Datasheet

• A REPEATED START and a STOP condition.
It is the user software’s responsibility to ensure that these illegal arbitration conditions never occur. This
implies that in multi-master systems, all data transfers must use the same composition of SLA+R/W and
data packets. In other words: All transmissions must contain the same number of data packets, otherwise
the result of the arbitration is undefined.
25.6 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.
The following figure 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 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, making sure that
TWINT is written to one,
and TWSTA is written to zero.
5. CheckTWSR 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. CheckTWSR 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
Hardware
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.
AVR 8-Bit Microcontroller
TWI - Two-wire Serial Interface
© 2017 Microchip Technology Inc.
Datasheet Complete
40001974A-page 222