ECE 477 Digital Systems Senior Design Project Spring 2006
Homework 6: Printed Circuit Board Layout Design
Due: Friday, February 24, at NOON
Team Code Name: D.E.A.Th ______Group No. 5 ____
Team Member Completing This Homework: Philip Smith ______
Evaluation:
Component/Criterion / Score / Multiplier / PointsIntroduction & Layout Considerations / 0 1 2 3 4 5 6 7 8 9 10 / X 3
Documentation for PCB Layout Design / 0 1 2 3 4 5 6 7 8 9 10 / X 5
List of References / 0 1 2 3 4 5 6 7 8 9 10 / X 1
Technical Writing Style / 0 1 2 3 4 5 6 7 8 9 10 / X 1
TOTAL
Comments:
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1.0 Introduction
The goal of the project is to develop an autonomous robot that will navigate through the use of GPS coordinates. The robot will be given GPS waypoints by way of a wireless connection. It will then generate a path and navigate through the waypoints to the terminal location.
The platform for the robot is an existing remote control tank. The size constraints imposed by this existing platform are 7 inches by 24 inches, allowing plenty of room for the printed circuit board. In addition to having a large area for our printed circuit board, most of the sizable components reside off of the board, further loosening size constraints. The SONAR module, GPS module, and speed sensors are all mounted exterior to the board, and as such will require headers to interface with the microcontroller. The existing tank systems, including drive motors, turret control, and gun firing mechanism, will all need to be interfaced with the board via headers.
2.0 PCB Layout Design Considerations
The overall circuit layout started by separating the digital, analog, and power supply sections of the schematic. Each section takes up roughly a third of the printed circuit board, with analog on the left side, power in the lower right corner and digital in the center. Special considerations were taken to increase the power and ground trace sizes, as they will have to carry large amounts of current. To size the traces an overestimate of the current requirements was used with an online trace calculator to determine the proper width for the traces. The trace widths can be seen below.
Power / Components / Current / Trace Needed / Trace Actual3.3 V / Micro, GPS, Freq-to-V,
RS-232 chip / 600 mA / 10.2 mils / 50 mils
5 V / SONAR, Servo, PLD / 300 mA / 3.4 mils / 25 mils
9 V / H-Bridge to motor (3) / 2 A / 52 mils / 70 mils
The remaining design considerations will be analyzed by system component, and organized in the following way:
1. Power Supply Layout
2. Microcontroller Layout
3. Communications Connector Layout (RJ45 and DB9 connectors)
4. H-Bridge Layout
5. Extra Component Layout
Power Supply Layout
Due to the voltage needs of the circuit, the printed circuit board will have two power supplies. These power supplies use a MAX1791 step down controller which has a myriad of design requirements [4]. Each MAX1791 chip needs both an analog and digital ground, which is connected at a single grounding point. These grounds are created through the use of copper pours, which serve as ground planes for the circuit. Copper pours are used for the 12 V battery connection to facilitate the current demands of both the power supplies. The power supply components are located in as close proximity to each other as possible to increase stability and efficiency of the power supply. In addition to close component placement, the traces between components have been made as short as possible, again increasing stability and efficiency. The internal connections of the power supply have increased trace sizes, allowing larger than normal amounts of current to flow. The power supplies have been located on the bottom right corner of the printed circuit board to reduce their interaction with the rest of the digital logic circuitry. In addition to the normal capacitor located between the output voltage and ground signals, each power supply includes two extra capacitors. These capacitors serve to recharge the bypass capacitors located throughout the circuit, and as such are located as close to the power supply as possible [6].
Microcontroller Layout
The microcontroller in our project, a Freescale 9S12NE64 [3], has been placed near the top center of the printed circuit board. This has been done to facilitate access to its 112 pins and allow proper placement of the RJ45 jack (see Communication Connector Layout section). The microcontoller’s bypass capacitors have been placed underneath the microcontroller to allow short connections to the microcontroller’s power pins. The crystal oscillator has also been placed close to the microcontroller, which will help stem clock signal distortion.
Communication Connector Layout
On the board there are two main communication connectors, an RJ45 connector and a DB9 connector. Both are located at the edge of the top of the board, allowing them to be close to their respective microcontroller pins and be on the edge of the board. The RJ45 connector carries a high data rate and as such its signals are easily distorted [6]. To reduce the errors in communications due to mechanical concerns, the traces to the RJ45 connector were made as short as possible, as close to equal length as possible, and did not cross planes. To reduce errors due to electrical concerns, no traces or components were placed on the board opposite to the RJ45 signal traces. Traces are laid beneath the end of the RJ45 connector, which is magnetically shielded, preventing any interference.
