Rainbow Electronics ATmega128L - Manual (Page 205)

205
ATmega128(L)
2467B–09/01
Figure 95. 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 $08 (See
Table 87).
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 acknowledgment 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 87.
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 follow-
ing 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 gen-
erated 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 X 1 0 X 1 0 X
TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
value 1 X 0 0 X 1 0 X
TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
value 1 X 0 0 X 1 0 X
TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
value 1 X 0 1 X 1 0 X
TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
value 1 X 1 0 X 1 0 X
Device 1
MASTER
TRANSMITTER
Device 2
SLAVE
RECEIVER
Device 3 Device n
SDA
SCL
........ R1 R2
V
CC