233
2545M–AVR–09/07
ATmega48/88/168
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 soft-
ware action. The appropriate action to be taken for each status code is detailed in Table 21-5.
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 trans-
fer. 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 expect-
ing 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
