ECE 477 Digital Systems Senior Design Project Rev 8/09
Homework 3: Design Constraint Analysis and Component Selection Rationale
Team Code Name: ______Big Brother______Group No. ___7___
Team Member Completing This Homework: ______Russell Willmot______
E-mail Address of Team Member: rwillmot @ 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:
Comments from the grader will be inserted here.
1.0 Introduction
PSSC’s:
1. An ability to display a pattern with rotating LEDs
2. An ability to control the direction in which the pattern is being displayed
3. An ability to track an RF beacon
4. An ability to change the pattern while the machine is in operation
5. An ability accept user-generated text to display
The basic operation of the Persistence of Vision machine is as follows. The user will carry a battery-powered transmitter which emits a signal at one frequency (1.2 GHz). Multiple antennas spaced one quarter wavelength apart will receive this tone, and the relative phase between the antennas will change depending on the direction of arrival of the incoming signal. By measuring this phase difference, the location of the transmitter can be calculated. A microcontroller (micro #1) will be on a stationary PCB and will take this information, as well as input from 5 pushbuttons and generate a pixel map of the image to be displayed on the LEDs. The micro will also interface with an OLED screen for user feedback.
The pixel map will be sent to an upper rotating PCB with a 2.4 GHz RF data link. A second microcontroller (micro #2) will constantly be taking the data from the 2.4 GHz receiver and store it in memory. An 8-bit data bus will connect this microcontroller to a third one (micro #3). Micro #3 will be connected to an IR sensor which will be used for speed feedback as well as a position reference for the upper board. To display an image, the third micro will send one column of data to shift registers, which control BJTs to drive the 32 RGB LEDs on the LED “post.” During its downtime, micro #3 will request the new pixel map data from micro #2.
2.0 Design Constraint Analysis
There are several things that need to be addressed in order to develop a working design. For this specific project, a thorough timing analysis is crucial. To display a visible image, signals need to be sent to the LEDs at precise times.
Each device that handles the image data needs to be able to support the necessary data rates per the timing analysis, and must have sufficient memory to hold the pixel maps. Proper interfacing and communication between these devices will allow for the data to be moved easily. Certain on-chip peripherals will be needed to interface with certain devices.
The upper spinning PCB will need to be battery-powered, so an analysis of the voltage and power requirements of the upper board will be needed to ensure that the device will be able to operate for a decent amount of time. Packaging is also crucial for the upper board because the weight of the devices on this board need to be placed in such a way that vibration is minimized.
Finally, since there is a large RF component to our project, link budget analyses need to be performed to ensure that the receivers will be able to receive the transmitted signals.
2.1 Computation Requirements
Each of the 3 microcontrollers is dedicated to performing specific computational tasks. The lower stationary board is required to generate a pixel map based on the user specified text or an image such as the time, temperature, Purdue Logo, etc. The pixel map is then transmitted to the upper board via the RF link. The lower stationary board is also interfaced with the phase detectors and uses the phase detector outputs to find which direction to display the image with the use of a look-up table. It also computes the speed of the motor based on the IR sensor feedback. The lower board also computes the temperature from the thermometer.
Microcontroller #2 is used to receive data (the pixel maps and the direction to display) from the lower board and send the data to Microcontroller #3 using a double buffer technique.
Microcontroller #3 sends the data to the shift registers, which drive the LEDs. This microcontroller performs the timing computations to send the data at the correct time so that the image will be sent in the direction of the RF beacon. Microcontroller #3 is also interfaced with the IR sensor that will allow it to calculate the reference point and the speed at which it should output the data to the shift registers.
2.2 Interface Requirements
Microcontroller 1 requires at least 21 general purpose I/O pins to interface with the OLED screen, the 2.4 GHz transceiver, 5 pushbuttons, 2 phase detectors, a motor, an analog thermometer, and an IR sensor. The OLED and the pushbuttons are included on the evaluation board and do not require any additional interfacing. The analog signals from the thermometer and the 2 phase detectors need to have a range of 0-3.3V or smaller in order to be read correctly by the microcontroller. The motor circuit needs to be optically isolated in order to protect the microcontroller.
