M41T66 datasheet(20/33 Pages) STMICROELECTRONICS

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Clock operation

M41T66

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3.5

Square wave output

The M41T66 offers the user a programmable square wave function which is output on the

SQW pin. RS3-RS0 bits located in 04h establish the square wave output frequency. These

frequencies are listed in Table 4. Once the selection of the SQW frequency has been

completed, the SQW pin can be turned on and off under software control with the square

wave enable bit (SQWE) located in register 0Ah.

The SQW output is an open drain output driver. The initial power-up default for the SQW

output is 32 KHz.

Note:

When the SQW is enabled and the ST bit is set (ST = 1), the square wave output could be

low or high and could drain current through the pull-up resistor.

Due to the output buffer circuitry used for the SQW output, this pin must not be taken to a

voltage greater than VCC. A diode is required on the SQW pin for SuperCap™ (or battery)

backup. A low threshold BAT42 diode is recommended (see Figure 4 on page 7).

3.6

Century bits

These two bits will increment in a binary fashion at the turn of the century, and handle all

leap years correctly. See Table 6 on page 22 for additional explanation.

Table 4.

Square wave output frequency

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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