Rainbow Electronics ATmega128RFA1 - Manual (Page 385)

385
8266A-MCU Wireless-12/09
ATmega128RFA1
Figure 25-10. Interfacing the Application to the TWI in a Typical Transmission
START SLA+W A Data A STOP
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 into TWDR, and
loads appropriate control signals into
TWCR, making 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,
making sure that TWINT is written to one
TWI bus
Indicates
TWINT set
Application
Action
TWI
Hardw
are
Action
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