Datasheet

ATmega48A/PA/88A/PA/168A/PA/328/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002061A-page 225
22.7 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
application. 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<<TWS)|
(1<<TWEN)
out TWCR, r16
TWCR =
(1<<TWINT)|(1<<TWST
A|
(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