After a repeated START condition (state 0x10) the 2-wire Serial Interface can access
the same Slave again, or a new Slave without transmitting a STOP condition. Repeated
START enables the Master to switch between Slaves, Master Transmitter mode and
Master Receiver mode without losing control of the bus.
Table 116. Status codes for Master Transmitter Mode
Status Code
(TWSR)
Prescaler Bits
are 0
Application Software Response
To/from TWDR To TWCR
STO TWINT
Status of the 2-wire Serial Bus
and 2-wire Serial Interface Hard-
ware
STA
0
TWEA
X
Next Action Taken by TWI Hardware
0x08
A START condition has been Load SLA+W
transmitted
0
1
SLA+W will be transmitted;
ACK or NOT ACK will be received
0x10
A
repeated START condition Load SLA+W or
0
0
0
0
1
1
X
X
SLA+W will be transmitted;
ACK or NOT ACK will be received
SLA+R will be transmitted;
has been transmitted
Load SLA+R
Logic will switch to Master Receiver mode
0x18
0x20
0x28
0x30
0x38
SLA+W has been transmitted;
ACK has been received
Load data byte or
0
0
1
X
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
No TWDR action or
No TWDR action or
1
0
0
1
1
1
X
X
No TWDR action
Load data byte or
1
0
1
0
1
1
X
X
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
SLA+W has been transmitted;
NOT ACK has been received
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
No TWDR action or
No TWDR action or
1
0
0
1
1
1
X
X
No TWDR action
1
0
1
0
1
1
X
X
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
Data byte has been transmitted; Load data byte or
ACK has been received
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
No TWDR action or
No TWDR action or
1
0
0
1
1
1
X
X
No TWDR action
1
0
1
0
1
1
X
X
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
Data byte has been transmitted; Load data byte or
NOT ACK has been received
No TWDR action or
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
1
0
0
1
1
1
X
X
No TWDR action or
No TWDR action
1
1
1
X
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
Arbitration lost in SLA+W or data No TWDR action or
bytes
0
1
0
0
1
1
X
X
2-wire Serial Bus will be released and not addressed
Slave mode entered
A START condition will be transmitted when the bus be-
comes free
No TWDR action
258
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATMEL [ ATMEL ]
