Rainbow Electronics ATmega128L - Manual (Page 213)

213
ATmega128(L)
2467B–09/01
The upper 7 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 ($00), 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 acknowledgment 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 90. The slave transmitter mode may also be entered if
arbitration is lost while the TWI is in the master mode (see state $B0).
If the TWEA bit is written to zero during a transfer, the TWI will transmit the last byte of
the transfer. State $C0 or state $C8 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 $C8 is entered if the master
demands additional data bytes (by transmitting ACK), even though the slave has trans-
mitted 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 1 0 0 0 1 0 X