Homework 6: Printed Circuit Board Layout Design Narrative Due: Friday, February 27, at NOON

Transcription

1 Homework 6: Printed Circuit Board Layout Design Narrative Due: Friday, February 27, at NOON Team Code Name: _Magic Wand Group No. 5 Team Member Completing This Homework: Michelle Zhang Address of Team Member: purdue.edu 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 body of the report should be 3-5 pages, not including this cover sheet, references, attachments or appendices. Evaluation: SCORE DESCRIPTION Excellent among the best papers submitted for this assignment. Very few 10 corrections needed for version submitted in Final Report. Very good all requirements aptly met. Minor additions/corrections needed for 9 version submitted in Final Report. Good all requirements considered and addressed. Several noteworthy 8 additions/corrections needed for version submitted in Final Report. Average all requirements basically met, but some revisions in content should 7 be made for the version submitted in the Final Report. Marginal all requirements met at a nominal level. Significant revisions in 6 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:

2 1.0 Introduction The Magic Wand is a hand writing recognition tool that uses acceleration to detect pen motion and displays the interpreted character to an LCD. The Magic Wand will consist of two main sections: the external attachment to a pen and the base station that analyze the acceleration data and displays the result. The pen attachment contains a PIC18LF2320 microcontroller, a MRF24J40MA transceiver, and an ADXL330 accelerometer. These devices work together to gather pen movement data and transmit the information to the base station for analysis. The base station uses a dspic33fj128gp204 microcontroller along with a MRF24J40MA transceiver to receive the transmitted acceleration data from the pen. The pen is constantly monitoring the incoming data to look for patterns related to character recognition. When a character is recognized, it is displayed on a LCD. 2.0 PCB Layout Design Considerations Overall While designing the PCBs, the major technical issue is the physical size of the PCB for the pen. In order to create a moderately user friendly and aesthetically pleasing device, the PCB on the pen needs to be narrow enough to allow a user to write and short enough as to not extended off the back of the writing device. The PCB for the base station is a simplistic design with little emphasis on special considerations and more emphasis on proper routing. The pen and the base station both use a wireless transceiver that require that no traces be routed underneath certain parts of the module and no metal devices be located within a certain region of the antenna. These constraints have forced the transceivers to be mounted on the edges of the PCBs: on the top of the pen PCB and on the side of the base station PCB. The only analog signals in the overall design reside on the pen, and come from the accelerometer. There are three analog signals for the three sensing directions of the accelerometer. The accelerometer itself is to be placed near the bottom of the PCB simply to eliminate extra movement that would be detected if the accelerometer were mounted up higher on the pen (due to an individual s typical writing style). With the accelerometer on the bottom of the PCB, the microcontroller was moved down towards the accelerometer to shorten the traces and thus eliminate the noise that would be present on longer analog signal traces. -1-

3 The standard width of power and ground traces on the pen and base station PCB will be 50 mills in width. The recommended width of 50 mills will be adequate for both traces due to the small power requirements of both boards. See the table below for the estimated current consumption [1 5]. Component Part #. Current Consumption Operating Voltage (V) Description (ma) Transceiver MRF24J40MA Microcontroller(pen) PIC18LF Microcontroller(base dspic33fj128gp station) Accelerometer ADXL LCD Display LCD16x4BL 4 5 Table 2.1 Current Consumption and Operating Voltage of major components 3.0 PCB Layout Design Considerations - Microcontroller The microcontroller on the pen is a Microchip PIC18LF2320. In order to reduce noise, the microcontroller is placed closer to the accelerometer with analog signals so the trace length between the two is minimized. Ideally, the decoupling capacitor for the microcontroller will be placed right next to the device itself on the same board layer. However, the physical size of the capacitor limits the amount of space near the microcontroller for other traces and components. As a compromise, the decoupling capacitor is placed underneath the device on the bottom layer and is connected to the pin using a via. The microcontroller on the base station is a Microchip dspic33fj128gp204. The size limitation on the pen does not apply to the base station. Bypass capacitors have enough room to be placed right next to the device on the same layer without using any via. There are no analog devices on the base station, so the microcontroller is placed right next to the transceiver and LCD header on the left side of the board. All power management related components are grouped together on the right side of the board. 4.0 PCB Layout Design Considerations - Power Supply The pen is powered using two AA batteries. All major components on the pen have an input voltage of 3.3V. The power management components are grouped together and located in the center of the board away from the analog device to reduce noise. The positive and negative -2-

