ATmega8U2/16U2/32U2
when stopping the clock will remain occupied, hence the peripheral should in most cases be dis-
abled 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.
9.9
Minimizing Power Consumption
There are several issues 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.
9.9.1
Analog Comparator
When entering Idle mode, the Analog Comparator should be disabled if not used. 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 dis-
abled in all sleep modes. Otherwise, the Internal Voltage Reference will be enabled,
independent of sleep mode. Refer to “Analog Comparator” on page 223 for details on how to
configure the Analog Comparator.
9.9.2
9.9.3
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 sig-
nificantly to the total current consumption. Refer to “Brown-out Detection” on page 50 for details
on how to configure the Brown-out Detector.
Internal Voltage Reference
The Internal Voltage Reference will be enabled when needed by the Brown-out Detection, or the
Analog Comparator. If these modules are disabled as described in the sections above, the inter-
nal 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 Voltage Reference” on
page 51 for details on the start-up time.
9.9.4
9.9.5
Watchdog Timer
If the Watchdog Timer is not needed in the application, the module should be turned off. If the
Watchdog Timer is enabled, 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 consump-
tion. Refer to “Interrupts” on page 64 for details on how to configure the Watchdog Timer.
Port Pins
When entering a sleep mode, all port pins should be configured to use minimum power. The
most important is then to ensure that no pins drive resistive loads. In sleep modes where the I/O
clock (clkI/O) is stopped, the input buffers of the device will be disabled. This ensures that no
power is consumed by the input logic when not needed. In some cases, the input logic is needed
for detecting wake-up conditions, and it will then be enabled. Refer to the section “Digital Input
Enable and Sleep Modes” on page 71 for details on which pins are enabled. If the input buffer is
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7799D–AVR–11/10
ATMEL [ ATMEL ]
