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: ______SLOW______Group No. _8_
Team Member Completing This Homework: ____Ayush Johari______
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 idea of the SLOW project is to design and create an LED panel (10 cascaded 5x7 LED dot matrices) where a user can login to an embedded web server through the internet to change the text message being displayed. Besides displaying scrolling messages, the panel would also be able to show the current time and outdoor temperature using a digital thermometer that the user can wire to the outside.
We chose a double-sided PCB so we can place the LED panel on the front and have all the other components on the back of the board making the size of the whole system look compact and organized. The system will be operated by a 12V DC adapter [6] that will be regulated down to exactly 3.3V for the microcontroller [2] and 5V for all the other internal components. Special emphasis has been made on reducing EMI, trace widths, signal routing, footprint design and board size constraints. Using the Design Rule Check function, we are able to make sure that our PCB is viable in that all the traces are of an appropriate width, enough space in between traces, and other PCB checks.
2.0 PCB Layout Design Considerations
A lot of considerations need to be made for the PCB layout to be successful. One of the goals to produce a successful PCB layout is reducing EMI as much as possible. A digital signal error could cause an input pin to read a ‘high’ value instead of a ‘low’ value, which potentially could prevent the circuit from working correctly. Problems caused by EMI can give us a lot of difficulty in debugging so it is important that we take proper care to reduce as much EMI as possible.
One of the physical constraints is the size of the board. There was an unexpected change in PCB width because the shift registers and Darlington transistors ended up taking up a lot of space, so extra room was needed. In this extra space we allocated, we would be able to place all the other analog components such as resistors and capacitors and even the microcontroller without much trouble. The original planned overall size of the design’s package was to be about the same size as the cascaded LED’s but in the end resulted in a board size of 15x4 inches. As discussed earlier, our main concern was to have the LED panel on the top of the board and have the rest of the components on the backside of the board. Another consideration to look at would be the fact that the LED panel would cover the entire front of the PCB, making a number of components through-hole which imply that we would need solders on the front side, therefore making it difficult to remove components for which the LED’s would have to be removed first. The microcontroller was placed at the center of the board with its pins facing in directions convenient for routing to its analog components such as the RJ-45 jack. The header is also placed in an area to make it easier to route to the shift registers.
The analog components that we have included are namely a 25 MHz crystal oscillator, a high voltage-high current Darlington transistor array [10] and a number of capacitors and bypass capacitors. Many of these components were required because they were specified as the minimum external circuitry for proper functionality of the MC9S12NE64 as stated in its datasheet. As these components are to be placed closer to the microcontroller, we made sure that they had short traces and also be correctly configured as per the microcontroller’s data sheet diagrams [2].
One of the most important concerns in the PCB layout would be the power supply circuitry. As we have two DC regulators which generate two regulated voltage of 3.3V and 5V. The 3.3V is only used by the microcontroller and the 5V is used by the rest of the circuit. For the layout, we made sure that the power regulators are kept on the top left corner of the board to prevent interference with the digital components and also the analog part of the circuit. Another reason would be the power dissipation by the regulators, so we left a lot of room around them for the regulators to dissipate heat. Trace widths have to be thick where needed for the power and ground signals to make sure the entire circuit gets enough current. When connecting a power or ground signal to a pin, the trace size is gradually decreased down to the size needed for the connecting point and this is actually done very near to the connecting point so as to ensure proper functionality.
As for the microcontroller, its external circuitry also required special attention as according to the data sheets. For optimal functionality of the MC9S12NE64, its bypass capacitors, PLL filter circuit and crystal all need to be as close as possible to the microcontroller without any vias. Lastly, the Ethernet, the RJ-45 also needs to be within 1 inch of the microcontroller pins, without using any vias, and while keeping the traces roughly around the same length. This is to make sure that no noise interferes with the Ethernet controller.
3.0 Summary
The overall PCB layout was done with great consideration keeping all the guidelines in the mind such as keeping the trace lengths small and routing signals at no acute angles and accommodating to all the Design Rule Check requirements. The general locations of most of the components on the PCB layout were determined by their location on the packaging design. Considering size as one of the constraint, we made sure that we placed all the parts at the right place making the board compact and organized and efficient.
List of References
[1] MC9S12NE64 Product Summary Page,
http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MC9S12NE64&srch=1
[2] MC9S12NE64 Data Sheet,
http://www.freescale.com/files/microcontrollers/doc/data_sheet/MC9S12NE64V1.pdf
[3] Implementing an Ethernet Interface with the MC9S12NE64,
http://www.freescale.com/files/microcontrollers/doc/app_note/AN2759.pdf
[4] DS18S20 Data Sheet,
http://pdfserv.maxim-ic.com/en/ds/DS18S20.pdf
[5] Digi-Key.com, LITE-ON INC LTP-2157AKR Data Sheet,
http://rocky.digikey.com/WebLib/Lite-on/Web%20Data/LTP2157AKx.pdf
[6] Wall Wart 12V DC adapter
http://www.apogeekits.com/ac_adapter_12vdc_eu.htm
[7] Power Regulators: MAX831 and MAX832
http://pdfserv.maxim-ic.com/en/ds/MAX830EVKIT-MAX833EVKIT.pdf
[8] Toshiba 16 Bit LED driver with shift register and Latch Ability
http://www.marktechopto.com/PDFs/Toshiba/ACF42D.pdf
[9] System Design and Layout Techniques
http://shay.ecn.purdue.edu/~dsml/ece477/Homework/Spr2006/AN1259.pdf
[10] High Voltage High Current Darlington Transistor
http://www.chipcatalog.com/Datasheet/B7E53F71FE64685A764E9999EAE9255C.htm
Appendix A: Routing Statistics Report
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