Datasheet

ATmega48A/PA/88A/PA/168A/PA/328/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002061A-page 235
To initiate the Slave Transmitter mode, TWAR and TWCR must be initialized as follows:
The upper seven bits are the address to which the 2-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 “1” (read), the TWI will
operate in ST mode, otherwise SR 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 22-5. The Slave Transmitter mode may also be entered if arbitration is lost while the TWI is in the Master
mode (see state 0xB0).
If the TWEA bit is written to zero during a transfer, the TWI will transmit the last byte of the transfer. State 0xC0
or state 0xC8 will be entered, depending on whether the Master Receiver transmits a NACK or ACK after the
final byte. The TWI is switched to the not addressed Slave mode, and will ignore the Master if it continues the
transfer. Thus the Master Receiver receives all “1” as serial data. State 0xC8 is entered if the Master demands
additional data bytes (by transmitting ACK), even though the Slave has transmitted the last byte (TWEA zero
and expecting NACK from the Master).
While TWEA is zero, the TWI does not respond to its own slave address. However, the 2-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 2-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 2-wire Serial Bus
clock as a clock source. The part will then wake up from sleep and the TWI will hold the SCL clock will low
during the wake up and until the TWINT Flag is cleared (by writing it to one). Further data transmission 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 2-wire Serial Interface Data Register – TWDR does not reflect the last byte present on the bus
when waking up from these sleep modes.
TWAR TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE
value Device’s Own Slave Address
TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
value 0 100010 X