ATMEGA128A_1 Datasheet, PDF (213/386 Pages) ATMEL Corporation – Write/Erase cycles: 10,000 Flash/100,000 EEPROM

8151G–AVR–07/10
Figure 21-11. Data Transfer in Master Transmitter Mode
ATmega128A
V_CC
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
value
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
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 transmitted, the TWINT flag is set by hard-
ware, and the status code in TWSR will be $08 (See Table 21-2). 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:
TWCR
value
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
1
X
0
0
X
1
0
X
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 status 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 21-2.
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 Regis-
ter. 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
value
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
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 generat-
ing a STOP condition or a repeated START condition. A STOP condition is generated by writing
the following value to TWCR:
TWCR
value
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
1
X
0
1
X
1
0
X
A REPEATED START condition is generated by writing the following value to TWCR:
TWCR
value
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
1
X
1
0
X
1
0
X
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