Datasheet

3. The application software should now examine the value of TWSR, to make sure that the START
condition was successfully transmitted. 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 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.
AVR 8-Bit Microcontroller
TWI - Two-wire Serial Interface
© 2017 Microchip Technology Inc.
Datasheet Complete
40001974A-page 223