ATmega16U4 datasheet(44/408 Pages) ATMEL

44 / 408 page

7766D–AVR–01/09

ATmega16U4/ATmega32U4

7.5

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.

7.6

Extended Standby Mode

When the SM2..0 bits are 111 and an external crystal/resonator clock option is selected, the

SLEEP instruction makes the MCU enter Extended Standby mode. For compatibility reasons

with AT90USB64/128 this mode is still present but since Timer 2 Asynchronous operation is not

present here, this mode is identical to Standby-mode.

Notes:

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

2. For INT6, only level interrupt.

3. Asynchronous USB interrupts are VBUSTI and WAKEUPI.

7.7

Power Reduction Register

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

als 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 “Supply Current of IO modules” on page 429 for examples. In all other

sleep modes, the clock is already stopped.

7.7.1

Power Reduction Register 0 - PRR0

Table 7-2.

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

Active Clock Domains

Oscillators

Wake-up Sources

Sleep Mode

Idle

XXX

ADCNRM

X(2)

XXX

Power-down

X(2)

XXX

Power-save

X(2)

XXX

Standby(1)

XX(2)

XXX

Extended

Standby

XX(2)

XXX

Bit

7654

PRTWI

PRTIM2

PRTIM0

–

PRTIM1

PRSPI

PRADC

PRR0

Read/Write

R/W

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