Starter Guide

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PCF8574 pin information
IIC communication protocol
Two bus lines, one data line SDA and one clock line SCL.
The IIC bus has three types of signals in the process of transmitting data:
start signal, end signal, and response signal.
Start signal: When SCL is high, SDA transfers from high level to low level
and starts transmitting data.
End signal: When SCL is high, SDA transfers from low level to high level and
ends transmitting data.