SPRUHZ7K August 2015 – April 2024 AM5706 , AM5708 , AM5716 , AM5718 , AM5718-HIREL
Under normal conditions, only one master device generates the clock signal, SCL. However, there are two or more master devices during the arbitration procedure, and the clock must be synchronized so that the data output can be compared. The wired-AND property of the clock line means that a device that first generates a low period of the clock line overrules the other devices. At this high/low transition, the clock generators of the other devices are forced to start generation of their own low period. The clock line is then held low by the device with the longest low period, while the other devices that finish their low periods must wait for the clock line to be released before starting their high periods. A synchronized signal on the clock line is thus obtained, where the slowest device determines the length of the low period and the fastest device determines the length of the high period. If a device pulls down the clock line for a longer time, the result is that all clock generators must enter the WAIT-state. In this way a slave can slow down a fast master and the slow device can create enough time to store a received byte or prepare a byte to be transmitted (clock stretching).
In case the SCL or SDA lines are stuck low, a bus clearing operation is supported:
Figure 24-10 shows clock synchronization.