Design Project - Purdue University



Homework 6: Printed Circuit Board Layout Design Narrative

Due: Friday, February 22, at NOON

Team Code Name: Touch 2 Order Group No. 13

Team Member Completing This Homework: Srichand Yella

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

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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:

Comments from the grader will be inserted here.

1. Introduction

The ‘Touch 2 Order’ is wirelessly integrated menu with a touch screen display made especially for use in restaurants. The user can select food items from the touch screen interfaced menu and pay the bill using the RFID card. When finished paying the bill the order is sent wirelessly to the kitchen server where is added to the order queue.

The major components for the device are the touch screen with LCD graphical controller, RFID reader, ZigBee wireless transceiver, DC-DC voltage converter and the 16-bit Freescale 9S12CE128 microcontroller. But the components mounting on the device would be ZigBee wireless transceiver, DC-DC voltage converter and the microcontroller. The PCB will also have headers for serial interface, power and general purpose I/O’s. The LCD graphical controller and the RFID reader are connected through the serial interfaced headers since they should be available for the user. All these parts are designed on the PCB efficiently taking the physical restrictions of the components and also the overall device into consideration.

2. PCB Layout Design Considerations – Overall

PCB Layout design involves many factors to be taken into consideration for it to be fully operational. One of the most important factors is noise reduction in the circuit and next is the efficient placement of the components on the PCB. The unwanted noise in the circuit can be reduced by placing decoupling capacitors in every design which will be either a low-inductance axial glass or of a multi-layer ceramic.

As said above next important factor is the placement of various components on the PCB. There is a need to separate the components on the basis of whether they are digital, analog or noisy circuits. The circuits involving the DC-DC voltage regulators for 5 V and 3.3 V and the power input are grouped together to one corner of the PCB. The microcontroller is connected to headers on all sides and is connected to other digital circuitry such as the XBee Pro wireless transceiver through the headers. Another important requirement for the successful functioning of a PCB is placing the headers or input pins on the edges of the board. This includes the headers for serial interface of the RFID reader and touch screen with LCD graphical controller and for the power connection. The overall PCB should also be packaged so that it is in right contact with the components connected which are not present on the PCB.

One more important factor that should be taken into consideration while designing a PCB is the tracing of the connections in the circuitry of the device. It is important to minimize the lengths of the trace connections to avoid any stray inductance and any interference with the rest of the circuitry. The width of the trace should also be considered. It should be well enough to handle the current flowing through it. While routing, right angle turns are not at all advisable as it may lead to the transmission of the reflections. It is also important to separate digital and analog trace lines. The high frequency lines should also be at a minimum distance to avoid any unwanted noise in the circuit. The trace lines of power and ground would also be running parallel to each other which reduce the noise to a larger extent.

3. PCB Layout Design Considerations – Microcontroller

For the proper functioning of the microcontroller in the PCB Layout the designing should be done with extra precision. The onboard voltage regulator for the microcontroller requires decoupling capacitors, central point grounding and low inductance connections between VSS1, VSS2, and VSSR. As shown in Figure 3.1 the decoupling capacitors should be placed meeting the requirements and the central point of ground should be connected to the VSSR pin.

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Figure 3.1 Placement of the Decoupling capacitors with respect to the microcontroller

The 9S12CE128 Microcontroller requires a external crystal circuit to control the internal clock generator. This circuitry will be separated from the rest to avoid any unnecessary contact with power and ground. The VSSPL, EXTAL and XTAL connection will be as short as possible. The power and ground traces would be running parallel to each other thereby reducing the possibility of production of any kind of noise from the power pins of the rest of the components of the circuit. This kind of unwanted noise may also affect the oscillator and its output. The VSSA and VDDA pins are connected to the central power for the microcontroller.

It is not safe to have traces running beneath the microcontroller or any other circuitry involved with it such as the decoupling capacitors or the external crystal as they are very sensitive. The pins of the microcontroller are connected to headers on all four sides. So the connections from the microcontroller would be leaving from those headers instead of directly leaving from the microcontroller.

4. PCB Layout Design Considerations - Power Supply

The main components for power supply that go onto the PCB include DC – DC linear converters, external voltage input port and a power header. All these components are placed on the top-left corner of the PCB to isolate any noise that they produce. The external voltage input port and the power header pins are mounted to the left edge of the board for easy connectivity from outside.

The DC – DC linear converters have an external circuit requirement on the PCB. Sufficient room is planned for these components on the PCB.

All the power components are being planned to be placed on the top side of the PCB. The power trace should be wide enough to carry enough current for the system. The ground trace should be wide to reduce any interference between components. The ground trace is planned to be as long as possible so that there is minimal interference across all the components. Both the traces will be run parallel to each other which will reduce the interference to minimum. All the ground traces will be grounded at a single point. The figure 4.1 below shows the DC – DC linear converter, LM 2679, pins.

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Figure 4.1: LM2679 DC – DC linear converter

VIN0, CB, Current Adj., FB, VSW0, GND and Softstart are the only pins that are used in building the circuit for DC – DC converters. Since all the pins are analog, there are no restrictions on routing them on the PCB, but they will be provided with adequate spacing so that there is no interference among the signals.

5. Summary

The ‘Touch 2 Order’ requires a fully functional PCB with all kinds of constraints considered for its operation. All these aspects were taken care of to produce a fully functional PCB Layout. Some of the main constraints were appropriate placement of the major components and headers for the rest of the components, headers for power and general purpose I/O’s including wiring considerations of the circuitry. By ensuring proper functionality of the PCB the overall circuit of the device would be perfectly operational.

List of References

[1] XBee RF module data sheet



[2] Freescale 16-Bit Microcontroller Data sheet



[3] Reach Technology touch screen SLCD5



[4] Sparkfun RFID data sheet ID-12



[5] Sparkfun RFID LF tags



[6] National Semiconductor DC to DC convertor



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NOTE: 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 20% of the individual component of the team member’s grade. The body of the report should be 3-5 pages, not including this cover sheet, references, attachments or appendices.

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