Microcontroller 2 requires at least 16 I/O pins to interface with the 2.4 GHz transceiver and to send pixel map data to Microcontroller 3. Micro #3 requires 13 I/O pins to interface with an IR sensor, shift registers, and to receive pixel map data from Micro #2. The shift registers will expand the I/O to drive 32 RGB LEDs. The shift registers must have parallel output.
2.3 On-Chip Peripheral Requirements
This persistence of vision machine will require three microcontrollers. Each microcontroller has different on-chip peripheral requirements. Microcontroller 1 needs three channels of 10-bit ATD for the two phase detectors and the analog thermometer. It also requires a 250 kbps SPI peripheral to transmit wireless data to Microcontroller 2. Microcontroller 2 requires a 250 kbps SPI peripheral (or faster) for gathering data from the 2.4 GHz wireless connection with Microcontroller 1. Microcontroller 3 needs an 8 Mbps SPI to send data to the LEDs. It also requires an internal timer peripheral for keeping track of the LED display offset.
2.4 Off-Chip Peripheral Requirements
Our design does not require the use of any off-chip peripherals.
2.5 Power Constraints
The upper rotating board and lower stationary board each have unique power restraints. The 120VAC from the wall outlet will be stepped-down and rectified to 5VDC using a wall wart. The 5VDC will provide power to the ARM evaluation board and the power circuitry for the motor.
The upper rotating board will be powered using batteries with voltage regulators. The battery power will be regulated by a switch mode power supply to drive the LEDs and a low drop out regulator to source the two upper rotating board microcontrollers. 3.3 VDC will be used for all components on the upper rotating board. The RGB LEDs can potentially require a large amount of current. The design uses 32 RGB LEDs, so effectively there are 96 LEDs that each requires about 20 mA, resulting in a total current requirement of 1.92 Amps if all of the LEDs are on at once. The batteries are rated at 2200 mAH, therefore the batteries can drive the LEDs for about 1 hour if all of the LEDs were running continuously.
2.6 RF Link Budgets
The location detection circuitry will use a transmitter that outputs 10 dBm of power at 1.2 GHz. The transmitter and receiver antennas will have a gain of 5 dBi. Assuming the user is standing 5 meters away from the device, the received power will be approximately -30 dBm. The input sensitivity of the AD8302 phase detector is -60 dBm, a sufficient margin.
The two 2.4 GHz chips for the RF data link can output a maximum power of 0 dBm. Since these two chips will be on the upper and lower PCBs, we can assume that their separation will be no more than 30 cm, approximately. Assuming the on-chip ceramic antennas are 3 dBi, the received power will be -24 dBm. The receiver sensitivity for these chips is -85 dBm.
2.7 Packaging Constraints
This device should be able to sit on top of a table and plug into a normal wall outlet. The transmitter needs to be small enough to fit in someone’s pocket or clip onto a belt. The upper PCB needs to be fixed securely and squarely to the motor shaft so that there is no “wobble” in the displayed image. Also, the weight on the upper spinning board needs to be distributed to reduce vibration as much as possible.
2.8 Cost Constraints
The team determined that the total budget should be no more than $350. The final product will compete with novelty POV machines, table LED clocks with thermometers, and scrolling LED signs. A simple POV kit without a motor or batteries can be purchased for $17.99 from MakerShed.com [1]. A standard LED clock with a thermometer display can be purchased for $13.99 from SourcingMap.com [2]. A scrolling LED display can be purchased for $169.95 from Batteryspace.com [3]. Since this product incorporates all of the features of these competitors in a single package in addition to the unique RF tracking feature, it should be able to sell retail for somewhere between $200 and $300.
3.0 Component Selection Rationale
Microcontroller #1 needs to have the most processing power and the minimum requirements for it are shown in Table 3.1. The A/D requirement is necessary for reading the angle output from the phase detectors. The phase detectors do not have a linear response to phase angle and the 10-bit resolution will allow for a ± 5° resolution of angle detection. The third channel of A/D is for the thermometer and 10-bit resolution is adequate. The SPI rate requirement of 250kbps is defined by the RF transceiver. It needs a 250kbps ± 200ppm SPI data and clock rate input. The 21 I/O pins are necessary to connect to all the peripherals as well as the user input buttons, output to the OLED screen, and connection of IR sensor and motor enable. Micro #1 will accept user input of text to display on the POV screen and it must process the text and generate a 3-bit color 96x32 pixel map of the image. The image will be about 1.2 kbytes and in order to generate the image the color layers must be separated—now 3.6 kbytes. At least a few images will be stored in flash memory for the startup presets, and 36 KB of flash would be adequate for image and program storage.
