Download Homework 6: Printed Circuit Board Layout Design

Homework 6: Printed Circuit Board Layout Design
Due: Friday, February 24, at NOON
Team Code Name: _____Rubber Ducky___________________________ Group No. _1_
Team Member Completing This Homework: ___Stephon Watson__________________
NOTE: This is the third in a series of four “design component” homework assignments, each
of which is to be completed by one team member. The completed homework will count for
10% of the team member’s individual grade. The report itself should be a minimum of five
pages, not including the cover sheet, references, or any of the attachments (statistics report).
Electronically submit the “.MAX” PCB layout file along with the “.doc” version of this
report zipped into one file.
Evaluation:
Component/Criterion
Score
Multiplier
Introduction & Layout Considerations
0 1 2 3 4 5 6 7 8 9 10
X3
Documentation for PCB Layout Design
0 1 2 3 4 5 6 7 8 9 10
X5
List of References
0 1 2 3 4 5 6 7 8 9 10
X1
Technical Writing Style
0 1 2 3 4 5 6 7 8 9 10
X1
Points
TOTAL
Comments:
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1.0 Introduction
The Rubber Ducky Hunting game is very similar to the vintage Nintendo game “Duck Hunt.” A
player will use a “gun” to shoot ducks that are displayed on the screen via an NTSC Video encoder on
the console. The gun houses an image sensor which captures an image of the TV screen and stores it
on the gun’s on-board memory. The image is processed on the gun’s on-board microcontroller, and
then compared to the image of the duck in memory. The results, either a hit or miss, are transmitted to
the console. The console does all of the video displaying and graphics, including projecting a
background and at least one target. There are separate PCBs for the console and gun. The gun PCB
must fit with a 2in. by 3in. area, so the components will be closer together. A few of the challenges in
designing the gun PCB include proper trace spacing and creating enough separation between the
analog parts and digital components. The console PCB does not have such a tight space constraint as
the gun PCB. There are many more components on the console board, especially sensitive analog
sections such as the audio and visual circuits.
2.0 PCB Layout Design Considerations
There are two PCB layouts for Rubber Ducky Hunting game, one for the gun and one for the
console. Both PCBs share similar considerations, although the console has a few unique considerations
due to the audio and video output signals. The components of the PCBs are divided by their
interaction with each other. Parts that are closely related, or had many routes between them, are placed
closer together (such as FIFO and the microcontroller on the gun). Digital and analog parts of the
circuits are separate from each other to minimize electromagnetic interference (EMI) that may cause
digital signals to fall outside of the DC noise margin for components. EMI can also interfere with
clocking signals and disrupt the precise timing the digital components need to function properly.
Below are the considerations for each PCB:
Common Considerations:
Low-dropout voltage regulators [1] are analog components with the largest amount of the current
on the circuit board. The power and ground traces from the LDOs are run in parallel so as to reduce
signal noise because the electromagnetic fields generated by each trace to partially cancel out each
other. Both PCBs have at least one clocking component also. Oscillators [2] and crystals [3] provide
timing for the digital components. The power and ground traces must be routed around both the
clocking components and the traces from the clocking components to the ICs. Electromagnetic fields
from the power traces can distort the clocking signals and disrupt the precise timing the digital
components need. The RS232 transceivers are near the edge of the boards because they are external
interfaces. Finally, passive components such as pull up resistors and bypass capacitors populate much
of both PCBs. According to many of the manufacturers’ data sheets and general rules of PCB design,
bypass capacitors and pull up resistors for the FIFO, latches, SRAM, microcontroller, RS232
transmitter, and LDO power regulator also need to be placed within a few centimeters of the devices.
Specific PCB Considerations:
PCB 1- Gun
The image sensor header, FIFO, RS232 transmitters and microcontroller will be placed near each
other and with a clear area between the components to reduce trace length and via usage. Traces run
from the image sensor header to the FIFO and from the FIFO to the microcontroller. The
microcontroller also sends signals to the RS232 transmitter. Orientation of each component is chosen
carefully to reduce convoluted routes around other components or traces. An example would be that
many of the FIFO pins that are connected to the image sensor are facing the image sensor header.
Similar considerations are made for the microcontroller.
Other design constraints for the Gun PCB included a significant size constraint. The desired size
is an area of about 6 square inches (roughly 2 inches by 3 inches), because the PCB must fit in the 2
inch diameter of the barrel of the gun. A header is included for one side of the microcontroller to
facilitate on board programming, debugging, circuit and logic analyzing, and a limited capacity for
expansion or changes to the design.
The LDO power regulator is in the lower left corner of the board so as to isolate this analog
part from the rest of the circuit, especially clocking signals. The rest of the gun circuit uses an LDOsupplied voltage of 3.3V at a max current of 250 mA, and the console uses a 5V regulated circuit.
