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 spe-
cial 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.
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AT90USB64/128
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ATMEL [ ATMEL ]
