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ATmega8A [DATASHEET]
8159E–AVR–02/2013
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 hard-
ware 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 condi-
tion 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.