ATmega128 datasheet(205/386 Pages) ATMEL

205 / 386 page

205

2467S–AVR–07/09

ATmega128

or cleared by manipulating the TWI Control Register (TWCR). When in Transmitter mode, the

value of the received (N)ACK bit can be determined by the value in the TWSR.

The START/STOP Controller is responsible for generation and detection of START, REPEATED

START, and STOP conditions. The START/STOP controller is able to detect START and STOP

conditions even when the AVR MCU is in one of the sleep modes, enabling the MCU to wake up

if addressed by a master.

If the TWI has initiated a transmission as master, the Arbitration Detection hardware continu-

ously monitors the transmission trying to determine if arbitration is in process. If the TWI has lost

an arbitration, the Control Unit is informed. Correct action can then be taken and appropriate

status codes generated.

Address Match Unit

The Address Match unit checks if received address bytes match the 7-bit address in the TWI

Address Register (TWAR). If the TWI General Call Recognition Enable (TWGCE) bit in the

TWAR is written to one, all incoming address bits will also be compared against the General Call

address. Upon an address match, the Control Unit is informed, allowing correct action to be

taken. The TWI may or may not acknowledge its address, depending on settings in the TWCR.

The Address Match unit is able to compare addresses even when the AVR MCU is in sleep

mode, enabling the MCU to wake up if addressed by a master. If another interrupt (e.g., INT0)

occurs during TWI Power-down address match and wakes up the CPU, the TWI aborts opera-

tion and return to it’s idle state. If this cause any problems, ensure that TWI Address Match is the

only enabled interrupt when entering Power-down.

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

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