M41ST87Y_08 datasheet(35/48 Pages) STMICROELECTRONICS

35 / 48 page

M41ST87Y, M41ST87W

Clock operation

35/48

Table 9.

Square wave output frequency

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 39).

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

Square wave bits

Square wave

RS3

RS2

RS1

RS0

Frequency

Units

0000

None

–

0001

32.768

kHz

0010

8.192

kHz

0011

4.096

kHz

0100

2.048

kHz

0101

1.024

kHz

0110

512

0111

256

1000

128

1001

1010

1011

11008

11014

11102

11111

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