Datasheet

187
ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
7. The application software should now examine the value of TWSR, to make sure that the data packet was
successfully transmitted, and that the value of the ACK bit was as expected. If TWSR indicates otherwise, the
application software might take some special action, like calling an error routine. Assuming that the status code is
as expected, the application must write a specific value to TWCR, instructing the TWI hardware to transmit a
STOP condition. Which value to write is described later on. However, it is important that the TWINT bit is set in the
value written. Writing a one to TWINT clears the flag. The TWI will not start any operation as long as the TWINT bit
in TWCR is set. Immediately after the application has cleared TWINT, the TWI will initiate transmission of the
STOP condition. Note that TWINT is NOT set after a STOP condition has been sent.
Even though this example is simple, it shows the principles involved in all TWI transmissions. These can be summarized as
follows:
● When the TWI has finished an operation and expects application response, the TWINT flag is set. The SCL line is
pulled low until TWINT is cleared.
● When the TWINT flag is set, the user must update all TWI registers with the value relevant for the next TWI bus cycle.
As an example, TWDR must be loaded with the value to be transmitted in the next bus cycle.
● After all TWI register updates and other pending application software tasks have been completed, TWCR is written.
When writing TWCR, the TWINT bit should be set. Writing a one to TWINT clears the flag. The TWI will then
commence executing whatever operation was specified by the TWCR setting.
In the following an assembly and C implementation of the example is given. Note that the code below assumes that several
definitions have been made, for example by using include-files.
Table 19-3. Code Example
No. 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