MCP79400 datasheet(32/62 Pages) MICROCHIP

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MCP79400/MCP79401/MCP79402

DS20005009F-page 32

 2011-2016 Microchip Technology Inc.

5.7

Battery Backup

The MCP7940X features a backup power supply input

(VBAT) that can be used to provide power to the

timekeeping circuitry, RTCC registers, and SRAM while

primary power is unavailable. The MCP7940X will

automatically switch to backup power when VCC falls

below VTRIP, and back to VCC when it is above VTRIP.

The VBATEN bit must be set to enable the VBAT input.

The following functionality is maintained while

operating on backup power:

• Timekeeping

•Alarms

• Alarm Output

• Digital Trimming

• RTCC Register and SRAM Contents

The following features are not available while operating

on backup power:

•I2C Communication

• Square Wave Clock Output

• General Purpose Output

5.7.1

POWER-FAIL TIME-STAMP

The MCP7940X includes a power-fail time-stamp

module that stores the minutes, hours, date, and month

when primary power is lost and when it is restored

(Figure 5-8). The PWRFAIL bit is also set to indicate

that a power failure occurred.

To utilize the power-fail time-stamp feature, a backup

power supply must be available with the VBAT input

enabled, and the oscillator should also be running to

ensure accurate functionality.

FIGURE 5-8:

POWER-FAIL TIME-STAMP TIMING

Note:

Throughout this section, references to the

Time-Stamp module are referred to

generically by the use of ‘x’ in place of the

specific

module

name.

Thus,

“PWRxxMIN” might refer to the minutes

Note 1:

The PWRFAIL bit must be cleared to log

new time-stamp data. This is to ensure

previous time-stamp data is not lost.

Clearing the PWRFAIL bit will clear all

time-stamp registers.

Power-Down

Power-Up

Time-Stamp

VCC

VTRIP

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