26.7.4 Slave Receiver Mode

In the Slave Receiver (SR) mode, a number of data bytes are received from a master transmitter (see figure below). All the status codes mentioned in this section assume that the prescaler bits are zero or are masked to zero.

To initiate the SR mode, the TWI (Slave) Address Register n (TWARn) and the TWI Control Register n (TWCRn) must be initialized as follows:

The upper seven bits of TWARn are the address to which the two-wire serial interface will respond when addressed by a master (TWARn.TWA[6:0]). If the LSB of TWARn is written to TWARn.TWGCI=1, the TWI n will respond to the general call address (0x00), otherwise, it will ignore the general call address.

TWCRn must hold a value of the type TWCRn=0100010x - TWCRn.TWEN must be written to '1' to enable the TWI. TWCRn.TWEA bit must be written to '1' to enable the acknowledgment of the device’s own slave address or the general call address. TWCRn.TWSTA and TWSTO must be written to zero.

When TWARn and TWCRn have been initialized, the TWI waits until it is addressed by its own slave address (or the general call address, if enabled) followed by the data direction bit. If the direction bit is '0' (write), the TWI will operate in SR mode, otherwise, ST mode is entered. After its own slave address and the write bit have been received, the TWINT flag is set and a valid status code can be read from TWSR. The status code is used to determine the appropriate software action, as detailed in the table below. The SR mode may be entered if arbitration is lost while the TWI is in the Master mode (see states 0x68 and 0x78).

If the TWCRn.TWEA bit is reset during a transfer, the TWI will return a "Not Acknowledge" ('1') to SDA after the next received data byte. This can be used to indicate that the slave is not able to receive any more bytes. While TWEA is zero, the TWI does not acknowledge its own slave address. However, the two-wire serial bus is still monitored and address recognition may resume at any time by setting TWEA. This implies that the TWEA bit may be used to temporarily isolate the TWI from the two-wire serial bus.

In all sleep modes other than the Idle mode, the clock system to the TWI is turned off. If the TWEA bit is set, the interface can still acknowledge its own slave address or the general call address by using the two-wire serial bus clock as a clock source. The part will then wake up from sleep and the TWI will hold the SCL clock low during the wake-up and until the TWINT flag is cleared (by writing '1' to it). Further data reception will be carried out as normal, with the AVR clocks running as normal. Observe that if the AVR is set up with a long start-up time, the SCL line may be held low for a long time, blocking other data transmissions.