Design Project - Purdue University



Homework 6: Printed Circuit Board Layout Design Narrative

Team Code Name: ____Blinkers ++_____________________________ Group No. __5___

Team Member Completing This Homework: ___Jacquelyn Dickerson_________________

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

Evaluation:

|SCORE |DESCRIPTION |

|10 |Excellent – among the best papers submitted for this assignment. Very few corrections needed for version submitted in |

| |Final Report. |

|9 |Very good – all requirements aptly met. Minor additions/corrections needed for version submitted in Final Report. |

|8 |Good – all requirements considered and addressed. Several noteworthy additions/corrections needed for version |

| |submitted in Final Report. |

|7 |Average – all requirements basically met, but some revisions in content should be made for the version submitted in the|

| |Final Report. |

|6 |Marginal – all requirements met at a nominal level. Significant revisions in content should be made for the version |

| |submitted in the Final Report. |

|* |Below the passing threshold – major revisions required to meet report requirements at a nominal level. Revise and |

| |resubmit. |

* Resubmissions are due within one week of the date of return, and will be awarded a score of “6” provided all report requirements have been met at a nominal level.

Comments:

1. Introduction

Blinkers++ is a next-generation inter-car communication system. Blinkers++ is composed of two parts, the multi-touch user interface and the LED interface for the LEDs along the perimeter of the vehicle. This creates the need for two separate PCBs. The user interface PCB will be mounted in a box that can support a 7”x8” PCB, which was chosen specifically based on the sizes of the touch surface, the dsPIC, and the RF transmitter. The LED interface will be mounted on the inside of a remote controlled Escalade. This will restrict the LED interfacing PCB to the interior dimensions of the vehicle.

2. PCB Layout Design Considerations - Overall

Blinkers++ has standard PCB concerns, such as needing large traces for power, but there are also a number of project specific criteria. One such criterion is that the RF devices and the CapSense chips are very sensitive to noise. This requires several measures to be taken to reduce noise. One way of doing that is using shorter traces. Another way is to distance the RF devices and CapSense (PSOC) chips from any noisy components, such as data and switching lines, and especially from one another. To create these shorter traces, PSOC chips are required to be placed as close as possible to their corresponding capacitive touch pads. The current plan is to place them in the center of the square of pads they control and on the underside of the board. More than any other factor, these short traces will reduce the noise on the PSOC chips.

Another one of the criteria is fitting the 14 LED drivers with headers for LEDs and the LED interface board on a single 60 square inch board. Another piece of constraining criteria is the task of making 14 LED Panel PCBs and an LED output control PCB using less than 60 sq. in. Once laid on the single 60 sq. in. board, the team will then manually segment the whole into the 15 individual PCBs. Each LED panel PCB requires one LED driver, 2 headers, 5 LEDs, and 5 jumpers, plus resistors and capacitors to be placed on the 0.5”x5” PCBs. This created a tough challenge to layout and subsequently to route.

3. PCB Layout Design Considerations - Microcontroller

Because Blinkers++ is a very complex system with three different types of microcontrollers, many things needed to be considered in the design of the multiple PCBs. This includes, but is not limited to, things such as minimizing noisy traces and making room for programming headers.

1. PCB Layout Design Considerations – Microcontroller - PSOC

Blinkers++ uses Cypress Semiconductors’ CapSense PSOC controllers to read the user’s inputs on the capacitive touch pads on the top side of the board. These controllers are very sensitive to noise and require short traces and minimal parallel traces. This requires them to be placed far from the power supply and the RF transmitter that is also sensitive to noise. The solution being implemented to minimize the traces, and thusly minimize noise, is right-angle headers. The headers will be placed as close to the PSOC as possible to further reduce noise.

The PSOC does not require any special decoupling capacitors, however it does utilize the standard decoupling capacitor at +5V. Other than the noise reducing measures taken above, the PSOC does not require specific trace sizing or layouts.

