Homework 6: Printed Circuit Board Layout Design Narrative

ECE 477Digital Systems Senior Design ProjectRev 9/12

Homework 6: Printed Circuit Board Layout Design Narrative

Team Code Name: ______Android Street Car______Group No. _ 12___

Team Member Completing This Homework: ____ Xirong Ye______

E-mail Address of Team Member: ______ye26 @ purdue.edu

Evaluation:

SEC

DESCRIPTION

MAX

SCORE

1.0 /

Introduction

5

2.0 /

PCB Layout Design Considerations - Overall

20

3.0 /

PCB Layout Design Considerations - Microcontroller

10

4.0 /

PCB Layout Design Considerations - Power Supply

5.0 /

Summary

6.0 /

List of References

App A /

PCB Layout Top & Bottom Copper Screenshot

App B /

PCB Layout To-Scale Component Side Layout

TOTAL

100

Comments:

Comments from the grader will be inserted here.

1.0Introduction

The Android Street Car is a robotic vehicle which can be controlled by the users with an Android application on the tablet. Video and image feedback in the front side camera, ultra sonic sensor data provided by a Raspberry Pi [3] board over WiFi link, and 360 degrees camera can support the user to capture the video and images.The GPS is installed to let the user to locate the location of the vehicle and able to control it. In order to manage and control the complicated robotic vehicle and able to make it function, we need to build up a "brain" for our robotic system. PCB will be the important role on this area. In our PCB layout design, we will be putting five essential components on our PCB. There are LPC1768 microcontroller, GPS, 5V regulator, 3V regulator[2] , and fuel gauge [1]. Many special considerations are required in order to design PCB layout especially on the microcontroller and power supply area.

This reportwill be discussing our project PCB layout design through three types of design considerations area. There areoverall system, microcontroller and power supply. At the end of the report, we will be demonstrating both PCB Layout To-Scale Component Side Layout and PCB Layout Top & Bottom Copper which are the exact design when our PCB has fabricated.

2.0PCB Layout Design Considerations - Overall

Android Street Car includes a massive digital circuit in order to provide the function that described previously. PCB layout which is "the brain" of the system will be the most important in overall. Therefore, it is crucial to take into all different special considerations into account. In order to assure the PCB for Android Street Car is functioning as what we expect, there are several important special considerations that will be discussed as the following.

First, the size of overall PCB will be the most important since the size of PCB deals with not only each individual components but also chassis. The chassis that we will be using is a 4WD with internal H-bridge and motors. The surface is made by aluminum. The platform for this vehicle is around 120mm by 290mm. Since PCB is the most important, in order to easily receive and sendthe data and information, the PCB will locate in the middle of the chassis’platform. In our vehicle design, we will put two ultra-sonic sensors on the side of two chassis. With the ultra sonic sensor, the PCB size will be reduced some amount in the width. Since we will be putting power supply, GPS and micro controller on our PCB and one Wi-Fi antenna and GPS antenna outside of the PCB, in order to prevent the signal overlapping or signal distracting, while we designing the PCB, with decreasing in the width, we will increasing the length instead. With all those considerations and all different size of each individual component, the size of our PCB will be roughly around 100mm by 160mm on the size part.

The second essential consideration is the placement for each important component. As you can see from the figure for PCB layout as below, we will be putting our microcontroller in the dead center of the PCB layout. The reason why we are doing this is because the microcontroller is "the heart" of the whole system. Therefore, we would like to have all data and signal can be sending, transferring and receiving not just only easily but also with the shortest amount of distance. Next, the header on the right side of the PCB layout and more than half on the bottom side of the PCB layout are the headers for all the external devices such as ultra-sonic sensor, Raspberry Pi [3], motor and H-bridge, GPS antenna and power system. The right side of the PCB will be located closer to the Raspberry Pi since there is a Wi-Fi connector on the raspberry PI. We do not want to let the PCB components to be affected by the Wi-Fi signals especially GPS. Additionally, it is a lot easier to connect all the external device headers from one location rather than coming from all different place of the PCB layout. If so, it will look extremely messy and not efficient. The next important thing is how to locate the remaining essential components on the left side and top side of the PCB layout figure as below. The easiest way without any consideration that we have come up with is to put components together with the order of the types. Based on this rule, we will be putting decoupling capacitor, diode, inductors and five essential components into its area. Since we have the power supply control system, we would like to put fuel gauge [1], 3.3V regulator, the connector of the battery and 5V regulator as close to each other as possible.Another consideration that we have on the placement is that since the power system control is analog system and EMI effect and noisemay affect to other signal, we decided to put GPS as far as we can from all other power supply control system. With those considerations and the placements for the headers, we decide to put the GPS to the left bottom corner of the overall PCB layout and the battery connector, fuel gauge, and 3.3V and 5V regulator will be on the top left corner and top side of the PCB layout design. Fuel gauge will be shared by both regulators, so we will be putting between the two regulators. And, since battery will create the most effect to other electronic components, its placement will locate the furthest side of the PCB layout. The remainingcomponents such as decoupling capacitor, inductor and diode will be separated power supply system control and GPS. It will be locate on the left center side as the figure shows. The reset pushbutton for the overall system will be located between the micro controller and the decoupling capacitor. In order to reduce the noise for each essential component, each component has its own decoupling capacitor which located between Vcc and ground of every essential component. Each decoupling capacitor value is determined based on its distance that will be putting from the components and how big the noise will affect the components. The diodes are located for in front of the 3.3V and 5V regulator in order to limit the direction of the current flow. All the routing and tracing are drawn with 90 degree turns in order to reduce the EMI effect. With all this overall PCB layout special considerations, PCB layout will able to function as what we expected.

