168
ATmega8A [DATASHEET]
8159E–AVR–02/2013
Figure 21-14. Formats and States in the Master Receiver Mode
Table 21-4.
Status codes for Master Receiver Mode
Status Code
(TWSR)
Prescaler Bits
are 0
Status of the Two-wire Serial
Bus and Two-wire Serial Inter-
face Hardware
Application Software Response
Next Action Taken by TWI Hardware
To/from TWDR
To TWCR
STA
STO
TWINT
TWEA
0x08
A START condition has been
transmitted
Load SLA+R
0
1
X
SLA+R will be transmitted
ACK or NOT ACK will be received
0x10
A repeated START condition
has been transmitted
Load SLA+R or
Load SLA+W
0
1
X
SLA+R will be transmitted
ACK or NOT ACK will be received
SLA+W will be transmitted
Logic will switch to Master Transmitter mode
0x38
Arbitration lost in SLA+R or NOT
ACK bit
No TWDR action or
No TWDR action
0
1
0
1
X
Two-wire Serial Bus will be released and not addressed
Slave mode will be entered
A START condition will be transmitted when the bus
becomes free
0x40
SLA+R has been transmitted;
ACK has been received
No TWDR action or
No TWDR action
0
1
0
1
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
0x48
SLA+R has been transmitted;
NOT ACK has been received
No TWDR action or
No TWDR action
1
0
1
0
1
X
Repeated START will be transmitted
STOP condition will be transmitted and TWSTO Flag will
be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
0x50
Data byte has been received;
ACK has been returned
Read data byte or
Read data byte
0
1
0
1
Data byte will be received and NOT ACK will be
returned
Data byte will be received and ACK will be returned
0x58
Data byte has been received;
NOT ACK has been returned
Read data byte or
Read data byte
1
0
1
0
1
X
Repeated START will be transmitted
STOP condition will be transmitted and TWSTO Flag will
be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
S
SLA
R
A
DATA
A
$08
$40
$50
SLA
R
$10
AP
$48
A or A
$38
Other master
continues
$38
Other master
continues
W
A
$68
Other master
continues
$78
$B0
To corresponding
states in slave mode
MR
MT
Successfull
reception
from a slave
receiver
Next transfer
started with a
repeated start
condition
Not acknowledge
received after the
slave address
Arbitration lost in slave
address or data byte
Arbitration lost and
addressed as slave
DATA
A
n
From master to slave
From slave to master
Any number of data bytes
and their associated acknowledge bits
This number (contained in TWSR) corresponds
to a defined state of the Two-Wire Serial Bus. The
prescaler bits are zero or masked to zero
P
DATA
A
$58
A
RS