2. PCB Layout Design Considerations – Microcontroller - dsPIC

The dsPIC does not have many special requirements. It requires several decoupling capacitors between each of the voltage drain and voltage source pins and the standard capacitor at +5V. Because Blinkers++ does not require an oscillator circuit, there are fewer concerns with routing or placement than would need to be addressed if one was being used. However, special care has been taken to insure that there are headers for the programming pins.

3. PCB Layout Design Considerations – Microcontroller - PIC

The PIC chosen currently only requires two decoupling capacitors. They are placed at the +5V pin and the USB voltage pin. The PIC is placed on the LED interface board and will generate the LED patterns and send them to the LED drivers. All of the headers of the LED drivers are hooked together, and therefore the placement of the PIC is somewhat flexible. The RF receiver is also on the LED board and is sensitive to the noise that can be produced by traces from the PIC and LED drivers; fortunately, these are relatively short traces and will not affect the receiver too much. This is why the PIC to driver traces need to be minimized. Fortunately, there is again no need for an oscillator circuit, and therefore this is possible without worrying about the placement of other parts or traces.

4. PCB Layout Design Considerations - Power Supply

Blinkers++ uses two different power supplies to drive each of the two main boards. There is a user interface power supply and a LED interface power supply. The user interface will utilize a standard wall wart supply, while the LED interface will use a 5V switching regulator.

1. PCB Layout Design Considerations - Power Supply – User Interface

The user interface will be powered by a typical wall wart supply. The current plan is to use a 12V battery, such as a car battery, and regulate it down to 5V. This will require a sizeable heat sink. It will also need to utilize thick power traces, probably around 55 mils. Fortunately, there are no specific requirements for ground planes or inductive/resistive paths. Also fortunately, there is a considerable amount of extra room on the base of the user interface board that will allow an increase both in the heat sink and in the trace widths if the need arises later.

2. PCB Layout Design Considerations - Power Supply – LED Interface

David Collins suggested that Blinkers++ utilize the 5V switching regulator from the PADS Tutorial. This was an extremely helpful solution. Not only was there a suggested layout for this power supply, but it will also do exactly what was needed for the LED driver interface. Luckily, there is no need for ground planes or resistive paths again. In this instance, there is also no need for a heat sink which is convenient due to the severe size limitations on the PCB. This switching regulator will run off of a lithium-polymer battery. The battery will not be placed on the PCB, but there will be headers on the PCB for which the battery can connect

5. Summary

Blinkers++ is an extremely complex project from the shear number of special made parts, such as each CapSense pad, to the number of LEDs being driven. These two constraints alone make the layout of the PCB difficult. Adding in the fact that the capacitive touch pads and the RF devices are sensitive to noise, the PCB was extremely difficult to route. There were many considerations taken in the routing and placement of parts, and hopefully this report has highlighted them.

List of References

1] Microchip Inc. “High Performance, 16-bit Digital Signal Controllers,” dsPIC33FJXXXMCX06/X08/X10 datasheet, Jul 2007 [Revised Mar. 2009]

[2] Cypress Semiconductors. “CapSense Applications,” CY8C20x36/46/66/96 datasheet, Oct 2008 [Revised July 2009]

[3] Texas Instruments, “16-Channel Fm+ I2C-Bus Constant-Current LED Sink Driver,” TLC59116 datasheet, Feb. 2008 [Revised Jul. 2008]

[4] Microchip Inc. “20-Pin USB Flash Microcontrollers with nanoWatt XLP Technology,” PIC18F13K50/14K50 datasheet, May 2008 [Revised Apr. 2009]

[5] Freescale Semiconductor. “Low G Micromachined Accelerometer,” MMA1270EG datasheet, April 2007

[6] Holy Stone Enterprise Co., Ltd, “SAW Resonator Transmitter/Receiver Module,” MO-SAWR-AS315M datasheet, Jun. 2004

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