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Having trouble reading value from I2C device with Python libs (OmegaExpansion / OnionI2C)



  • Hello, I'm working on writing a driver for the Adafruit MPR121 capacitive touch sensor using the Onion python i2c libraries (OmegaExpansion onionI2C). For some reason that I haven't been able to figure out yet, reading from a register address always returns 0x00. There's a sequence of register writes to reset the device, which results in a specific value 0x24 stored in one of the registers. I stripped all my code down to the bare minimum, but no luck calling this from python. The command line tools i2cdetect i2cget i2cset work correctly, so I know the breakout itself is working.

    • Using the command line tools
    > i2cdetect -y 0
         0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
    00:          -- -- -- -- -- -- -- -- -- -- -- -- --
    10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
    20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
    30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
    40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
    50: -- -- -- -- -- -- -- -- -- -- 5a -- -- -- -- --
    60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
    70: -- -- -- -- -- -- -- --
    
    > i2cset -y 0 0x5a 0x5e 0x00
    > i2cset -y 0 0x5a 0x80 0x63
    > i2cset -y 0 0x5a 0x5e 0x8f
    > i2cset -y 0 0x5a 0x5e 0x00
    > i2cget -y 0 0x5a 0x5d
    0x24
    
    • My code using the python onionI2C library
    from OmegaExpansion import onionI2C
    
    i2c = onionI2C.OnionI2C()
    i2c.setVerbosity(2)
    
    i2c.writeByte(0x5a, 0x5e, 0x00)
    i2c.writeByte(0x5a, 0x80, 0x63)
    i2c.writeByte(0x5a, 0x5e, 0x8f)
    i2c.writeByte(0x5a, 0x5e, 0x00)
    
    i2c.readBytes(0x5a, 0x5d, 1)
    

    ...and the output

    > python touchTest.py
    onion-i2c:: writing buffer:
    	buffer[0]: 0x5e
    	buffer[1]: 0x00
    onion-i2c:: writing buffer:
    	buffer[0]: 0x80
    	buffer[1]: 0x63
    onion-i2c:: writing buffer:
    	buffer[0]: 0x5e
    	buffer[1]: 0x8f
    onion-i2c:: writing buffer:
    	buffer[0]: 0x5e
    	buffer[1]: 0x00
    onion-i2c:: Reading 1 byte from device 0x5a: addr = 0x5d	read 1 byte, value: 0x00
    

    I'm pretty stumped and not sure what else to troubleshoot. Any ideas from the community on other directions I can take? Thanks in advance for any help


  • administrators

    @cmelnick It's possible the onion-i2c:: debug readings aren't 100% correct.

    Can you try changing your code when you read to this:

    val = i2c.readBytes(0x5a, 0x5d, 1)
    print(val)
    

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