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114

XMEGA A [MANUAL]

8077I–AVR–11/2012

10.

Battery Backup System

10.1

Features

Integrated battery backup system ensuring continuos, real-time clock during main power failure

Battery backup power supply from dedicated V

BAT pin to power:

– One 32-bit real-time counter

– One ultra low power 32.768kHz crystal oscillator with failure detection monitor

– Two battery backup registers

Automatic power switching between main power and battery backup power:

– Switching from main power to battery backup power at main power loss

– Switching from battery backup power to main power at main power return

10.2

Overview

Many applications require a real-time clock that keeps running continuously, even in the event of a main power loss or

failure. The battery backup system includes functions for this through automatic power switching between main power

and a battery backup power supply. No external components are required. Figure 10-1 on page 115 shows an overview

of the system.

On devices with a battery backup system, a backup battery can be connected to the dedicated VBAT power pin. If the

main power is lost, the backup battery will continue and power the real-time counter (RTC), a 32.768kHz crystal oscillator

with failure detection monitor, and two backup registers. The battery backup system does not provide power to other

parts of the volatile memory in the device, such as SRAM and I/O registers outside the system.

The device uses its BOD to detect main power loss and switch to power from the VBAT pin. After main power is restored,

the battery back system will automatically switch back to being powered from the main power again. The backup battery

is drained only when main power is not present, and this ensures maximum battery life.

On devices with the battery backup system, the RTC will keep running in all sleep modes.

10.3

Battery Backup System

The battery backup system consists of a VBAT power supervisor, a power switch, a crystal oscillator with failure monitor, a

32-bit real-time counter (RTC), and two backup registers.

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