Datasheet

189
ATmega48PA/88PA/168PA [DATASHEET]
9223F–AVR–04/14
22.6 Using the TWI
The AVR TWI is byte-oriented and interrupt based. Interrupts are issued after all bus events, like reception of a byte or
transmission of a START condition. Because the TWI is interrupt-based, the application software is free to carry on other
operations during a TWI byte transfer. Note that the TWI interrupt enable (TWIE) bit in TWCR together with the global
interrupt enable bit in SREG allow the application to decide whether or not assertion of the TWINT flag should generate an
interrupt request. If the TWIE bit is cleared, the application must poll the TWINT flag in order to detect actions on the TWI
bus.
When the TWINT flag is asserted, the TWI has finished an operation and awaits application response. In this case, the TWI
status register (TWSR) contains a value indicating the current state of the TWI bus. The application software can then
decide how the TWI should behave in the next TWI bus cycle by manipulating the TWCR and TWDR registers.
Figure 22-10 is a simple example of how the application can interface to the TWI hardware. In this example, a master wishes
to transmit a single data byte to a slave. This description is quite abstract, a more detailed explanation follows later in this
section. A simple code example implementing the desired behavior is also presented.
Figure 22-10. 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.
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, making 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, 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,
makin sure that TWINT is
written to one