ATMEGA88 datasheet(37/349 Pages) ATMEL

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ATmega48/88/168

2545D–AVR–07/04

The Timer/Counter2 can be clocked both synchronously and asynchronously in Power-

save mode. If Timer/Counter2 is not using the asynchronous clock, the Timer/Counter

Oscillator is stopped during sleep. If Timer/Counter2 is not using the synchronous clock,

the clock source is stopped during sleep. Note that even if the synchronous clock is run-

ning in Power-save, this clock is only available for Timer/Counter2.

Standby Mode

When the SM2..0 bits are 110 and an external crystal/resonator clock option is selected,

the SLEEP instruction makes the MCU enter Standby mode. This mode is identical to

Power-down with the exception that the Oscillator is kept running. From Standby mode,

the device wakes up in six clock cycles.

Notes:

1. Only recommended with external crystal or resonator selected as clock source.

2. If Timer/Counter2 is running in asynchronous mode.

3. For INT1 and INT0, only level interrupt.

Power Reduction

The Power Reduction Register, PRR, provides a method to stop the clock to individual

peripherals to reduce power consumption. The current state of the peripheral is frozen

and the I/O registers can not be read or written. Resources used by the peripheral when

stopping the clock will remain occupied, hence the peripheral should in most cases be

disabled before stopping the clock. Waking up a module, which is done by clearing the

bit in PRR, puts the module in the same state as before shutdown.

Module shutdown can be used in Idle mode and Active mode to significantly reduce the

overall power consumption. See “Power-Down Supply Current” on page 306 for exam-

ples. In all other sleep modes, the clock is already stopped.

Power Reduction Register -

PRR

• Bit 7 - PRTWI: Power Reduction TWI

Writing a logic one to this bit shuts down the TWI by stopping the clock to the module.

When waking up the TWI again, the TWI should be re initialized to ensure proper

operation.

Table 19. Active Clock Domains and Wake-up Sources in the Different Sleep Modes.

Active Clock Domains

Oscillators

Wake-up Sources

Sleep Mode

Idle

XXX

X(2)

XXX

ADC Noise

Reduction

X(2)

X(3)

Power-down

X(3)

Power-save

X(3)

Standby(1)

XX(3)

Bit

543

PRTWI

PRTIM2

PRTIM0

–

PRTIM1

PRSPI

PRUSART0

PRADC

PRR

Read/Write

R/W

Initial Value

ATMEGA88 Datasheet(PDF) 37 Page - ATMEL Corporation

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