4 terminals, from the battery, are wired to the board through 1 pin headers. The current passes through a 0.1 ohm current sensing resistor (used by the fuel gauge) and goes to the boost [6]. The boost outputs 3.3V [7] and powers the fuel gauge. A 50 mils power rail is on top of the board to supply the input voltage of all the major components. All power and ground traces are placed first since they are the most important and difficult to deal with. As the power traces travel down to the microcontroller and the accelerometer, the traces are running at 15 mils and 12 mils respectively due to the smaller size of the pads. All other signal traces are running at 12 mils as suggested by the fabrication house. The base station is powered using either a 3.7V Li-Ion battery or 5V unregulated voltage coming from the wall wart. The current from the battery goes through a 0.1 ohms current sensing resistor [6] and a Schottky diode that acts as a switch between battery and wall wart. When plugged in, a 5V unregulated voltage that comes from the wall wart is passed through a linear voltage regulator that steps it down to 4V [8] and powers the battery charger before passing through another Schottky diode. The two anodes of the two Schottky diodes are connected, and they direct the current to a 5V boost and a 3.3V buck/boost connected in parallel [7, 9]. The outputs of both converters are connected to inversely biased Zener diodes to help attenuate the voltage peaks generated by the switching actions inside the chips. The LCD is powered at 5V, and it is placed right beneath the 5V boost [7], a 50 mils trace could run almost straight down to the LCD header, as illustrated in Figure 1.1. All other components are powered at 3.3V, a power rail of 50 mils will run on the top of the board to supply the input voltage, as illustrated in Figure Summary In conclusion, the magic wand will have two PCB boards: one small and narrow 4.1 in. x 1.2 in. board for the pen and another 4.1 in. x 1.5 in. board for the base station. With noise reduction, analog and digital components separation, proper trace routing and bypass capacitor placement all taken into consideration, the resulting PCB board should be reliable and manageable. ALL TEAM MEMBERS should read Motorola Application Note AN1259 (posted on course web site) before you begin your PCB layout. -3-

5 List of References [1] Microchip, MRF24J40MA Data Sheet, [Online Document], Available: [cited: February 26, 2009] [2] Microchip, PIC18F2220/2320/4220/4320 Datasheet, [Online Document], Available: [cited: February 26, 2009] [3] Microchip, dspic33fj32gp302/304, dspic33fj64gpx02/x04, and dspic33fj128gpx02/x04 Data Sheet, [Online Document], Available: [cited: February 26, 2009] [4] Analog Devices, ADXL330 Data Sheet Rev A, 09/2006, [Online Document], Available: [cited: February 26, 2009] [5] Futurlec, LED Display LCD16x4BL, [Online Document], Available: [cited: February 26, 2009] [6] Linear Technology, LTC4150 Coulomb Counter / Battery Gas Gauge, [Online Document], Available: 4,D1556 [cited: February 26, 2009] [7] Linear Technology, LT1302/LT Micropower High Output Current Step-Up Adjustable and Fixed 5V DC/DC Converters, [Online Document], Available: 60,P1029,D3678 [cited: February 26, 2009] [8] Texas Instruments, Fast-Transient Response 3-A Low Dropout Voltage Regulators, [Online Document], Available: [cited: February 26, 2009] [9] Texas instruments, High Efficient Single Inductor Buck-Boost Converter w/1-a Switches, [Online Document], Available: [cited: February 26, 2009] 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. -4-

6 Appendix A: LT V boost P o w e r LCD header Figure 1.1 Base Station 5V Power Rail 3.3 V Power Rail TPS Figures 1.1 Base Station 3.3 V Power Rail -5-

7 3.3 V Boost 5V Boost Microchip Transceiver dspic uc Battery Charger Voltage Regulator Fuel Gauge ICD 2 Header LCD header Battery Figure 1.3 Base Station Layout Transceiver 3.3V Boost Fuel Gauge Microcontroller accelero meter Battery Figure 1.4 Pen Layout -6-

8 Figure 1.5 Pen semi-routed top layer Figure 1.6 Pen semi-routed bottom layer -7-