3.0PCB Layout Design Considerations - Microcontroller

For this project, mbedlpc1768[4] was chosen as the microcontroller used in the design. The microcontroller will be put near the center of the PCB in consideration of easier wiring and protection. The external oscillator is used which is located under the microcontroller to reduce the noise. It would be sufficient for this project. Power supply bypass capacitors are placed between the microcontroller and the 3.3V regulator as suggested. [2] Since lpc1768 has totally 100 pins available where 70 pins can be used as general purpose I/O pins, some unconnected pins are wired to pin headers for future uses. PWM pins and some of general purpose I/O pins are connected to pin headers for the connection of motor and H-bridge which are put near the right edge of the PCB. There are also pins connected to a reset button which will be able to reset the microcontroller. To make sure the microcontroller can operate at a normal temperature and have good performance, it is put away from regulators. As long as the temperature does not exceed 60℃, the power assumption of microcontroller is under the normal situation.

4.0PCB Layout Design Considerations - Power Supply

Two separate switching regulators are used in this design to provide DC power to the Raspberry Pi [3] and the LPC1768 MCU [4]. Since the Raspberry Pi, or RPi, is not on the customized PCB, the regulated power output is routed to the edge of the PCB and connected to a header to outside the board. As the RPiitself has bulk capacitor and bypass capacitors on its own board, the need of a bulk capacitor and bypass capacitor for the 5V DC output is eliminated. The bulk capacitor and bypass capacitor for the 3.3V power is connected close to the output of the switching regulator to reduce noise. Decoupling capacitors are provided near the IC devices to reduce noises in the circuit.

This design incorporates single-point grounding, as a ground plane is not feasible for this two layer PCB. Both the 5V and 3.3V power supply circuits use the same ground, which comes from the battery supply outside the board. As there is no analog circuit in this PCB design, a separate analog ground is not needed and not provided. The 5V power supply trace, alongwith its ground trace, is routed at the edge to connect to headers as stated above. The 3.3V positive voltage trace and its ground trace are routed around the board to provide power to the MCU at the center and the GPS module at the corner. The two traces are designed to be in most cases in parallel to reduce noise.

5.0Summary

In this report, we have to take different types of special considerations into account while designing the PCB for the robotic vehicle Android Street Car. Those special considerations include the size of the chassis and different size for each individual components, EMI, signal overlapping or interrupting, and special specification for both microcontroller and power supply control system.With those special considerations, the PCB layout does not just simply include major components on it. It includes physical placement of those five essential components, decoupling capacitor and inductor placement, overall PCB connection with the things that are interfacing with, power supply control placement, and overall PCB circuit tracing. Overall, with those considerations and designing layout taking into account, we are assure that our PCB can function as what we expect once it is fabricating out.

6.0 List of References

[1] Maxim Integrated. “MAX1704 Fuel Gauge” 2012.[Online]. Available: [Accessed: Jan 13, 2013]

[2] National Semiconductor. “LM2671 Switching Regulator” 2003.[Online]. Available: [Accessed: Jan 13, 2013]

[3] RaspberryPi.org. “Raspberry Pi Start Guide” 2012. [Online]. Available: [Accessed: Feb 10, 2013]

[4]NXP Semiconductors. “LPC1769/68/67/66/65/64/63 Datasheet” 2012.[Online]. Available:

[Accessed: Feb 8, 2013]

Appendix A: PCB Layout Top & Bottom Copper

Fig.1 Overall PCB Layout

Appendix B: PCB Layout To-Scale Component Side Layout

Fig.2 PCB Layout-GPS

Fig3. PCB Layout-Microcontroller

Fig. 4 PCB Layout-Regulators

Fig.5 PCB Layout-Fuel Gauge

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