Datasheet

99
XMEGA B [DATASHEET]
8291B–AVR–01/2013
8.3.5 Extended Standby Mode
Extended standby mode is identical to power-save mode, with the exception that the enabled system clock sources are
kept running while the CPU and peripheral clocks are stopped. This reduces the wake-up time.
8.4 Power Reduction Registers
The power reduction (PR) registers provide a method to stop the clock to individual peripherals. When this is done, the
current state of the peripheral is frozen and the associated I/O registers cannot be read or written. Resources used by the
peripheral will remain occupied; hence, the peripheral should be disabled before stopping the clock. Enabling the clock to
a peripheral again puts the peripheral in the same state as before it was stopped. This can be used in idle mode and
active modes to reduce the overall power consumption. In all other sleep modes, the peripheral clock is already stopped.
Not all devices have all the peripherals associated with a bit in the power reduction registers. Setting a power reduction
bit for a peripheral that is not available will have no effect.
8.5 Minimizing Power Consumption
There are several possibilities to consider when trying to minimize the power consumption in an AVR MCU controlled
system. In general, correct sleep modes should be selected and used to ensure that only the modules required for the
application are operating.
All unneeded functions should be disabled. In particular, the following modules may need special consideration when
trying to achieve the lowest possible power consumption.
8.5.1 Analog-to-Digital Converter - ADC
When entering idle mode, the ADC should be disabled if not used. In other sleep modes, the ADC is automatically
disabled. 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 326 for details on ADC operation.
8.5.2 Analog Comparator - AC
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 disabled in all sleep modes. Otherwise, the internal voltage reference will be
enabled, irrespective of sleep mode. Refer to “AC – Analog Comparator” on page 352 for details on how to configure the
analog comparator.
8.5.3 Brownout Detector
If the brownout detector is not needed by the application, this module should be turned off. If the brownout detector is
enabled by the BODLEVEL fuses, it will be enabled in all sleep modes, and always consume power. In the deeper sleep
modes, it can be turned off and set in sampled mode to reduce current consumption. Refer to “Brownout Detection” on
page 109 for details on how to configure the brownout detector.
8.5.4 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. Refer to “WDT – Watchdog Timer” on page 115
for details on how to configure the watchdog timer.
8.5.5 Port Pins
When entering a sleep mode, all port pins should be configured to use minimum power. Most important is to ensure that
no pins drive resistive loads. In sleep modes where the Peripheral Clock (Clk
PER
) 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.