I2C sda data line curious developer
2232 Views - Created 14/03/2017
The 0x12 would be the register address for GPIOA which is the first port on the MCP23017 expander chip. When writing to the MCP23017 you normally have to send three bytes, the address of the chip, the register to access and the value to write.
If you look in the python library you can see there is a list of the different registers that are available on the MCP23017 along with their function.
The datasheet for the MCP23017 goes into more detail about how the I2C communication works and what each function does.
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