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

cmelnick

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

Lazar Demin

@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)