Two unique bus conditions are used for marking the beginning (START) and end (STOP) of a transaction. The master issues a START condition (S) by indicating a high-to-low transition on the SDA line while the SCL line is kept high. The master completes the transaction by issuing a STOP condition (P), indicated by a low-to-high transition on the SDA line while SCL line is kept high.
Figure 1. START and STOP Conditions
Multiple START conditions can be issued during a single transaction. A START condition that is not directly following a STOP condition is called a repeated START condition (Sr).