Datasheet

Bits 3 to 0: Rate Selector Bits (RS3 to RS0). These

four rate-selection bits select one of the 13 taps on the

15-stage divider or disable the divider output. The tap

selected can be used to generate an output square

wave (SQW pin) and/or a periodic interrupt. The user

can do one of the following:

1) Enable the interrupt with the PIE bit;

2) Enable the SQW output pin with the SQWE or E32k

bits;

3) Enable both at the same time and the same rate; or

4) Enable neither.

Table 4 lists the periodic interrupt rates and the square-

wave frequencies that can be chosen with the RS bits.

DS17285/DS17287/DS17485/DS17487/DS17885/DS17887

Real-Time Clocks

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Table 4. Periodic Interrupt Rate and Square-Wave Output Frequency

EXT REG B SELECT BITS REGISTER A

E32K RS3 RS2 RS1 RS0

PERIODIC INTERRUPT

RATE

SQW OUTPUT FREQUENCY

0 0 0 0 0 None None

0 0 0 0 1 3.90625ms 256Hz

0 0 0 1 0 7.8125ms 128Hz

0 0 0 1 1 122.070µs 8.192kHz

0 0 1 0 0 244.141µs 4.096kHz

0 0 1 0 1 488.281µs 2.048kHz

0 0 1 1 0 976.5625µs 1.024kHz

0 0 1 1 1 1.953125ms 512Hz

0 1 0 0 0 3.90625ms 256Hz

0 1 0 0 1 7.8125ms 128Hz

0 1 0 1 0 15.625ms 64Hz

0 1 0 1 1 31.25ms 32Hz

0 1 1 0 0 62.5ms 16Hz

0 1 1 0 1 125ms 8Hz

0 1 1 1 0 250ms 4Hz

0 1 1 1 1 500ms 2Hz

1 X X X X * 32.768kHz

RS3 to RS0 determine periodic interrupt rates as listed for E32K = 0.