M41ST87YSS6F datasheet(37/52 Pages) STMICROELECTRONICS

37 / 52 page

M41ST87Y, M41ST87W

Clock operation

Doc ID 9497 Rev 8

37/52

3.6

Full-time 32 kHz square wave output

The M41ST87Y/W offers the user a special 32kHz square wave function which defaults to

output on the F32k pin (pin 21) as long as VCC ≥ VSO, and the oscillator is running (ST bit =

'0'). This function is available within one second (typ) of initial power-up and can only be

disabled by setting the 32 kE bit to '0' or the ST bit to '1.' If not used, the F32k pin should be

disconnected and allowed to float.

Note:

The F32k pin is an open drain which requires an external pull-up resistor.

3.7

Power-on reset

The M41ST87Y/W 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.

Note:

A power-on reset will result in resetting the following control bits to '0': OFIE, AFE, ABE,

SQWE, FT, WDS, BMB0-BMB4, RB0, RB1, TIE1, and TIE2 (see Table 13 on page 41).

3.8

Reset inputs (RSTIN1 & RSTIN2)

The M41ST87Y/W provides two independent inputs which can generate an output reset.

The function of these resets is identical to a reset generated by a power cycle. Table 10 and

Figure 26 illustrate the AC reset characteristics of this function. Pulses shorter than tR1 and

tR2 will not generate a reset condition. RSTIN1 and RSTIN2 are each internally pulled up to

VCC through a 100 kΩ resistor. Note that RSTIN1 triggers on the falling edge while RSTIN2

triggers on the rising edge.

Note:

RSTIN2 is available only in the SOX28 (MX) package.

Figure 26.

RSTIN1 & RSTIN2 timing waveforms

AI07072

RSTIN1

RST

RSTIN2

tR1

trec

Hi-Z

tR2

trec

Hi-Z

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