26.3.2.6 Clock and Clock Stretching
All devices connected to the bus are allowed to stretch the low period of the clock to slow down the overall clock frequency or to insert Wait states while processing data. A device that needs to stretch the clock can do this by holding/forcing the SCL line low after it detects a low level on the line.
Three types of clock stretching can be defined, as shown in Figure 26-9.
If a slave device is in Sleep mode and a Start condition is detected, the clock stretching normally works during the wake-up period. For AVR devices, the clock stretching will be either directly before or after the ACK/NACK bit, as AVR devices do not need to wake-up for transactions that are not addressed to it.
A slave device can slow down the bus frequency by stretching the clock periodically on a bit level. This allows the slave to run at a lower system clock frequency. However, the overall performance of the bus will be reduced accordingly. Both the master and slave device can randomly stretch the clock on a byte level basis before and after the ACK/NACK bit. This provides time to process incoming or prepare outgoing data or perform other time-critical tasks.
In the case where the slave is stretching the clock, the master will be forced into a Wait state until the slave is ready, and vice versa.