3 power reduction register, 4 minimizing power consumption, 1 analog to digital converter – Rainbow Electronics ATmega128RFA1 User Manual
Page 159: 2 analog comparator, 3 brown-out detector, 4 internal voltage reference
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8266A-MCU Wireless-12/09
ATmega128RFA1
12.3 Power Reduction Register
The Power Reduction Register (PRR), see
"PRR0 – Power Reduction Register0" on
"PRR1 – Power Reduction Register 1" on page 168
and
Reduction Register 2" on page 168
, provide 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 can not be read or written. Resources used by the peripheral when
stopping the clock will remain occupied. Hence the peripheral unit 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 the shutdown. Exceptions
are the SRAM blocks and the radio transceiver. The SRAM is shut down by a DRT
switch and the radio transceiver is in reset state if its respective power reduction bit is
set.
Module shutdown can be used in Idle mode and Active mode to significantly reduce the
overall power consumption. See chapter
"Typical Characteristics" on page 510
for
examples. In all other sleep modes, the clock is already stopped.
12.4 Minimizing Power Consumption
There are several issues to consider when trying minimizing 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.
12.4.1 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. Refer to chapter
for details on ADC operation.
12.4.2 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 also 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
for details on how to configure the Analog Comparator.
12.4.3 Brown-out Detector
If the Brown-out Detector is enabled by the BODLEVEL Fuses, it will be disabled in
Deep-sleep mode. Refer to
"Brown-out Detection" on page 178
for details on how to
configure the Brown-out Detector. It is recommended to enable the Brown-out Detector.
12.4.4 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 not
consume 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