muse Capstone Course: Wireless Sensor Networks

Transcription

1 muse Capstone Course: Wireless Sensor Networks Experiment ADC: Sensing and Analog-to-Digital Conversion Objectives 1. Get familiar with the CLIO wireless sensor node development platform. 2. See how to interface an analog peripheral board to the ez430 on the CLIO. 3. Understand analog-to-digital conversion. 4. Learn how to use an ADC on a microcontroller. What You Will Need CLIO board TI ez430-rf2500 MSP430 Wireless Development Tool with two-row 18-pin female header attached to the bottom and with each of the 18 contacts soldered to the ez430 on the top of the ez430. If you do not have experience soldering, get practice on junk boards, or better yet get help. CLIO-SL light sensor board For Part II: (a) a standard laboratory function generator (a function generator with a high-impedance load option is best); (b) a cable to connect the function generator to the CLIO a BNC-to-alligator cable works well. You will also need wire to connect the ez430 I/O terminals on the CLIO to one of the breakout terminals on the CLIO. You have two options: 1. jumper cables with built in header pin-compatible terminals; one possibility is is Jumper Wires Premium 6 F/F Pack of 10, sku: PRT from sparkfun.com; see info.php?products id= gauge or 28-gauge solid (non-stranded) wire and a wire-wrap tool A USB cable to connect your PC to the ez430 when plugged into the CLIO board. In addition, you will need what you used in Experiment 1: CLIO Quick Start: a computer running Windows (XP or Vista) 1

2 TI s Code Composer Studio Microcontroller integrated development environment (installed as in Experiment 1: CLIO Quick Start). Notes Hardware Mechanical. When the ez430 is connected to the CLIO via the 18-pin header, be careful when the ez430 is connected to the PC via the USB cable the tiny 6-pin connector that connects the ez430 debugging interface (the part with the USB connector) to the target is very fragile. Hardware Electrical Power. You must make sure that you set the power jumper on CLIO to the correct source (either USB or battery) or damage could result. See the box labeled Must disconnect USB cable if CLIO battery enabled. Software. We ve provided a function punadc10 WaveformSample() that configures and controls the ADC10 peripheral on the MSP430. An example call is p_unresults = punadc10_waveformsample(ksample_50, 200, RES_10_BITS); It takes three arguments: 1. Sampling Rate: KSAMPLE xx; see adc10.h in the hardware subdirectory for which rates are selectable. 2. Number of Samples: You can choose the number of samples from 1 to Resolution: The function provides emulation of any resolution from 1 to 10 bits via RES xx BITS; see adc10.h As you can see from the code, punadc10 WaveformSample() returns a pointer to (i.e., the address of) an array where the function has stored the samples. Also, note that this function selects a reference voltage of 2.5 V for the ADC10. In the demo code, look for the line for( unloopcount = 0x0000; unloopcount < 200; unloopcount++) The second number, in this case 200, needs to be the same as the number of samples specified in the call to punadc10 WaveformSample(). Otherwise the samples will not be fully printed to the screen. 2

3 The data from the ez430 using this function is in ADC counts, which will be an integer in the range {0,..., 1023} at full 10-bit resolution. At other resolutions, the function automatically outputs the midpoint (on a scale of 0 to 1023) of the quantization bin. For example, one-bit quantization means there will be two possible output levels: 256 and 768. Procedure Part I: Measuring light with the CLIO and CLIO-SL 1. Plug the CLIO-SL board on the CLIO so that its pin 1 is connected to pin 1 of one of the CLIO I/O headers. On CLIO-SL, pin 1 is marked with a white dot. See Figure 1 for a photo and description of the set-up, and Figure 2 for a schematic of CLIO-SL. Make sure that you have connected all three jumper wires as shown in Figure 1: Analog light signal: the amplified output of the photodetector on CLIO-SL (pin 7 of the CLIO-SL female header) is connected to the the physical pin P2.0/ACLK/A0/OA010 of the MSP430 via the CLIO I/O header pin (pin 3) with the same label. Power (V DD ): Connect pin 1 of the CLIO-SL female header to any Vdd Bus pin on CLIO. Ground: Connect pin 9 of the CLIO-SL female header to any GND Bus pin on CLIO. 2. Start up CCS, open a new workspace, and unzip the project within CCS using the procedure described in Experiment RSSI. In the code, make sure the ADC is configured for a sampling rate of 50 Ks/s, and to take 50 samples at the highest (10-bit) resolution. 3. Place CLIO/CLIO-SL so that a flourescent light source is shining on the photodiode. Note : you may need to adjust the distance to get a good signal. 4. Start up Hyperterminal (or Putty) and compile/download the program to the ez430. The program is designed to take the samples when you hit Enter in Hyperterminal. 5. Run the program on the ez430 target and import the data into Matlab and plot the waveform. 6. Use the simplefft MATLAB code (see Appendix: MATLAB FFT code) to plot the power spectrum of the waveform. Notice that this code plots the spectrum in db relative to frequency bin with the highest power. In your report, include these plots and interpret the results you see in both time and frequency domains, and explain how they relate. 7. (Optional extra credit) Capture samples from sunlight and/or an incandescent light bulb. Using MATLAB, plot and analyze your data. 3

