36.3.3 Start Condition

All I²C transmissions begin with a Start condition. The Start condition is used to synchronize the SCL signals between the host and client devices. The I²C Specification defines a Start condition as a transition of the SDA line from a logic high level (Idle state) to a logic low level (Active state) while the SCL line is at a logic high (see figure below). A Start condition is always generated by the host and is initiated by either writing to the Start (S) bit or by writing to the I²C Transmit Buffer (I2CxTXB) register, depending on the Address Buffer Disable (ABD) bit setting.

When the I²C module is configured in Host mode, module hardware waits until the bus is free (Idle state). Module hardware checks the Bus Free Status (BFRE) bit to ensure the bus is idle before initiating a Start condition. When the BFRE bit is set, the bus is considered idle, and indicates that the SCL and SDA lines have been in a logic high state for the amount of I²C clock cycles as selected by the Bus Free Time Selection (BFRET) bits. When a Start condition is detected on the bus, module hardware clears the BFRE bit, indicating an active bus.