Datasheet

52
ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]
2549Q–AVR–02/2014
11.8 Power Reduction Register
The Power Reduction Register (PRR), see “PRR0 – Power Reduction Register 0” on page 55 and “PRR1 – Power
Reduction Register 1” on page 56, provides a method for stopping the clock to individual peripherals to reduce
power consumption.
Note that when the clock for a peripheral is stopped, then:
• The current state of the peripheral is frozen
• The associated registers can not be read or written
• Resources used by the peripherals (for example I/O pin, etc.) will remain occupied
The peripheral should in most cases be disabled before stopping the clock. Waking up a module, which is done by
cleaning the bit in PRR, puts the module in the same state as before shutdown. Module shutdown can be used in
Idle mode or Active mode to significantly reduce the overall power consumption. See “Power-down Supply Cur-
rent” on page 380 for examples. In all other sleep modes, the clock is already stopped.
11.9 Minimizing Power Consumption
There are several issues to consider when trying to minimize the power consumption in an AV R 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.
11.9.1 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.
Refer to “ADC – Analog to Digital Converter” on page 268 for details on ADC operation.
11.9.2 Analog Comparator
W hen entering Idle mode, the Analog Comparator should be disabled if not used. W hen 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 sleep mode. Refer to “AC – Analog Comparator” on page 265 for details on how to
configure the Analog Comparator.
11.9.3 Brown-out Detector
If the Brown-out Detector is not needed by the application, this module should be turned off. If the Brown-out
Detector 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. Refer to
“Brown-out Detection” on page 59 for details on how to configure the Brown-out Detector.
11.9.4 Internal Voltage Reference
The Internal Voltage Reference will be enabled when needed by the Brown-out Detection, the Analog Comparator
or the 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 turned on again, the user must allow the reference to start up
before the output is used. If the reference is kept on in sleep mode, the output can be used immediately. Refer to
“Internal V oltage Reference” on page 60 for details on the start-up time.