PIC18F2450-I/ML【PDF 19/241 页】IC PIC MCU FLASH 8KX16 28QFN

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115

XMEGA A [MANUAL]

8077I–AVR–11/2012

Figure 10-1. Battery backup system and its power domain implementation.

10.3.1 Power Supervisor

The power supervisor monitors the voltage on the VBAT pin. It performs three main functions:

The power-on detection (BBPOD) function detects when power is applied to the VBAT pin, i.e., when the backup battery is

inserted. When this happens the battery backup power-on detection flag (BBPODF) is set and the power switch is

disconnected to prevent the backup battery from being drained before the device is configured.

The brown-out detection (BBBOD) function monitors the VBAT voltage level when the system is powered from the VBAT

pin. If the VBAT voltage drops below a threshold voltage, the battery backup bod flag (BBBODF) is set. The BBBOD

samples the VBAT voltage level at around a 1Hz rate, and is designed for detecting slow voltage changes. The BBBOD is

turned off when the device runs from the main power.

The power detection (BBPWR) function controls the VBAT voltage after a reset. If no voltage is present on the VBAT pin,

the battery backup power flag will be set. This indicates that the backup battery is not present or has been drained.

BBPODF, BBBODF, and the BBPWR flag are later referred to as the power supervision flags.

10.3.2 Power Switch

The power switch switches between main power and the VBAT pin to power the system. This happens automatically, and

is controlled from the main BOD in the device.

10.3.3 Crystal Oscillator with Failure Monitor

The crystal oscillator (XOSC) supports connection of a external 32.768kHz crystal. It provides a prescaled clock output

selectable to 1.024kHz or 1Hz. The crystal oscillator is designed for ultra low power consumption and by default is

configured for low ESR and load capacitance crystals. It is possible to enable a high ESR mode to drive crystals with high

ESR or load capacitance, but this will increase current consumption. The crystal oscillator failure monitor will detect if the

crystal is permanently or temporarily stopped and then set the crystal oscillator failure flag.

Power

switch

VDD

VBAT

TOSC1

TOSC2

VBAT

power

supervisor

Crystal

Oscillator

RTC

Failure

monitor

Backup

Registers

Main

power

supervision

Oscillator &

sleep

controller

FLASH,

EEPROM

& Fuses

Watchdog w/

Ooscillator

Internal

RAM

GPIO

XTAL2

XTAL1

OCD &

Programming

Interface

Level

shifters

Isolation

CPU

Peripherals

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