ATmega328P datasheet(225/448 Pages) ATMEL

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8161D–AVR–10/09

ATmega48PA/88PA/168PA/328P

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.

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.

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 condi-

tion. 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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