ATMEga64L ATMEL Corporation, ATMEga64L Datasheet - Page 208

• Manufacturer Part Number: ATMEga64L
• Description: 8-bit AVR Microcontroller with 64K Bytes In-System Programmable Flash
• Manufacturer: ATMEL Corporation

Using the TWI
Figure 95. Interfacing the Application to the TWI in a Typical Transmission
208
TWI bus
1. Application writes
to TWCR to initiate
transmission of
ATmega64(L)
START
START condition sent
Status code indicates
2. TWINT set.
START
TWDR, and loads appropriate control
signals into TWCR, making sure that
3. Check TWSR to see if START was
sent. Application loads SLA+W into
TWINT is written to one, and
TWSTA is written to zero.
• Bit 0 – TWGCE: TWI General Call Recognition Enable Bit
If set, this bit enables the recognition of a General Call given over the Two-wire Serial
Bus.
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 applica-
tion 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 95 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 descrip-
tion is quite abstract, a more detailed explanation follows later in this section. A simple
code example implementing the desired behavior is also presented.
1. The first step in a TWI transmission is to transmit a START condition. This is
2. When the START condition has been transmitted, the TWINT flag in TWCR is
SLA+W
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.
set, and TWSR is updated with a status code indicating that the START condition
has successfully been sent.
Status code indicates
SLA+W sent, ACK
4. TWINT set.
received
A
5. Check TWSR to see if SLA+W was
and loads appropriate control signals
Application loads data into TWDR,
into TWCR, making sure that
sent and ACK received.
TWINT is written to one
Data
data sent, ACK received
Status code indicates
6. TWINT set.
A
7. Check TWSR to see if data was sent
and ACK received. Application loads
STOP into TWCR, making sure that
appropriate control signals to send
TWINT is written to one
STOP
TWINT set
2490G–AVR–03/04
Indicates