Bus Collision During a Start Condition

During a Start condition, a bus collision occurs if:

  1. 1.SDA or SCL are sampled low at the beginning of the Start condition (see Figure 1).
  2. 2.SCL is sampled low before SDA is asserted low (see Figure 2).

During a Start condition, both the SDA and the SCL pins are monitored.

If the SDA pin is already low, or the SCL pin is already low, then all of the following occur:

Figure 1. Bus Collision During Start Condition (SDA Only)

The Start condition begins with the SDA and SCL pins deasserted. When the SDA pin is sampled high, the Baud Rate Generator is loaded and counts down. If the SCL pin is sampled low while SDA is high, a bus collision occurs because it is assumed that another host is attempting to drive a data ‘1’ during the Start condition.

Figure 2. Bus Collision During Start Condition (SCL = 0)

If the SDA pin is sampled low during this count, the BRG is reset and the SDA line is asserted early (see Figure 3). If, however, a ‘1’ is sampled on the SDA pin, the SDA pin is asserted low at the end of the BRG count. The Baud Rate Generator is then reloaded and counts down to zero; if the SCL pin is sampled as ‘0’ during this time, a bus collision does not occur. At the end of the BRG count, the SCL pin is asserted low.

Figure 3. BRG Reset Due to SDA Arbitration During Start Condition
Important: The reason that a bus collision is not a factor during a Start condition is that no two bus hosts can assert a Start condition at the exact same time. Therefore, one host will always assert SDA before the other. This condition does not cause a bus collision because the two hosts must be allowed to arbitrate the first address following the Start condition. If the address is the same, arbitration must be allowed to continue into the data portion, Repeated Start or Stop conditions.