M41ST85WMH6F datasheet(26/43 Pages) STMICROELECTRONICS

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Clock operation

M41ST85W

26/43

DocID7531 Rev 12

3.7

Power-on reset

The M41ST85W continuously monitors VCC. When VCC falls to the power fail detect trip

point, the RST pulls low (open drain) and remains low on power-up for trec after VCC passes

VPFD(max). The RST pin is an open drain output and an appropriate pull-up resistor should

be chosen to control rise time.

3.8

Reset inputs (RSTIN1 & RSTIN2)

The M41ST85W provides two independent inputs which can generate an output reset. The

duration and function of these resets is identical to a reset generated by a power cycle.

Table 5

and Figure 17 illustrate the AC reset characteristics of this function. Pulses shorter

than tRLRH1 and tRLRH2 will not generate a reset condition. RSTIN1 and RSTIN2 are each

internally pulled up to VCC through a 100 kΩ resistor.

Figure 17. RSTIN1 & RSTIN2 timing waveforms

Note:

With pull-up resistor

Table 5. Reset AC characteristics

3.9

Power-fail input/output

The power-fail input (PFI) is compared to an internal reference voltage (1.25 V). 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 undervoltage detector to signal a failing power supply.

Typically PFI is connected through an external voltage divider (see Figure 5 on page 11) to

either the unregulated DC input (if it is available) or the regulated output of the VCC

AI03665

RSTIN2

RST

(1)

RSTIN1

tRLRH1

tRLRH2

tR1HRH

tR2HRH

Symbol

Parameter(1)

Min

Max

Unit

tRLRH1

(2)

RSTIN1 low to RSTIN1 high

200

tRLRH2

(3)

RSTIN2 low to RSTIN2 high

100

tR1HRH

(4)

RSTIN1 high to RST high

200

tR2HRH

(4)

RSTIN2 high to RST high

200

1. Valid for ambient operating temperature: TA = –40 to 85°C; VCC = 2.7 to 3.6 V (except where noted).

2. Pulse width less than 50 ns will result in no RESET (for noise immunity).

3. Pulse width less than 20 ms will result in no RESET (for noise immunity).

4. Programmable (see Table on page 28).

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