ATmega64A
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
After a repeated START condition (state 0x10) the Two-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 21-2. Status Codes for Master Transmitter Mode
Status Code
(TWSR)
Prescaler Bits
are 0
Application Software Response
To TWCR
Status of the Two-wire Serial
Bus and Two-wire Serial Inter-
face Hardware
STA
STO
TWINT
TWEA
X
Next Action Taken by TWI Hardware
To/from TWDR
Load SLA+W
0x08
0x10
A START condition has been
transmitted
0
0
1
SLA+W will be transmitted;
ACK or NOT ACK will be received
A repeated START condition
has been transmitted
Load SLA+W or
Load SLA+R
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;
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
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
Data byte has been transmit-
ted;
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
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
Data byte has been transmit-
ted;
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
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 bytes
No TWDR action or
No TWDR action
0
1
0
0
1
1
X
X
Two-wire Serial Bus will be released and not ad-
dressed slave mode entered
A START condition will be transmitted when the bus
becomes free
214
8160C–AVR–07/09
ATMEL [ ATMEL ]
