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7530J–AVR–03/12
Atmel ATmega48/88/168 Automotive
19.5.5
Control Unit
The Control unit monitors the TWI bus and generates responses corresponding to settings in the
TWI Control Register (TWCR). When an event requiring the attention of the application occurs
on the TWI bus, the TWI Interrupt Flag (TWINT) is asserted. In the next clock cycle, the TWI Sta-
tus Register (TWSR) is updated with a status code identifying the event. The TWSR only
contains relevant status information when the TWI Interrupt Flag is asserted. At all other times,
the TWSR contains a special status code indicating that no relevant status information is avail-
able. As long as the TWINT Flag is set, the SCL line is held low. This allows the application
software to complete its tasks before allowing the TWI transmission to continue.
The TWINT Flag is set in the following situations:
After the TWI has transmitted a START/REPEATED START condition.
After the TWI has transmitted SLA+R/W.
After the TWI has transmitted an address byte.
After the TWI has lost arbitration.
After the TWI has been addressed by own slave address or general call.
After the TWI has received a data byte.
After a STOP or REPEATED START has been received while still addressed as a Slave.
When a bus error has occurred due to an illegal START or STOP condition.
19.6
TWI Register Description
19.6.1
TWI Bit Rate Register – TWBR
Bits 7..0 – TWI Bit Rate Register
TWBR selects the division factor for the bit rate generator. The bit rate generator is a frequency
19.6.2
TWI Control Register – TWCR
The TWCR is used to control the operation of the TWI. It is used to enable the TWI, to initiate a
Master access by applying a START condition to the bus, to generate a Receiver acknowledge,
to generate a stop condition, and to control halting of the bus while the data to be written to the
bus are written to the TWDR. It also indicates a write collision if data is attempted written to
TWDR while the register is inaccessible.
Bit
76543210
TWBR7
TWBR6
TWBR5
TWBR4
TWBR3
TWBR2
TWBR1
TWBR0
TWBR
Read/Write
R/W
Initial Value
00000000
Bit
76543210
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
TWCR
Read/Write
R/W
R
R/W
R
R/W
Initial Value
00000000