Design Project - Purdue University



Homework 6: Printed Circuit Board Layout Design Narrative

Due: Friday, September 22, at NOON

Team Code Name: __ Wirelessly Integrated Menu System__ ______ Group No. __2___

Team Member Completing This Homework: ________ Ryan Coppa________________

Evaluation:

|Component/Criterion |Score |Multiplier |Points |

|Introduction |0 1 2 3 4 5 6 7 8 9 10 |X 1 | |

|Layout Considerations - Overall |0 1 2 3 4 5 6 7 8 9 10 |X 3 | |

|Layout Considerations - Microcontroller |0 1 2 3 4 5 6 7 8 9 10 |X 2 | |

|Layout Considerations – Power Supply |0 1 2 3 4 5 6 7 8 9 10 |X 2 | |

|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:

1. Introduction

The Wirelessly Integrated Menu System (WIMS) will provide restaurant patrons with a fast and convenient way to place their food and beverage order. The user will be presented with a customized LCD touch-sensitive menu of the restaurant’s offerings. Once the customer’s choices have been made, the order will be sent wirelessly to the kitchen and placed in a queue.

The main components of the WIMS include a microcontroller, an LCD touch panel with a controller board, an embedded wireless adapter, a magnetic card reader, a switch mode DC-DC controller, a battery charging controller, a coulomb-counting fuel gauge, and a battery pack. Only four of these devices will be mounted directly to the PCB. The PCB will also include several interface components: two DB9 headers, multiple general purpose input/output (GPIO) headers, a header for power, two Molex connectors, and a BDM connector. These parts will be arranged and connected in a manner to minimize noise emission and reception whilst taking into account the physical restrictions of the packaging.

2. PCB Layout Design Considerations – Overall

There are many factors that contribute to a successful PCB layout. Two of the general desires are to reduce sources of unwanted noise in the circuit and to efficiently place the components. Reducing noise in the system can be achieved by including decoupling capacitors near every IC in the design. These capacitors will be placed across the power leads on all the ICs, and will be constructed of either low-inductance axial glass or of a multi-layer ceramic [1]. In order to handle the voltage spike that occurs during transistor state change, these capacitors supply the instantaneous current required.

Placement of components on the PCB is dependent upon the type of device to be placed. Components need to be grouped in the following categories: digital, analog, and power. To this effect, all of the power components will be grouped together. These components include the DC-DC switched mode regulator, the power header, the battery charging IC, and the coulomb counting fuel gauge, will be located in the upper left corner of the board, separate from the other components. In addition to grouping the IC’s by type, all input or output devices will be placed near the edge of the board. This consideration includes the RS-232 headers, the Molex connectors, the power header, and the BDM connection. The physical location of the PCB in the final packaging also needs to be taken into account. For example, the Molex connector on the PCB is located near the Molex connector on the SLCD. This placement will allow the use of the shortest possible cable to make the connection, because the overall length of the cable is decreased, less area is vulnerable to accepting interference.

Physical components are not the only elements that need to be examined. The traces play an important role in the proper operation of the circuit. Trace lengths should always be minimized in order to reduce stray inductance that causes interference with other parts of the circuit. Trace widths need to be adequate to handle the current in each trace. Right angle routing must be avoided as this can lead to transmission reflections or potentially inadequate trace widths in corners. Analog and digital traces must be kept separate to prevent interference. Sensitive traces, notably the external oscillator and high frequency traces communicating with the WiPort, must be kept clear of possible noise sources. Common sources of noise include analog input pins and voltage reference pins on the microcontroller. [1]

3. PCB Layout Design Considerations – Microcontroller

To ensure proper operation of the microcontroller, special care must be taken with respect to the PCB layout. The onboard voltage regulator has special requirements, including decoupling capacitors, central point grounding, and low ohmic, low inductance connections between VSS1, VSS2, and VSSR [2]. Table 3.1, taken from the MC9S12E128 data sheet, gives the values that will be used for the decoupling capacitors. Figure 3.1 shows where these capacitors will be placed. As mentioned earlier, the central point of ground will be the VSSR pin [2].

|Component |Purpose |Type |Value |

|C1 |VDD1 filter cap |Ceramic X7R |100-220 nF |

|C2 |VDD2 filter cap |Ceramic X7R |100-220 nF |

|C3 |VDDA filter cap |Ceramic X7R |100 nF |

|C4 |VDDR filter cap |X7R/tantalum |>= 100 nF |

|C5 |VDDPLL filter cap |Ceramic X7R |100 nF |

|C6 |VDDX filter cap |X7R/tantalum |>= 100 nF |

Table 3.1 - Decoupling Capacitors [2]

[pic]

Figure 3.1 – Decoupling Capacitor Placement [1]

The microcontroller requires the use of an external crystal to control the internal clock generator circuitry. An isolated space on the PCB will be designated for the crystal circuitry that will be void of power and ground traces in order to reduce possible interference. Also, the traces of VSSPL, EXTAL, and XTAL will be as short as possible [1] [2]. In addition, the ground signal of the crystal circuit will be connected to the ground pin using the shortest allowable trace, and the power and ground pins of the crystal will be routed directly to the power posts of the PCB [1]. Running the power traces in this manner will reduce the possibility of noise being introduced from the power pins of other devices in the circuit. Unwanted noise could affect the power input of the oscillator which would result in undesirable oscillator output.

