ATTINY88_14 Datasheet, PDF (138/302 Pages) ATMEL Corporation – 8-bit AVR Microcontroller with 8K Bytes In-System

Figure 15-10. Interfacing the Application to the TWI in a Typical Transmission
1. Application
writes to TWCR to
initiate
transmission of
START
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 START
SLA+W
A
Data
A
STOP
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
Indicates
TWINT set
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 success-
fully 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 appli-
cation 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 success-
fully 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 spe-
cial 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
138 ATtiny48/88
8008H–AVR–04/11