ATmega32U2 datasheet(44/314 Pages) ATMEL

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7799B–AVR–07/09

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

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.

9.9.3

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

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.

9.9.5

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 (clk

I/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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