Datasheet

TWINT bit in TWCRn is set. Immediately after the application has cleared TWINT, the TWI n will
initiate transmission of the START condition.
2. When the START condition has been transmitted, the TWINT Flag in TWCRn is set, and TWSRn is
updated with a status code indicating that the START condition has successfully been sent.
3. The application software should now examine the value of TWSRn, to make sure that the START
condition was successfully transmitted. If TWSRn 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 load SLA+W into TWDR. Remember that TWDRn is used both for
address and data. After TWDRn has been loaded with the desired SLA+W, a specific value must be
written to TWCRn, instructing the TWI n hardware to transmit the SLA+W present in TWDRn.
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 TWCRn 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 TWCRn is set, and TWSRn 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 TWSRn, to make sure that the address
packet was successfully transmitted, and that the value of the ACK bit was as expected. If TWSRn
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 load a data packet into
TWDRn. Subsequently, a specific value must be written to TWCRn, instructing the TWI n hardware
to transmit the data packet present in TWDRn. 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 n will not start any operation as long as the TWINT bit in TWCRn is set. Immediately after
the application has cleared TWINT, the TWI will initiate transmission of the data packet.
6. When the data packet has been transmitted, the TWINT Flag in TWCRn is set, and TWSRn 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 TWSRn, 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
TWCRn, instructing the TWI n 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 n will not start any operation as long as the TWINT bit in
TWCRn 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 n Registers with the value relevant for
the next TWI n bus cycle. As an example, TWDRn must be loaded with the value to be transmitted
in the next bus cycle.
• After all TWI n Register updates and other pending application software tasks have been
completed, TWCRn is written. When writing TWCRn, the TWINT bit should be set. Writing a one to
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
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