Both LDO regulators required that bypass capacitors be situated within one centimeter of the LDO
device. The proximity of the bypass capacitors to the LDO blocks much of the area for the traces, so
there are only a few available exit paths for the ground and power traces. Because the project’s
maximum current output is 500 mA and maximum voltage of 5 V, a temperature increase of about 10
degrees C, and a trace thickness of 1 oz/ft2, the power and ground traces can be smaller than most
other PCB layouts. According to [4], the traces can be as small as 3.5 mils, which all traces on both
PCBs are more than five times above that minimum width. The trace width for the power and ground
traces is 20 mils in most places, sometimes going as thin as 10 mils to fit on the pins of some of the
microchips.
The image sensor headboard header is in the upper left hand corner of the board because the
ribbon cable connecting the sensor to the board will come from the left of the PCB. A few
components had through-holes, such as the diode and the 10F capacitor. All other parts, except
headers, are surface mount devices. The RS 232 transceiver is in the upper right corner of the PCB
because the connector pins need to be near the edge of the board and bundled together as a ribbon
cable. The digital components occupy the middle section of the board, with plenty of space separating
the power circuitry and the digital parts.
The focus of the layout, after laying power and ground, are the traces leading from the
microcontroller to the FIFO and the image headboard. At first, the microcontroller and the FIFO were
on the top of the board. However, due to the extended travel routes needed for some traces, the
microcontroller is now on the bottom layer. The challenge is that the microcontroller, the main
component of the board, has many traces coming in, which often blocks the possible routes from other
components to the header pins. Vias are used to circumvent chokepoints and blocked paths.
There are no DRC errors on the gun PCB. There is, however, a discrepancy in the routing
statistics. Although OrCad indicates only 98.44% routed, the final “partials” are connected to ground
or power traces and can be verified visually.
PCB 2- Console Considerations
There are a few components that connect to external wires. The power jack must be mounted on
edge of board, since it supplies power to both console and gun through an LDO. The audio and video
connectors are located near the edge of the board to facilitate as well. The A/V components are
physically located on the bottom layer and the southeast corner of the board. The physical separation
of the A/V circuits from other components will help reduce EMI. They also need to be located on the
edge of the board because they are outputting signals to a TV screen and a speaker.
Another constraint is that the console PCB must fit within the dimensions of 5.385 in by 3in
based on the packaging design of a box of 5.5 inches long and 3.5 inches wide. There are more ICs on
this board which also makes the console more difficult to layout compared to the gun PCB.
The dual-port SRAM’s layout is very complicated. The SRAM receives address inputs from the
microcontroller and the CPLD and outputs pixel data to the CPLD. As a quad-package chip, nearly
every side has to interface with the microcontroller or the CPLD. Similarly, the CPLD is another
complex device on the board. The CPLD is mounted on the PCB via a socket. This means the pins can
be connected on either the top or bottom layer. However, since the pins are very close to each other,
neighboring traces alternate on different layers.
The microcontroller presents a significant challenge as well. Data and address pins are spread out
over all sides of the microcontroller. Other challenges are the fact that the traces from neighboring pins
have to cross over one another before connecting to the latches or SRAM. This requires the use of
many vias to correct the order of the traces.
There are no DRC errors on the console PCB. Similar to the gun PCB, the “unrouted” traces are
not significant and represent nets that are connected in reality but the program doesn’t recognize those
connections.
3.0 Summary
The Rubber Ducky Hunting Game consists of two PCBs with microcontrollers on each. Each
board has its own constraints to take into account, including specific landing placement for bypass
capacitors, proximity to other components, or connectivity. The gun PCB layout is simple than the
console layout. There are more components, both analog and digital, that increased the difficulty of the
console PCB.
Be sure to read Motorola Application Note AN1259 (posted on course web
site) before you begin your PCB layout.
List of References
[1] National Semiconductor (2005) LP3997 Micropower 250mA LDO Regularo with Error
Flag/Power-On-Reset. Available: http://www.national.com/ds/LP/LP3997.pdf
[2] CTS CMOS/TTL Oscillator; Model CB3. Available:
http://shay.ecn.purdue.edu/~477grp1/Clock/CB3-LV-Series.pdf
[3] ECS, Inc. CSM-7 SMD Quartz Crystal. Available:
http://shay.ecn.purdue.edu/~477grp1/Clock/CSM-7.pdf
[4] PCB Trace Width Calculator (2006) Available:
http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/
IMPORTANT: Use standard IEEE format for references, and CITE ALL REFERENCES
listed in the body of your report. Provide “live” links to all data sheets utilized.
Appendix A: Routing Statistics Report
Copy-and-paste your routing statistics report here.
GUN PCB:
Console PCB: