Start and Stop Conditions

Prior to any transaction on the bus, a START condition needs to be issued on the bus. The start condition acts as a signal to all connected IC's that something is about to be transmitted on the bus. As a result, all connected chips will listen to the bus.

After a message has been completed, a STOP condition is sent. This is the signal for all devices on the bus that the bus is available again (idle). If a chip was accessed and has received data during the last transaction, it will now process this information (if not already processed during the reception of the message).

Start Start waveform The chip issuing the Start condition first pulls the SDA (data) line low, and next pulls the SCL (clock) line low.
Stop Stop waveform The Bus Master first releases the SCL and then the SDA line.

A few notes about start and stop conditions: