Data Sheet

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ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
must load SLA+W into TWDR. Remember that TWDR is used both for address and data. After TWDR has
been loaded with the desired SLA+W, a specific value must be written to TWCR, instructing the TWI
hardware to transmit the SLA+W 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 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
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 TWDR. Subsequently, a
specific 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
transmission 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
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.