Datasheet

14.9 Power Reduction Register
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 cannot 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 corresponding bit in the 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.
14.10 Minimizing Power Consumption
There are several possibilities to consider when trying to minimize the power consumption in an AVR
controlled system. In general, sleep modes should be used as much as possible, and the sleep mode
should be selected so that as few as possible of the device’s functions are operating. All functions not
needed should be disabled. In particular, the following modules may need special consideration when
trying to achieve the lowest possible power consumption.
14.10.1 Analog-to-Digital Converter
If enabled, the ADC will be enabled in all sleep modes. To save power, the ADC should be disabled
before entering any sleep mode. When the ADC is turned off and on again, the next conversion will be an
extended conversion.
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28. Analog-to-Digital Converter (ADC)
14.10.2 Analog Comparator
When entering Idle mode, the analog comparator should be disabled if not used. When entering ADC
Noise Reduction mode, the analog comparator should be disabled. In other sleep modes, the analog
comparator is automatically disabled. However, if the analog comparator is set up to use the internal
voltage reference as input, the analog comparator should be disabled in all sleep modes. Otherwise, the
internal voltage reference will be enabled, independent of the sleep mode.
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27. Analog Comparator (AC)
14.10.3 Brown-Out Detector
If the Brown-Out Detector (BOD) is not needed by the application, this module should be turned off. If the
BOD is enabled by the BODLEVEL fuses, it will be enabled in all sleep modes, and hence, always
consume power. In the deeper sleep modes, this will contribute significantly to the total current
consumption.
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15. System Control and Reset
14.10.4 Internal Voltage Reference
The internal voltage reference will be enabled when needed by the Brown-out Detection, the analog
comparator or the Analog-to-Digital Converter (ADC). If these modules are disabled as described in the
sections above, the internal voltage reference will be disabled and it will not be consuming power. When
ATmega48A/88A/168A
Power Management and Sleep Modes
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Datasheet Complete
DS40002007A-page 71