Datasheet

ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
198
19.8.4 Slave Transmitter Mode
In the slave transmitter mode, a number of data bytes are transmitted to a master receiver (see Figure 19-18). All the status
codes mentioned in this section assume that the prescaler bits are zero or are masked to zero.
Figure 19-18. Data Transfer in Slave Transmitter Mode
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 19-7 on page 199. 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
Device 1
Slave
Transmitter
SDA
V
CC
SCL
Device 3 Device n........ R1 R2
Device 2
Master
Receiver