M41ST87YMX6F datasheet(38/52 Pages) STMICROELECTRONICS

38 / 52 page

Clock operation

M41ST87Y, M41ST87W

38/52

Doc ID 9497 Rev 8

Table 10.

Reset AC characteristics

3.9

Power-fail comparators (1 and 2)

Two power-fail inputs (PFI1 and PFI2) are compared to an internal reference voltage (1.25V).

If either PFI1 or PFI2 is less than the power-fail threshold (VPFI), the associated power-fail

output (PFO1 or PFO2) will go low. This function is intended for use as an under-voltage

detector to signal a failing power supply. Typically PFI1 and PFI2 are connected through

external voltage dividers (see Figure 5 on page 11) 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 PFI1 or PFI2 falls below VPFI several milliseconds before the

regulated VCC input to the M41ST87Y/W or the microprocessor drops below the minimum

operating voltage, thus providing an early warning of power failure.

During battery back-up, the power-fail comparator turns off and PFO1 and PFO2 go (or

remain) low. This occurs after VCC drops below VPFD(min). When power returns, PFO1 and

PFO2 are 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 PFO1 and PFO2 follow PFI1 and PFI2. If the comparator is

unused, PFI1 or PFI2 should be connected to VSS and the associated PFO1 or PFO2 left

unconnected.

3.10

Power-fail outputs

The PFO1 and PFO2 outputs are programmable as N-channel, open drain output drivers, or

full-CMOS output drivers. By setting the power-fail output open drain bit (PFOD) to a '1,' the

output will be configured as open drain (with IOL as specified in Table 17 on page 44). When

PFOD is set to '0,' the outputs will be configured as full-CMOS (sink and source current as

specified in Table 17 on page 23).

Note:

When configured as open drain (PFOD = '1'), PFO1 and PFO2 will require an external pull-

up resistor.

Symbol

Parameter(1)

Valid for ambient operating temperature: TA = –40 to 85 °C; VCC = 4.5 to 5.5 V or 2.7 to 3.6 V (except

where noted).

Min

Max

Unit

tR1

(2)

Pulse widths of less than 100 ns will result in no RESET (for noise immunity).

RSTIN1 low to RST low (min pulse width)

100

200

tR2

(2)

RSTIN2 low to RSTIN2 high (min pulse width)

100

200

trec

(3)

Programmable (see Table 12 on page 40). Same function as power-on reset.

RSTIN1 or RSTIN2 high to RST high

98(3)

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