Design Project



Homework 6: Printed Circuit Board Layout Design

Due: Friday, February 24, at NOON

Team Code Name: ____Spot Dash__________________________ Group No. __14____

Team Member Completing This Homework: Yuan-Jiun (David) Sung

Evaluation:

|Component/Criterion |Score |Multiplier |Points |

|Introduction & 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. Introduction

The driver interface system for the solar racing car provides relevant information to the driver regarding the operation condition of the car. Some key parameters to display include battery voltage, car speed, solar power, text messages from the race team, and so forth. The driver interface system communicates with other devices on the car via CANbus network to receive information on parameters to display.

The driver interface PCB board is separated into seven major sections which are DC/DC converter, PIC18 microcontroller, CANbus interface, programmable logic device (PLD), optical isolator, bicolor LED, and temperature sensor. All the components within each functional block are powered by a DC-DC converter and are close to each other to minimize the power consumption. The physical size of each component must fit into the designated PCB board size which is 4.5 inch by 2.75 inch. All the passive components should be at the bottom layer and active components at the top layer for easier debugging, soldering and interfacing purposes. Finally, the driver interface PCB board needs to interface with several external devices such as push button and CANbus. Hence, all the MTE right angles components must be placed near the edge of the board.

2. PCB Layout Design Considerations

The driver interface PCB was arranged into the following major sections:

1. DC/DC converter (Analog component) [1] - This is a high efficiency DC/DC converter which takes the 12V input voltage from the battery pack and steps down to 5V output voltage.

2. PIC18 microcontroller (Digital component) [2] – The major component of our PCB board which require 5V operating voltage and 12MHz operating frequency.

3. CANbus interface (Digital component) [3] – Two connectors used for CAN to provide flexibility of adding additional nodes to the car. However, these two connectors aren't required for CAN bus. This device also requires 5V operating voltage.

4. Programmable logic device (PLD) (Digital component) [4] – The PLD requires a 44 pins PLCC socket which operates at 5V input voltage. This PLD provides extra I/O pins to our microcontroller.

5. Bicolor LED (Digital component) [5] - These bicolor LEDs require a 5V operating voltage and 330Ω current limiting resistors. These LEDs are used for programming purposes as well as for displaying information regarding the connectivity of the entire driver interface system.

6. Optical isolator (Digital component) [6, 7] - This optical isolator is required for our push button interface which operates at 5V input voltage. The push button is connected to a 2x7 MTE right angle programming header.

7. Temperature sensor (Analog component) [8] - One of the few analog components on our PCB board which requires a 5V operating voltage and must be separated from digital components to avoid mixed signals or noisy signals.

Several layout design considerations were carefully analyzed and considered by the team before the start of the layout processes. First of all, the team placed decoupling capacitors under the microcontroller to provide instantaneous current to the ICs. This design could help lower power consumption and balance out the inductive losses of the traces. Crystal was also placed near the microcontroller to keep the clock signal from being coupled into other signals on the board. It also reduced EMI since the traces act as antennas, the shorter the traces the better [9].

According to the IC datasheets, analog components must be placed as far away from the digital components as possible to avoid mixed signals. However, since there are only two major analog components (DC/DC converter and temperature sensors), the team decided that it is easy to separate the digital components from the analog components and this issue was not a major concern. Nevertheless, the components within each functional block (digital and analog blocks) should be placed as close as possible to minimize the length of the traces. The design consideration is that the longer the track length, the greater its resistance, capacitance and inductance. All of these factors could result in undesired results on the team’s circuit board such as noisy signals and EMI. Also, to avoid placement errors, the team used insertion outline on some footprints to pass the design rule check (DRC thinks the boarder of the footprint is a trace if the insertion outline is not enabled) [9, 10].

