Datasheet

ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
186
Figure 19-11. Interfacing the Application to the TWI in a Typical Transmission
1. The first step in a TWI transmission is to transmit a START condition. This is done by writing a specific value into
TWCR, instructing the TWI hardware to transmit a START 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 START condition.
2. When the START condition has been transmitted, the TWINT flag in TWCR is set, and TWSR is updated with a
status code indicating that the START condition has successfully been sent.
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.
START
TWI
Hardware
Action
Application
Action
TWI bus
Indicates
TWINT set
SLA + W A A STOPData
1. Application
writes to TWCR to
initiate
transmission of
START
2. TWINT set.
Status code indicates
START condition sent
4. TWINT set.
Status code indicates
SLA + W sent,
ACK received
6. TWINT set.
Status code indicates
data sent, ACK received
3. Check TWSR to see if START was
sent. Application loads SLA + W into
TWDR, and loads appropriate control
signals into TWCR, makin sure that
TWINT is written to one,
and TWSTA is written to zero.
5. Check TWSR to see if SLA + W was
sent and ACK received.
Application loads data intoTWDR, and
loads appropriate control signals into
TWCR, makin sure that TWINT is
written to one
7. Check TWSR to see if data was sent
and ACK received.
Application loads appropriate control
signals to send STOP into TWCR,
makin sure that TWINT is
written to one