6.5
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.
Table 6-2.
Active Clock Domains and Wake-up Sources in the Different Sleep Modes.
Oscillator
Active Clock Domains
s
Wake-up Sources
Sleep
Mode
Idle
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
ADC
Noise
X(2)
Reduction
Power-do
wn
X(2)
X(2)
X
X
Standby(1)
X
Notes: 1. Only recommended with external crystal or resonator selected as clock source.
2. Only level interrupt.
6.6
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.
A full predictible behaviour of a peripheral is not guaranteed during and after a cycle of stopping
and starting of its clock. So its recommended to stop a peripheral before stopping its clock with
PRR register.
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.
42
ATmega16/32/64/M1/C1
7647F–AVR–04/09
ATMEL [ ATMEL ]