When laying out the traces, the team used 40 mils wide power and ground traces to provide plenty of margins. Since the power and ground traces will take the most space on the PCB board, the team decided to route these traces first. The power and ground traces could most likely be smaller than 10 mils without issues but 40 mils traces were used for the higher current applications and 20-24 mils traces for the lower current areas and 12 mils for hard to route areas. The wider the trace the less the trace temperature will rise. Most signal traces were 12 mils with the minimum tolerance of 8 mils. The space between two traces was greater than 8 mils to avoid short circuit [9, 10].

Since the PCB board will be mounting into a plastic enclosure, the team took the positions of the four mounting holes into design considerations. These mounting hole locations are 1.375 inch on top and bottom of each side from the center of the box. To fit into the plastic enclosure, the size of the PCB board size was also considered by the team. The box is around 5 inch by 3.5 inch in dimensions, so the estimation of a board size 4.5 inch by 2.75 inch should fit the enclosure nicely.

The active components were purposely placed by the team on the top layer and the passive components on the bottom layer. The main purposes of this configuration were for easier interfacing, soldering, and debugging of the PCB board. Another reason was for industry preparation where it is more expensive to have larger components on the back due to the way manufacturers solder the boards. It was also easier for the team to identify problems and assembly the board if the active components were placed on the top. All the MTE connectors (RS232 connector, CANbus connector, etc.) were also arranged to hang over the boundary for easier accessibility from the outside of the plastic enclosure.

While the team considered most critical considerations for the PCB board, there were also some common design constraints that the team didn’t include. For instance, the heat sink was not taken into consideration since there are only two analog components and both of them are highly efficient. According to the datasheet, the DC/DC converter has efficiency of 94% and the temperature sensor’s quiescent current is less 10 micro ampere which gives rise to less 2˚C in still air. Hence, temperature constraint was not part of the team’s considerations. Another common consideration, copper pours, was also not utilized by the team. Copper pours are used to provide shielding and a good ground. Since most of the team’s components are digital components, this design was not required [9, 10].

3. Summary

There are some major concerns when drawing our PCB layout. The analog signals must be separated from the digital signal to avoid noisy circuit. Each component should group into different functional building blocks if possible. The four mounting holes on our circuit board should be placed first and in the exact corresponding positions to avoid any broken traces. All the passives should be at the bottom layer and actives should be at the top layer. All the MTE connector must place closer to the edge of the boarder for easier cable connections. Lastly, there are other constraints such as EMI reduction and heat sink consideration were not considered after carefully analyzing and arranging our circuits.

Final Layout Documents:

Appendix A: Routing Statistics Report

Figure 1. Final PCB layout

Figure 2. Final PCB layout – Top Cover

Figure 3. Final PCB layout – Bottom Cover

Figure 4. Final PCB layout – Component Placement

List of References

[1] “High Efficiency Step-Down and Inverting DC/DC Converter”, Linear Technology, [Online Document],

3.3_1174-5.pdf

[2] “PIC18F2380/2580/4480/4580 Data Sheet”, Microchip, [Online Document],



[3] “2.54mm (.100") Pitch C-Grid Breakaway Header”, Molex, [Online Document],



[4] “STANDARD PLCC SOCKETS For Through Board Mount”, MILL-MAX, [Online Document],



[5] “LED GREEN/RED BICOLOR 1210 SMD”, LITEON, [Online Document],



[6] “Optoisolators”, Panasonic, [Online Document],



[7] “BUZZER 2.048KHZ 12MM PC MOUNT”, GUI, [Online Document],

(42).pdf

[8] “LM19 2.4V, 10μA, TO-92 Temperature Sensor”, National Semiconductor, [Online Document]



[9] “System Design and Layout Techniques for Noise Reduction in MCU-Based Systems”, MOTOROLA, [Online Document],



[10] “PCB Design Tutorial”, David Jones, [Online Document]



Appendix A: Routing Statistics Report

Figure 1. Final PCB layout

Figure 2. Final PCB layout – Top Cover

Figure 3. Final PCB layout – Bottom Cover

Figure 3. Final PCB layout – Component Placement

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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.

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·CJh1-ZCJhòJjCJ-jhwTCJNOTE: 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.

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