121
8331B–AVR–03/12
Atmel AVR XMEGA AU
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 auto-
matic power switching between main power and a battery backup power supply. No external
components are required.
shows an overview of the system.
On devices with a battery backup system, a backup battery can be connected to the dedicated
V
BAT
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 mem-
ory 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 V
BAT
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.