No traces will be run underneath the connection area to the microcontroller or underneath the areas occupied by C7, C8, C10 or Q1, due to the sensitive nature of the oscillator circuit [2]. The traces connecting the microcontroller to the rest of the circuit will remain as large as possible, progressively decreasing in width as they near the microcontroller pin in order to reduce overall inductance [1]. Central power to the microcontroller will be run to the VDDA and VSSA pins [2].

4. PCB Layout Design Considerations - Power Supply

The power supply circuit that will be implemented includes a DC-DC switch mode controller, a battery charging IC, a power header, and an external voltage input port. These components will be isolated in the upper-left corner of the PCB to reduce the noise emission to the rest of the circuit. To facilitate easy mounting of the external power connectors, the power header and the external voltage input port will be located near the edge of the board.

The DC-DC switch mode controller, the Maxim MAX8546, and the battery charging IC, the Maxim MAX713, bring their own unique requirements to the PCB layout. These ICs require extensive external operating circuitry, and sufficient room will be provided on the PCB for these layouts [3] [4] [5].

In order to achieve low-switching losses and stable operation from the DC-DC switch mode controller, the following requirements must be met. All of the power components will be mounted on the top side of the board with their ground terminals flush against each other and the power and analog grounds connected as close as possible to pin seven of the MAX8546 [3]. The length of the high-current paths, power traces, and load connections will be as short as possible [3]. The LX and GND connections will be made using Kelvin sense connections in order to guarantee the current-limit accuracy [3]. Kelvin sense connections can be achieved by routing the power on the top side of the PCB and routing the ground on the bottom side of the PCB [3]. When the switching nodes (BST, LX, DH, and DL) are routed, care will be taken to keep them away from sensitive analog areas (COMP and FB). These pins can be identified in Figure 4.0 below.

[pic]

Figure 4.0 – DC-DC Switch Mode Controller Pin out [3]

In order to properly support the decoupling capacitors dispersed throughout the board, a bulk capacitor will be placed across the power and ground leads, as close to the battery terminal as possible. This capacitor will be either tantalum electrolytic or metalized polycarbonate [1]. In parallel with the bulk capacitor is a .1 μF ceramic disk capacitor used to decouple high frequency noise at the terminals [1].

The power and ground traces are key aspects of a successful PCB layout. These traces are the foundation of the entire circuit, and proper design must be ensured. When routing the power and ground traces, it is important to run them parallel to each other; this will make certain that the least amount of interference will be produced. Ideally, one would utilize one of the layers on the PCB as a ground plane; however the WIMS is limited to two layers. Due to these constraints, a grounding technique called single-point grounding will be utilized. This technique requires that all of the ground traces terminate at a single point [1]. The width needs to be sufficiently large to handle the required current draw. The largest source of power draw on the PCB is the backlight inverter that requires 5 W of power. However, the current draw is only 230 mA DC [5]. Following the recommendations set in the module 2 lecture notes, the width of the power and ground traces will be 100 mills [6].

5. Summary

Without a successful design of the printed circuit board, the success of the WIMS will be in jeopardy. Many different aspects of the design must be examined to ensure that the desired result is produced. These requirements include proper placement of the decoupling capacitors, appropriate component locations, and ensuring that power, ground, and signal lines are run in a manner that promotes proper circuit operation.

List of References

1] Motorola, “Motorola Semiconductor Application Note AN1259”, [Online Document], 2005 September, Available HTTP:

2] Freescale, “MC9S12E128 Data Sheet”, [Online Document], 2005 September, Available HTTP:

3] Maxim, “Maxim MAX1967 Data Sheet”, [Online Document], 2005, Available HTTP:

4] Maxim, “Maxim MAX713 Data Sheet”, [Online Document], 2002, Available HTTP:

5] Hitachi, “Hitachi Backlight Inverter Data Sheet”, [Online Document], December 1999, Available HTTP:

6] D.G. Meyer, “Module 2: Digital System Design Considerations and PCB Layout Basics”, [Online Document], 2006, Available HTTP:

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