Datasheet

ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
200
Figure 19-19. Formats and States in the Slave Transmitter Mode
19.8.5 Miscellaneous States
There are two status codes that do not correspond to a defined TWI state, see Table 19-8.
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.
S
Reception of the own
slave address and one
or more data bytes
Last data byte transmitted.
Switched to not adressed
slave (TWEA = “0”
Arbitration lost as master
and addressed as slave
From master to slave
Any number of data bytes
and their associated acknowledge bits
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
From slave to master
SLA R A DATAAP or S
A
DATA
All 1’s
ADATA
$A8
$B0
$B8 $C0
$C8
A
P or SA
n
Table 19-8. Miscellaneous States
Status Code
(TWSR)
Prescaler
Bits are 0
Status of the 2-wire
Serial Bus and 2-wire
Serial Interface
Hardware
Application Software Response
Next Action Taken by TWI Hardware
To/from TWDR To TWCR
STA STO TWINT TWEA
0xF8 No relevant state
information available;
TWINT = “0”
No TWDR action No TWCR action Wait or proceed current transfer
0x00 Bus error due to an illegal
START or STOP condition
No TWDR action 0 1 1 X Only the internal hardware is affected, no
STOP condition is sent on the bus. In all
cases, the bus is released and TWSTO is
cleared.