Protocol Mechanism
A hardware synchronization protocol allows a slower slave device to pause transaction steps on a serial bus by holding the clock line low. By executing asynchronous clock stretching, the slave prevents the master device from sending further data until internal buffers are cleared. The slave subsequently releases the clock line to resume communication once the hardware processing completes.
Timing Distortion
The physical holding of the clock line introduces variable clock periods that differ from the nominal bus frequency. In many scenarios, asynchronous clock stretching alters the expected rise and fall patterns, complicating the timing analysis of the serial interface. This behavior requires the master to continuously sample the clock state, which can degrade overall throughput when processing cycles are extended.
Device Recovery
Slave devices must release the serial clock line within a defined period to prevent permanent bus lockup and recover from state machine exceptions. If asynchronous clock stretching persists indefinitely, the serial bus remains stalled, which prevents communication with other attached peripherals on the shared line. Correct implementation of an independent watchdog timer resolves this condition by resetting the internal state of the slave interface whenever the clock line is held low beyond a specified millisecond threshold, allowing the master to reclaim control.
Interface Boundary
Hardware controllers enforce a maximum limit on how long the bus clock may be held low. When the duration of asynchronous clock stretching exceeds this threshold, the transaction is aborted and a bus error is flagged. Designers must verify this limit against worst-case slave response times under maximum system load.
