Datasheet

45
7766E–AVR–04/10
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. In all other sleep modes, the clock is already stopped.
Table 7-2. Active Clock Domains and Wake-up Sources in the Different Sleep Modes.
Active Clock Domains Oscillators Wake-up Sources
Sleep Mode
clk
CPU
clk
FLASH
clk
IO
clk
ADC
Main Clock Source Enabled INT6, INT3:0 and Pin Change TWI Address Match SPM/ EEPROM Ready ADC WDT Interrupt Other I/O USB Synchronous Interrupts USB Asynchronous Interrupts
(3)
Idle X X X XXXXXXXX
ADCNRM X X X
(2)
XXXX XX
Power-down X
(2)
XXX
Power-save X
(2)
XXX
Standby
(1)
XX
(2)
XXX
Extended
Standby
XX
(2)
XXX