Rainbow Electronics ATmega32L - Manual (Page 178)

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ATmega32(L)
2503C–AVR–10/02
• 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.
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 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 85 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 behaviour is also presented.
Figure 85. 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.
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
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
Hardware
Action
3. Check TWSR to see if START was
sendt. 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