ATmega48/88/168
Figure 21-18. Formats and States in the Slave Transmitter Mode
Reception of the own
slave address and one or
more data bytes
S
SLA
R
A
DATA
A
DATA
A
P or S
$A8
A
$B8
$C0
Arbitration lost as master
and addressed as slave
$B0
Last data byte transmitted.
Switched to not addressed
slave (TWEA = '0')
A
All 1's
P or S
$C8
Any number of data bytes
and their associated acknowledge bits
From master to slave
From slave to master
DATA
A
This number (contained in TWSR) corresponds
to a defined state of the 2-Wire Serial Bus. The
prescaler bits are zero or masked to zero
n
21.7.5
Miscellaneous States
There are two status codes that do not correspond to a defined TWI state, see Table 21-6.
Status 0xF8 indicates that no relevant information is available because the TWINT Flag is not
set. This occurs between other states, and when the TWI is not involved in a serial transfer.
Status 0x00 indicates that a bus error has occurred during a 2-wire Serial Bus transfer. A bus
error occurs when a START or STOP condition occurs at an illegal position in the format frame.
Examples of such illegal positions are during the serial transfer of an address byte, a data byte,
or an acknowledge bit. When a bus error occurs, TWINT is set. To recover from a bus error, the
TWSTO Flag must set and TWINT must be cleared by writing a logic one to it. This causes the
TWI to enter the not addressed Slave mode and to clear the TWSTO Flag (no other bits in
TWCR are affected). The SDA and SCL lines are released, and no STOP condition is
transmitted.
Table 21-6. Miscellaneous States
Status Code
(TWSR)
Prescaler Bits
are 0
Application Software Response
To TWCR
Status of the 2-wire Serial Bus
and 2-wire Serial Interface
Hardware
To/from TWDR
STA
STO
TWIN
T
TWE
A
Next Action Taken by TWI Hardware
Wait or proceed current transfer
0xF8
0x00
No relevant state information
available; TWINT = “0”
No TWDR action
No TWDR action
No TWCR action
Bus error due to an illegal
START or STOP condition
0
1
1
X
Only the internal hardware is affected, no STOP condi-
tion is sent on the bus. In all cases, the bus is released
and TWSTO is cleared.
21.7.6
Combining Several TWI Modes
In some cases, several TWI modes must be combined in order to complete the desired action.
Consider for example reading data from a serial EEPROM. Typically, such a transfer involves
the following steps:
1. The transfer must be initiated.
2. The EEPROM must be instructed what location should be read.
3. The reading must be performed.
4. The transfer must be finished.
235
2545M–AVR–09/07
ATMEL [ ATMEL ]