H-Bridge Circuit Layout
The three h-bridge circuits located on the printed circuit board control the turret and drive motors [2]. Each h-bridge circuit requires a corresponding optoisolator to safely separate it from the microcontroller. Since our circuit uses a separate 9 volt battery to power the motors, the h-bridges are not connected to the onboard power supplies. The motor battery connector is mounted just above the h-bridges, allowing easy access. The h-bridges are separated in the upper left corner of the board to isolate them and the motor battery circuitry from the digital logic of the microcontroller.
Extra Component Layout
For use in speed control, our circuit uses frequency to voltage converters, which are located below the h-bridges on the bottom left corner of the printed circuit board. They are placed there to separate their analog function from the digital logic of the microcontroller. In the same corner of the board is the servo controller, which consists of a transistor and servo header. These are located next to the edge of the board to allow easy access to the header. Since our design requires a great number of headers for key connections, the layout attempted to place them towards the edges of the board, granting easier access. In addition to the key component headers, our layout includes headers for most of the microcontrollers’ pins. These headers were located as close to the microcontroller as possible, reducing the amount of vias needed to route the board. These headers also serve to replace vias in multiple instances, allowing a trace to change board sides.
3.0 Summary
The D.E.A.Th printed circuit board is laid out in a way to facilitate the number of external connections needed for operation. The individual board components are separated into analog, digital, and power categories to minimize harmful interactions. The power supplies and the RJ45 connector required the most careful layout as they are sensitive components which are vital to the success of our project.
List of References
[1] Pulse (2006) PE-53632 Datasheet Available:
http://www.pulseeng.com/pdf/PC2005_37.pdf
[2/22/2006]
[2] National Semiconductor. (2006) LMD18200T Datasheet. Available:
http://www.national.com/ds/LM/LMD18200.pdf
[2/22/2006]
[3] Freescale Semiconductor. (2006) MC9S12NE64 Datasheet Available: http://www.freescale.com/files/microcontrollers/doc/data_sheet/MC9S12NE64V1.pdf
[2/23/2006]
[4] Dallas Semiconductor Maxim (2006) Maxim 1791 Datasheet Available:
http://pdfserv.maxim-ic.com/en/ds/MAX1762-MAX1791.pdf
[2/21/2006]
[5] National Semiconductor (2006) SuperSOT-3 Package Dimensions
http://www.fairchildsemi.com/products/discrete/pdf/ssot3_dim.pdf
[2/22/2006]
[6] Motorola (2006) Application Note AN1259 Available:
http://shay.ecn.purdue.edu/~dsml/ece477/Homework/Spr2006/AN1259.pdf
[2/20/2006]
[7] David L. Jones (2004) PCB Design Tutorial
http://www.pcb123.com/tutorials/PDF%20Documents/PCBDesignTutorialRevA.pdf
[2/20/2006]
[8] Patrick Frantz () Preparing for OrCAD Layout Available:
http://cnx.org/content/m11677/latest/
[2/14/2006]
Appendix A: Routing Statistics Report
****************************************************
* STATISTICS REPORT *
* *
* N:\ALLEGRO_PROJECT\PCB1\LAYOUT\PCB_LAYOUT-6.MAX *
* Sun Mar 26 19:59:21 2006 *
****************************************************
STATISTIC ENABLED TOTAL
------
Board Area 28.0 28.0
Equivalent IC's 51.1 51.1
Sq. inches per IC 0.55 0.55
# of pins 767 767
Layers 4 28
Design Rule Errors 1 1
Time Used 26:33 26:33
% Placed 100.00% 100.00%
Placed 264 264
Off board 0 0
Unplaced 0 0
Clustered 0 0
Routed 551 551
% Routed 100.00% 100.00%
Unrouted 0 0
% Unrouted 0.00% 0.00%
Partials 0 0
% Partials 0.00% 0.00%
Vias 159 159
Test Points 0 0
Vias per Conn 0.29 0.29
Segments 2229 2229
Connections 551 551
Nets 205 205
Components 264 264
Footprints 171 171
Padstacks 79 79
Obstacles 574 574
Theoretical Dist 295.6 295.6
Routed Dist 287.5 287.5
Unrouted Dist 0.8 0.8
-1-