Figure 86. Data Transfer in Master Transmitter Mode
VCC
Device 1
MASTER
TRANSMITTER
Device 2
SLAVE
RECEIVER
Device 3
Device n
R1
R2
........
SDA
SCL
A START condition is sent by writing the following value to TWCR:
TWCR
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
1
X
1
0
X
1
0
X
TWEN must be set to enable the Two-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 Two-wire Serial Bus and generate a START
condition as soon as the bus becomes free. After a START condition has been transmit-
ted, the TWINT Flag is set by hardware, and the status code in TWSR will be $08 (See
Table 74). In order to enter MT mode, SLA+W must be transmitted. This is done by writ-
ing 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:
TWCR
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
1
X
0
0
X
1
0
X
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 sta-
tus codes in master mode are $18, $20, or $38. The appropriate action to be taken for
each of these status codes is detailed in Table 74.
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:
TWCR
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
1
X
0
0
X
1
0
X
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 gen-
erated by writing the following value to TWCR:
TWCR
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
1
X
0
1
X
1
0
X
A REPEATED START condition is generated by writing the following value to TWCR:
TWCR
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
Value
1
X
1
0
X
1
0
X
184
ATmega32(L)
2503J–AVR–10/06
ATMEL [ ATMEL ]
