During normal operation the data on SDA must be stable during the time when SCL is High.

There are three unique states for the SMBus:

START: A high-to-low transition on SDA while SCL is High indicates a message START condition.

STOP: A low-to-high transition on SDA while SCL is High indicates a message STOP condition.

IDLE: If SCL and SDA are both High for a time exceeding t_BUF from the last detected STOP condition or if they are High for a total exceeding the maximum specification for t_HIGH then the bus will transfer to the IDLE state.