ATmega8(L)
TWA6
R/W
1
TWA5
R/W
1
TWA4
R/W
1
TWA3
R/W
1
TWA2
R/W
1
TWA1
R/W
1
TWA0
R/W
1
TWGCE
R/W
0
TWAR
Read/Write
Initial Value
The TWAR should be loaded with the 7-bit Slave address (in the seven most significant
bits of TWAR) to which the TWI will respond when programmed as a Slave Transmitter
or Receiver, and not needed in the Master modes. In multimaster systems, TWAR must
be set in masters which can be addressed as Slaves by other Masters.
The LSB of TWAR is used to enable recognition of the general call address (0x00).
There is an associated address comparator that looks for the slave address (or general
call address if enabled) in the received serial address. If a match is found, an interrupt
request is generated.
• Bits 7..1 – TWA: TWI (Slave) Address Register
These seven bits constitute the slave address of the TWI unit.
• 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 appli-
cation 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 77 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.
171
2486M–AVR–12/03
ATMEL [ ATMEL ]
