ATmega16 datasheet(178/315 Pages) ATMEL

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178

ATmega16(L)

2466E–AVR–10/02

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.

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.

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 opera-

tion 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 success-

fully 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 appli-

cation 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 transmis-

sions. 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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