Datasheet

203
ATtiny828 [DATASHEET]
8371A–AVR–08/12
Given that the slave acknowledges the address, the master can start receiving data from the slave. There are no
limitations to the number of data packets that can be transferred. The slave transmits the data while the master signals
ACK or NACK after each data byte. The master terminates the transfer with a NACK before issuing a STOP condition.
Figure 84 illustrates a combined transaction. A combined transaction consists of several read and write transactions
separated by a Repeated START conditions (Sr).
Figure 84. Combined Transaction
19.3.7 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 85.
Figure 85. Clock Stretching
If the device is in a sleep mode and a START condition is detected the clock is stretched during the wake-up period for
the device.
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 performing 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.
19.3.8 Arbitration
A master can only start a bus transaction if it has detected that the bus is idle. As the TWI bus is a multi master bus, it is
possible that two devices initiate a transaction at the same time. This results in multiple masters owning the bus
simultaneously. This is solved using an arbitration scheme where the master loses control of the bus if it is not able to
transmit a high level on the SDA line. The masters who lose arbitration must then wait until the bus becomes idle (i.e.
wait for a STOP condition) before attempting to reacquire bus ownership. Slave devices are not involved in the arbitration
procedure.