Both microcontroller options meet the requirements of the POV system, but the ARM Cortex M3 made by Texas Instruments (Luminary Micro) micro has a few distinct features that make it the winner. It has an optional evaluation board that includes an OLED screen, and an integrated Ethernet port with magnetics. It is also programmable with the LabVIEW for ARM software that allows for easy interfacing and has useful tools for generating the images on the OLED and POV display.
Microcontrollers #2 and #3 are both going to be mounted on top of the spinning disc and they are much smaller than µC #1. Their minimum requirements are shown in Table 3.2.
The requirements for microcontrollers #2 and #3 are very similar and instead of the team learning two more micros, the PIC24FJ32 was chosen for both #2 and #3. This micro is fast enough to handle the data throughput and has enough memory to move the pixel maps around as well as control the interrupts and timers for updating the POV display.
4.0 Summary
This persistence of vision machine design requires careful consideration of timing, data throughput, data storage, interfacing, mechanical assembly, and wireless communication channels.
List of References
[1] http://www.makershed.com/ProductDetails.asp?ProductCode=MKAD1
[2] http://www.sourcingmap.com/digital-led-desktop-bedside-alarm-clock-thermometer-slim-design-p-5053.html
[3] http://www.batteryspace.com/4x26multicolorledscrollingdisplayboard-bringgreetinghumoralerttoyouroffice.aspx
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ECE 477 Digital Systems Senior Design Project Fall2008
Appendix A: Parts List Spreadsheet
Vendor / Manufacturer / Part No. / Description / Unit Cost /Qty
/ Total CostMouser / TI / SN74AHC594 / 8-bit serial-in parallel-out shift register with storage register / 0.48 / 12 / 5.76
Mouser / TI / EKT -LM3S8962 / 32-bit ARM microcontroller and eval / 104.42 / 1 / $0 (donated)
Mouser / Fairchild / FMB3906 / PNP BJT / 0.133 / 48 / 6.38
Linear Technology / Linear Technology / LT1765EFE-3.3#PBF / 3A 1.25 MHz Step-dn Converter / 6.89 / 1 / 6.89
Mouser / TI / TPS77833D / 3.3V LDO 750-mA / 2.47 / 2 / 4.94
Mouser / Omron / G5NB-1A4-E-DC5 / 5V coil 3A 120V pcb relay / 1.68 / 1 / 1.68
Mouser / Fairchild / FOD817B3S / Opto-isolator / 0.27 / 1 / 0.27
Digikey / Analog Devices / AD8302ARUZ / RF phase detector / 22.77 / 2 / $0 (samples)
Sparkfun / Nordic /
nRF2401A
/ 2.4 GHz RF transceiver w/ on-chip antenna / 24.95 / 2 / 49.90Office Depot / Energizer / NH15-2450 / AA NiMH 2450 mAH battery (8-pack) / 22.99 / 1 / 22.99
MicrochipDirect / Microchip / PIC24FJ32GA002-I/SO / 16-bit microcontroller / 2.87 / 2 / 5.74
BestBuy / Lasko / 3733 / Box Fan (for the motor) / 15.66 / 1 / 15.66
Mouser / Onsemiconductor / MGSF2N02ELT1G / N Channel Mosfet / 0.28 / 1 / 0.28
Pinecomputer / Pinecomputer / Antenna-1200-5db / 1.2 GHz, 5 dBi receiver antenna / 15.00 / 4 / 60.00
Maxim / Maxim / MAX2754 / 1.2 GHz Oscillator (for transmitter) / 3.78 / 1 / 3.78
Freescale / Freescale / MBC13916 / 1.2 GHz Amplifier (for transmitter) / 0.23 / 1 / 0.23
Microchip Direct / Microchip / TC1047AVNBTR / Linear Analog output temperature sensor / 0.62 / 1 / 0.62
TOTAL / $185.12
Appendix B: Updated Block Diagram