Raspberry Pi Analog to Digital Conversion Experiments and Howto

Software Support

I wrote Python functions to support the multiplexed SPI ADC devices using RPi.GPIO and SPIDEV, however any software that has support for the MCP3208 can easily be modified to support the multiplexer.

SPIMux / Analog Data Acquisition @ https://Mikronauts.com

(Top view of the data acquisition board – click on the image for a larger version)

Simply select the SPI device 0-7 by setting GPIO’s 23,24,25 to the binary value of 0-7 (GPIO23 is the LSB, GPIO25 is the MSB) before communicating with the selected SPI device.

The code below selects SPI device 1 on /CE1, and reads all eight channels every 15 seconds. This ADC is configured for 0-8.192V in 2mV steps with voltage dividers.

I made the following connections:

Channel 0: Pi’s 3.3V supply from the Pi GPIO header

Channel 1: not used

Channel 2: 4.096 voltage reference

Channel 3: Pi’s 5V supply from the GPIO header

Channel 4: not used

Channel 5: Sharp 4cm-30cm distance sensor

Channel 6: not used

Channel 7: SirMorph short distance IR sensor

CSV creation sample code:

    spi_device(1)
    mul=0.002

    csv = time.asctime()  # start each csv line with a time stamp

    for chan in range(0,8):
        x = readadc(chan)
        csv = csv + "," + str(x*mul)
        print chan, x*mul

    csv = csv + "\n"

    f = open("csv.txt","a")
    f.write(csv)
    f.close()

(Click on analogmux.py to download the source code)

I ran some timing tests, and the Raspberry Pi Model B was able to read approximately 10,000 samples per second using the code above (not printing or saving the values, just reading them).

Results

SPIMux / Analog Data Acquisition @ https://Mikronauts.com

(Bottom of Data Acquisition board – click on the image for a larger version)

Here is a sample of the CSV file produced by analogmux.py:

Fri Mar 20 12:37:09 2015,3.312,0.006,4.096,4.906,0.006,0.318,0.006,0.022
Fri Mar 20 12:37:27 2015,3.312,0.006,4.096,4.906,0.006,0.164,0.006,0.02
Fri Mar 20 12:37:44 2015,3.312,0.006,4.096,4.896,0.006,0.24,0.006,0.02
Fri Mar 20 12:38:01 2015,3.312,0.006,4.096,4.906,0.006,0.24,0.006,0.02
Fri Mar 20 12:38:17 2015,3.312,0.006,4.096,4.902,0.006,0.238,0.006,0.022
Fri Mar 20 12:38:33 2015,3.312,0.006,4.096,4.906,0.006,0.24,0.006,0.02
Fri Mar 20 12:38:49 2015,3.314,0.006,4.096,4.906,0.006,0.162,0.006,0.022
Fri Mar 20 12:39:04 2015,3.314,0.006,4.096,4.902,0.006,0.16,0.006,0.02
Fri Mar 20 12:39:20 2015,3.312,0.006,4.096,4.908,0.006,0.182,0.006,0.022
Fri Mar 20 12:39:36 2015,3.312,0.006,4.096,4.906,0.006,0.182,0.006,0.02
Fri Mar 20 12:39:53 2015,3.312,0.006,4.096,4.904,0.006,0.18,0.006,0.022
Fri Mar 20 12:40:08 2015,3.312,0.006,4.096,4.904,0.006,0.18,0.006,0.022
Fri Mar 20 12:40:26 2015,3.312,0.006,4.096,4.906,0.006,0.162,0.006,0.02
Fri Mar 20 12:40:42 2015,3.312,0.006,4.096,4.872,0.006,0.336,0.006,0.018

I imported the CSV file into LibreOffice Calc, and with a few mouse clicks generated a nice chart of the Sharp distance sensor data that was recorded while I was working on some code:

SPI Multiplexer @ https://Mikronauts.com

(click on the image for a larger version)

Matlab, GNUPlot, SciLab and other software could also be used to generate charts.

Conclusion

I hope you enjoyed this article!

I used the SPI address de-multiplexer for MCP3208 analog to digital converters, but you can actually use the same approach for any mix of eight SPI devices. Just remember to switch to the correct device before communicating with it!

Quite a few of my future articles will feature data gathered by this system, I’ve found it quite handy already, and instead of CSV files, I am planning to save the data in an SQL data base.

Please feel free to post questions about this project on my forums, or on the thread I made for it on the Raspberry Pi forums.

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Article Index:

  1. Introduction, Problems: Conversion Error, Number of Channels, Input Voltage Range, Schematic
  2. Software Support, Sample Code, Results, Conclusion

Pages: 1 2

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