routing

complexity. Also, due to the fact that two different power rails (5V and 3.3V) are needed for

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components on the PCB, routing signals through and around the PCB will be very complex. To

avoid routing problems, the layout of the RGB drivers, 7-Segment drivers, and multiplexers will

be planned before any component placement is done. Appendix C shows the placement of the

RGB drivers, 7-Segment drivers, and Hall Effect sensors. Every hexagon has one multiplexer

for its 8 Hall Effect sensors. The drivers and multiplexers were placed so that signals are routed

as short a distance from their destination and the components are placed as evenly throughout

the board as possible. The power and ground rail placement was also decided early in the layout

planning process.

Traces must be sized according to the amount of current they will be required to handle. The

power and ground rails will be the widest rails. The 5V rail will provide approximately 2.4A at

max draw (1.7A to the RGB LEDs and 1.0A to the Raspberry Pi) [1] [2] [3]. To handle this

current, a trace with a minimum width of 30 mils will be needed [4]. However, due the length of

the traces, wider traces will be used. The trace that runs the 5V rail all the way around the board

will be much larger (60 mils); another option is to make use of a large fly wire to route the

unregulated 5V rail around the board instead of using a large trace. If there is room in the final

PCB layout for the large trace it will be used instead of a fly wire to make the board population

process a little simpler. The 3.3V rail will provide approximately 1.1A at max draw. Thus, a

trace with a minimum width of 15 mils should be used [4]. As with the 5V traces, the 3.3V

traces will also be made wider (40 mils) due to their length. All of the smaller power/ground

traces that run the width of the board will be 24 mils wide.

3.0 PCB Layout Design Considerations - Microcontroller

The Atmel AVR32 will draw approximately 19 mA of current at peak draw [5]. Thus, no

special considerations are needed for the size of the power traces running to the microcontroller.

The oscillator circuit and decoupling capacitors will need to be located near the microcontroller.

The microcontroller requires a total of four decoupling capacitors, thus their placement and

routing will need to be planned carefully [5].

The microcontroller will be placed where there is adequate space to position all of the

necessary components (oscillator circuit, decoupling capacitors) and where all of the

multiplexers and LED drivers can route easily. Routing of signals from all 19 hexagons to the

microcontroller will be very difficult, so effective placement of the microcontroller is crucial to a

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successful layout. The microcontroller is currently in the center of the board; however, after

routing many of the long traces on the board it looks like moving the micro to the outside of the

board will help with the routing process. Another benefit of moving the micro to the outside is

that there will be more room for the decoupling capacitors and the oscillator circuit. The EMI of

the oscillator circuit can be handled better with the extra space also.

As the PCB layout is finalized, the choice of microcontroller placement will be very

important. The microcontroller reset will be placed in close proximity of the micro to prevent

noise from inadvertently resetting the micro. As you can see, component placement for the

microcontroller is a vital step to a successful layout of our PCB due to the number of

components and signals present.

4.0 PCB Layout Design Considerations - Power Supply

A 5V unregulated signal is input to the power supply; the input comes from an external

switch mode power supply that is connected to the PCB via a barrel jack. The barrel jack is

located on the edge of the board for easy accessibility. The external supply is capable of

providing up to 4A of current; our entire board (including the Raspberry Pi) can theoretically

draw about 3.5A at max draw [6]. Thus, traces at least 100 mils wide will be used in the power

supply circuit [4]. The traces will be made as wide as possible to help reduce EMI produced.

Also, a copper pour will be placed throughout the power supply area to help decrease noise in the

circuit.

Power will be routed through the board with power and ground rails running across the

board. The ground rails will made wide to decrease noise. All of our board components are

digital, so EMI should not be a huge issue. Also, because of this only the power supply circuitry

has to be separated from the rest of the board. A micro USB adapter will route the 5V

unregulated input from the board to the Raspberry Pi [3]. Because 3.3V and 5V are needed

throughout the board, a large 5V rail will be run around the board where it can then be run

throughout the board to eliminate the need to cross the different power rails.

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5.0 Summary

In this report the major design criteria for the PCB layout of the project have been

outlined. These criteria will be followed closely to help ensure a successful result. Proper

planning of how to place components and route their signals is toughest part of this design. The

PCB layout will be done very carefully and thoroughly to yield the best possible result.

The layout is not 100 percent done at this point; but the power and ground rails have been

laid out, the long signals have been routed, and some hexagons have hand routing. The images

included in the appendices of this report are the result of both hand and auto routing. More hand

routing will be done for the final version of the layout. The power supply circuitry has not been

completed totally and will be reworked prior to the design review on Monday. Carefully

analyzing the design as it progresses will ensure a successful design. If needed, auto route will be

used for some of the connections; but most of the connections will be done by hand so that the

design will meet all of the necessary constraints.

The appendix of this report includes images of the layout and the layout planning

process. Appendix A includes images of the top and bottom copper PCB layout. Appendix B

includes a to-scale illustration of the component side PCB layout. Appendix C includes images

showing a high level view of the placement of the 7-segment drivers, RGB LED drivers, and

Hall Effect sensors. The placement of these parts was a crucial part of the PCB layout planning.

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6.0 List of References

[1] Cree, “Cree® PLCC4 3 in 1 SMD LED CLV1L-FKB,” [Online]. Available:

http://www.cree.com/~/media/Files/Cree/LED%20Components%20and%20Modules/HB/D

ata%20Sheets/CLV1L%20FKB%201238.pdf. [Accessed 21 2 2013]

[2] Omron, “LED Control IC W2RF004RM,” 8 2012. [Online]. Available:

http://components.omron.com/components/web/pdflib.nsf/0/A59093552C8ED1C186257A

5D0077B1D2/$file/W2RV004RM_0812.pdf. [Accessed 21 2 2013].

[3] Broadcom Corporation, “BCM2835 ARM Peripherals,” 2 2012. [Online]. Available:

http://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pdf.

[Accessed 21 2 2013].

[4] Purdue ECE 477 Staff, “PCB Design Specifications,” 18 7 2011. [Online]. Available:

https://engineering.purdue.edu/ece477/Homework/CommonRefs/Tutorials/PCB/PCB%20D

esign%20Specifications.pdf. [Accessed 21 2 2013].

[5] Atmel, “32-bit ATMEL AVR Microcontroller,” 3 2012 [Online]. Available:

http://www.atmel.com/Images/doc32059.pdf. [Accessed 13 2 2013].

[6] Triad Magnetics, “External Switchmode Power Supplies - WSU Series,” 3 2012. [Online].

Available: http://triadmagnetics.com/pdf/WSU-Series%20datasheet%20(2012).pdf.

[Accessed 21 2 2013].

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Appendix A: PCB Layout Top & Bottom Copper

Figure 1. PCB Layout Top and Bottom Copper.

Note: High Resolution Image – zoom to see details.

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Figure 2. PCB Layout Top Copper.

Note: High Resolution Image – zoom to see details.

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Figure 3. PCB Layout Bottom Copper.

Note: High Resolution Image – zoom to see details.

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Appendix B: PCB Layout To-Scale Component Side Layout

Figure 4. Component Layout – Not to scale.

Note: High Resolution Image – zoom to see details.

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Figure 5. To Scale Component Layout View 1.

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Figure 6. To Scale Layout with Microcontroller.

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Figure 7. To Scale Layout with Power Supply.

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Appendix C: Driver and Sensor Placement

Figure 8. RGB LED Driver Placement.

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Figure 9. Seven Segment Display Driver Placement.

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Figure 10. Hall Effect Sensor Placement.