M41ST95W datasheet(22/35 Pages) STMICROELECTRONICS

22 / 35 page

M41ST95Y*, M41ST95W

22/35

Power-fail INPUT/OUTPUT

The Power-Fail Input (PFI) is compared to an in-

ternal reference voltage (1.25V). If PFI is less than

the power-fail threshold (VPFI), the Power-Fail

Output (PFO) will go low. This function is intended

for use as an under-voltage detector to signal a

failing power supply. Typically PFI is connected

through an external voltage divider (see Figure 17)

to either the unregulated DC input (if it is available)

or the regulated output of the VCC regulator. The

voltage divider can be set up such that the voltage

at PFI falls below VPFI several milliseconds before

the regulated VCC input to the M41ST95Y/W or the

microprocessor drops below the minimum operat-

ing voltage.

During battery back-up, the power-fail comparator

turns off and PFO goes (or remains) low. This oc-

curs after VCC drops below VPFD(min). When pow-

er returns, PFO is forced high, irrespective of VPFI

for the write protect time (tREC), which is the time

from VPFD(max) until the inputs are recognized. At

the end of this time, the power-fail comparator is

enabled and PFO follows PFI. If the comparator is

unused, PFI should be connected to VSS and PFO

left unconnected.

Figure 17. Power-Fail Comparator Hookup

Note: 1. Available only in 28-pin, 300mil SOIC (MX) package.

AI06373

VCC

VOUT

VCC

PFO

SDI

SCL

M41ST95Y/W

WDI

RSTIN2

RSTIN1

PFI

VSS

VBAT

IRQ/FT/OUT

SQW

RST

SDO

Unregulated

Voltage

Regulator

VCC

VIN

ECON

Pushbutton

Reset

To RST

To LED Display

To NMI

To INT

F32k

(1)

VCC

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