162
ATmega8A [DATASHEET]
8159E–AVR–02/2013
21.6.1
Transmission Modes
The TWI can operate in one of four major modes. These are named Master Transmitter (MT), Master Receiver
(MR), Slave Transmitter (ST) and Slave Receiver (SR). Several of these modes can be used in the same applica-
tion. As an example, the TWI can use MT mode to write data into a TWI EEPROM, MR mode to read the data back
from the EEPROM. If other masters are present in the system, some of these might transmit data to the TWI, and
then SR mode would be used. It is the application software that decides which modes are legal.
Assembly Code Example
C Example
Comments
1
ldi
r16, (1<<TWINT)|(1<<TWSTA)|
(1<<TWEN)
out
TWCR, r16
TWCR = (1<<TWINT)|(1<<TWSTA)|
(1<<TWEN)
Send START condition
2
wait1:
in
r16,TWCR
sbrs
r16,TWINT
rjmp
wait1
while
(!(TWCR & (1<<TWINT)))
;
Wait for TWINT Flag set. This
indicates that the START condition
has been transmitted
3
in
r16,TWSR
andi
r16, 0xF8
cpi
r16, START
brne
ERROR
if
((TWSR & 0xF8) != START)
ERROR();
Check value of TWI Status
Register. Mask prescaler bits. If
status different from START go to
ERROR
ldi
r16, SLA_W
out
TWDR, r16
ldi
r16, (1<<TWINT) | (1<<TWEN)
out
TWCR, r16
TWDR = SLA_W;
TWCR = (1<<TWINT) | (1<<TWEN);
Load SLA_W into TWDR Register.
Clear TWINT bit in TWCR to start
transmission of address
4
wait2:
in
r16,TWCR
sbrs
r16,TWINT
rjmp
wait2
while
(!(TWCR & (1<<TWINT)))
;
Wait for TWINT Flag set. This
indicates that the SLA+W has been
transmitted, and ACK/NACK has
been received.
5
in
r16,TWSR
andi
r16, 0xF8
cpi
r16, MT_SLA_ACK
brne
ERROR
if
((TWSR & 0xF8) !=
MT_SLA_ACK)
ERROR();
Check value of TWI Status
Register. Mask prescaler bits. If
status different from MT_SLA_ACK
go to ERROR
ldi
r16, DATA
out
TWDR, r16
ldi
r16, (1<<TWINT) | (1<<TWEN)
out
TWCR, r16
TWDR = DATA;
TWCR = (1<<TWINT) | (1<<TWEN);
Load DATA into TWDR Register.
Clear TWINT bit in TWCR to start
transmission of data
6
wait3:
in
r16,TWCR
sbrs
r16,TWINT
rjmp
wait3
while
(!(TWCR & (1<<TWINT)))
;
Wait for TWINT Flag set. This
indicates that the DATA has been
transmitted, and ACK/NACK has
been received.
7
in
r16,TWSR
andi
r16, 0xF8
cpi
r16, MT_DATA_ACK
brne
ERROR
if
((TWSR & 0xF8) !=
MT_DATA_ACK)
ERROR();
Check value of TWI Status
Register. Mask prescaler bits. If
status different from
MT_DATA_ACK go to ERROR
ldi
r16, (1<<TWINT)|(1<<TWEN)|
(1<<TWSTO)
out
TWCR, r16
TWCR = (1<<TWINT)|(1<<TWEN)|
(1<<TWSTO);
Transmit STOP condition