Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 229
The upper seven bits are the address to which the two-wire Serial Interface will respond when addressed by a
Master. If the LSB is set, the TWI will respond to the general call address (0x00), otherwise it will ignore the
general call address.
TWEN must be written to one to enable the TWI. The TWEA bit must be written to one to enable the
acknowledgement of the device’s own slave address or the general call address. TWSTA and TWSTO must be
written to zero.
When TWAR and TWCR 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. The appropriate action to be taken for each status code is detailed in
Table 21-5 on page 230. The Slave Receiver mode may also be entered if arbitration is lost while the TWI is in
the Master mode (see states 0x68 and 0x78).
If the 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 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 it to one). 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.
Note that the two-wire Serial Interface Data Register – TWDR does not reflect the last byte present on the bus
when waking up from these Sleep modes.
TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
value
0 100010 X