4 Figure 1: CLIO (upper left) and CLIO-SL (lower right) with correct jumpers for measuring light. Note black jumper for analog input to the MSP430 ADC10 from pin 3 of CLIO breakout header to pin 7 of the female header for the CLIO-SL. The red and white jumpers supply VDD and ground respectively to CLIOSL. Since CLIO is connected to the PC via USB, the the power jumper at center connects the two left-hand side pins of the central three-pin power connector. (Ignore resistors and wires in CLIO prototyping area). 4

5 Figure 2: Schematic for CLIO-SL light sensor board. Note pin numbering on HDR1 in upper right. 5

6 Part II: Understanding Quantization 1. Set up the function generator. Do the following in the given order. Set up the function generator for a high-impedance load. It is very important to set up the function generator to expect a high impedance load resistance. If you do not do this, most function generators default to expect 50 Ω load resistance and will incorrectly set the output voltage. See Appendix: Setting Up the Function Generator for an example. Set the amplitude to 2.5 V peak-to-peak. Set the DC offset to 1.25 V. Set the frequency to 5 KHz. Check that the settings are correct (use an oscilloscope if available) to prevent damaging the MSP Connect the function generator to the CLIO. (You will not need the CLIO-SL board.) A good method is to use a BNC-to-alligator cable to connect the function generator output to the ADC input pin and CLIO ground. 3. Set up the ADC for a sampling rate of 50 Ks/s, and to take 200 samples at the highest (10-bit) resolution. As in Part I, collect the data and plot time- and frequency-domain waveforms, and make sure you understand the causes of what you see you will include your plots, and describe and interpret this data in your report. 4. Repeat the previous step for resolutions of 1, 4, and 8 bits. How does the data look when compared with the full-resolution (10-bit) quantization? In your report, discuss the effects of A/D resolution in the time and frequency domains. Answer the following question: Do the effects of quantization that you see appear to be noise-like? Why or why not? What to Hand In, and When Hand in: a report that succinctly describes your work. Make sure that your plots have clearly labeled axes and descriptive captions. Include them as numbered figures and reference them in your report. Due date: Ask your instructor. For More Information 6

7 Helpful documentation for this and other CLIO projects: ez430-rf2500 Development Tool User s Guide CLIO and CLIO-SL schematics and layouts Appendix: Setting Up the Function Generator If you have an HP/Agilent 33120A function/arbitrary waveform generator, use the following steps (check the instruction manual if you have another manufacturer/model): 1. Power up the function generator without any connections. 2. Press the blue Shift Key, then the Menu Key right above it. 3. Press the Right Arrow Key until the display reads D: SYS MENU (should be 3 presses) 4. Press the Down Arrow Key until the display flashes PARAMETERS and then reads 50 OHM (should be 2 presses) 5. Press the Right Arrow Key and the display should change to HIGH Z (should be 1 press) 6. Press the Enter Key, the display should flash ENTERED and the device should return to normal mode. Note: If your function generator does not have a high-impedance load option, you can place a 50 Ω resistance in parallel with the ADC input on the CLIO. You can use a 47 or 51 Ω resistor or two 100 Ω resistors in parallel. Because the ADC input has a much higher impedance, the total impedance seen by the function generator will be approximately 50 Ω. This approach was tested using two 100 Ω resistors in parallel on the prototyping area of the CLIO board (Figure 1). Appendix: MATLAB FFT code function simplefft(samplerate, data) simplefft(samplerate, data) This function serves as a simple wrapper to MATLAB s fft function. samplerate - This is the sampling rate for data in samples/sec. Thus 50KSamp/sec would be 50E+3 data - vector containing the ADC samples taken from the ez430 7

8 Code orginally from a demo application provided by Dr. Paul Flikkema. Modified on 6 November 2009 by Kenji Yamamoto delta = 1/sampleRate; Totsamp = length(data); vector of time samples t = (0:Totsamp-1)*delta; plot(t,data); xlabel( Time (s) ); ylabel( ADC Result ); title( Sampled Waveform ); nf = 0:Totsamp/2; Nf = Totsamp/2; zf = (1/length(data))*fft(data); spec = zf.* conj(zf); spec = spec(1:nf+1); high = max(spec); logspec = 10*log(spec/high); scale = samplerate*(1/totsamp)*nf; figure; plot(scale,logspec, -xb ); axis([0 scale(length(scale)) ]) xlabel( Frequency (Hz) ); ylabel( Power (dbc) ); title( Signal Power Spectrum ) stop 8