Datasheet

189
ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
Figure 19-12. Data Transfer in Master Transmitter Mode
A START condition is sent by writing the following value to TWCR:
TWEN must be set to enable the 2-wire serial interface, TWSTA must be written to one to transmit a START condition and
TWINT must be written to one to clear the TWINT flag. The TWI will then test the 2-wire serial bus and generate a START
condition as soon as the bus becomes free. After a START condition has been transmitted, the TWINT flag is set by
hardware, and the status code in TWSR will be 0x08 (see Table 19-4 on page 190). In order to enter MT mode, SLA+W must
be transmitted. This is done by writing SLA+W to TWDR. Thereafter the TWINT bit should be cleared (by writing it to one) to
continue the transfer. This is accomplished by writing the following value to TWCR:
When SLA+W have been transmitted and an acknowledgement bit has been received, TWINT is set again and a number of
status codes in TWSR are possible. Possible status codes in master mode are 0x18, 0x20, or 0x38. The appropriate action
to be taken for each of these status codes is detailed in Table 19-4 on page 190.
When SLA+W has been successfully transmitted, a data packet should be transmitted. This is done by writing the data byte
to TWDR. TWDR must only be written when TWINT is high. If not, the access will be discarded, and the write collision bit
(TWWC) will be set in the TWCR register. After updating TWDR, the TWINT bit should be cleared (by writing it to one) to
continue the transfer. This is accomplished by writing the following value to TWCR:
This scheme is repeated until the last byte has been sent and the transfer is ended by generating a STOP condition or a
repeated START condition. A STOP condition is generated by writing the following value to TWCR:
A REPEATED START condition is generated by writing the following value to TWCR:
TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
value 1
X10X10 X
TWCR TWINT
TWEA TWSTA TWSTO TWWC TWEN – TWIE
value 1
X00X10 X
TWCR TWINT
TWEA TWSTA TWSTO TWWC TWEN – TWIE
value 1
X00X10 X
TWCR TWINT
TWEA TWSTA TWSTO TWWC TWEN – TWIE
value 1
X01X10 X
TWCR TWINT
TWEA TWSTA TWSTO TWWC TWEN – TWIE
value 1
X10X10 X
Device 1
Master
Transmitter
SDA
SCL
V
CC
Device 3 Device n ........ R1 R2
Device 2
Slave
Receiver