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116

XMEGA A [MANUAL]

8077I–AVR–11/2012

10.3.4 32-bit Real-time Counter

The 32-bit real-time counter (RTC) will count each clock output from the crystal oscillator. It provides a one-millisecond or

one-second resolution, depending on the crystal oscillator clock output selection. For more details on the 32-bit RTC,

refer to the “RTC32 – 32-bit Real-Time Counter” on page 195.

10.3.5 Backup Registers

The two backup registers can be used to store volatile data parameters when Vcc is not present.

10.4

Configuration

During device initialization, the battery backup system and RTC must be configured before they can be used. The

recommended configuration sequence is:

Apply a reset

Set the access enable bit

Optionally configure the oscillator output and ESR selection

Optionally enable the crystal oscillator failure monitor and the required delay before continuing configuration

Enable the crystal oscillator

Wait until the crystal oscillator ready flag is set

Configure and enable the RTC

10.5

Operation

The main BOD monitors the main voltage (Vcc) level and controls the power switching. This must always be enabled. In

active and idle modes, the BOD must be in continuos mode. In deep sleep modes, the BOD can be in continuos or

sampled mode. The system is designed as a power backup system for the RTC. Reset sources other than the BOD and

power loss (i.e. external reset, watchdog reset, and software reset) must be treated as a system reset. In this case, the

device state should be treated as unknown and lead to complete re-initialization, including battery backup system

configuration.

10.5.1 Main Power Loss

When Vcc drops below the programmed BOD threshold voltage, the device will:

Switch the battery backup system to be powered from the VBAT pin and enable the BBBOD.

Ignore any input signals to the system to prevent accidental or partial configuration.

Stretch the 1Hz / 1.024kHz clock signal to avoid a clock edge when switching is active.

Reset the part of the device not powered from the VBAT pin.

The battery backup system will continue to run as normal during the power switch and afterwards. When main power is

lost, it is not possible to access or read the status from the registers.

10.5.2 Main Power Restore and Start-up Sequence

At every startup after main power is restored, the software should:

Control the main reset source to determine that a POR or BOD took place.

Check for power on the VBAT pin by reading the BBPWR flag.

Read the power supervisor flags to determine further software action:

If all power supervision flags are cleared, the battery backup system runs as normal. The software should

enable access to the battery backup system and check the crystal oscillator failure flag. If the flag is set, the

software should assume that the RTC counter value is invalid and take appropriate action.

If any power supervision flags are set, it indicates the battery backup system has lost power sometime

during the period when the rest of the device was unpowererd. Software should assume that the

configuration and RTC value are invalid and take appropriate action.

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