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ATmega64A [DATASHEET]
8160D–AVR–02/2013
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 address packet.
4.
When the address packet has been transmitted, the TWINT flag in TWCR is set, and TWSR is updated with
a status code indicating that the address packet has successfully been sent. The status code will also
reflect whether a slave acknowledged the packet or not.
5.
The application software should now examine the value of TWSR, to make sure that the address packet
was successfully transmitted, and that the value of the ACK bit was as expected. If TWSR indicates other-
wise, the application software might take some special action, like calling an error routine. Assuming that
the status code is as expected, the application must load a data packet into TWDR. Subsequently, a spe-
cific value must be written to TWCR, instructing the TWI hardware to transmit the data packet present in
TWDR. 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 trans-
mission of the data packet.
6.
When the data packet has been transmitted, the TWINT flag in TWCR is set, and TWSR is updated with a
status code indicating that the data packet has successfully been sent. The status code will also reflect
whether a slave acknowledged the packet or not.
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 sta-
tus 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 oper-
ation 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 sum-
marized 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.
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