176
ATmega8A [DATASHEET]
8159E–AVR–02/2013
21.8
Register Description
21.8.1
TWBR – TWI Bit Rate Register
Bits 7:0 – TWI Bit Rate Register
TWBR selects the division factor for the bit rate generator. The bit rate generator is a frequency divider which gen-
rates.
21.8.2
TWCR – TWI Control Register
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 7 – TWINT: TWI Interrupt Flag
This bit is set by hardware when the TWI has finished its current job and expects application software response. If
the I-bit in SREG and TWIE in TWCR are set, the MCU will jump to the TWI Interrupt Vector. While the TWINT Flag
is set, the SCL low period is stretched. The TWINT Flag must be cleared by software by writing a logic one to it.
Note that this flag is not automatically cleared by hardware when executing the interrupt routine. Also note that
clearing this flag starts the operation of the TWI, so all accesses to the TWI Address Register (TWAR), TWI Status
Register (TWSR), and TWI Data Register (TWDR) must be complete before clearing this flag.
Bit 6 – TWEA: TWI Enable Acknowledge Bit
The TWEA bit controls the generation of the acknowledge pulse. If the TWEA bit is written to one, the ACK pulse is
generated on the TWI bus if the following conditions are met:
1.
The device’s own slave address has been received.
2.
A general call has been received, while the TWGCE bit in the TWAR is set.
3.
A data byte has been received in Master Receiver or Slave Receiver mode.
By writing the TWEA bit to zero, the device can be virtually disconnected from the Two-wire Serial Bus temporarily.
Address recognition can then be resumed by writing the TWEA bit to one again.
Bit 5 – TWSTA: TWI START Condition Bit
The application writes the TWSTA bit to one when it desires to become a Master on the Two-wire Serial Bus. The
TWI hardware checks if the bus is available, and generates a START condition on the bus if it is free. However, if
the bus is not free, the TWI waits until a STOP condition is detected, and then generates a new START condition to
claim the bus Master status. TWSTA must be cleared by software when the START condition has been
transmitted.
Bit
7654
3210
TWBR7
TWBR6
TWBR5
TWBR4
TWBR3
TWBR2
TWBR1
TWBR0
TWBR
Read/Write
R/W
Initial Value
0000
Bit
7654
3210
TWINT
TWEA
TWSTA
TWSTO
TWWC
TWEN
–
TWIE
TWCR
Read/Write
R/W
R
R/W
R
R/W
Initial Value
0000