Oscillator control bits, Square wave output selection, Periodic interrupt selection – Rainbow Electronics DS12C887 User Manual

Page 11

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DS12C887

11 of 19

OSCILLATOR CONTROL BITS

When the DS12C887 is shipped from the factory, the internal oscillator is turned off. This feature
prevents the lithium energy cell from being used until it is installed in a system. A pattern of 010 in bits 4
through 6 of Register A will turn the oscillator on and enable the countdown chain. A pattern of 11X will
turn the oscillator on, but holds the countdown chain of the oscillator in reset. All other combinations of
bits 4 through 6 keep the oscillator off.

SQUARE WAVE OUTPUT SELECTION

Thirteen of the 15 divider taps are made available to a 1-of-15 selector, as shown in the block diagram of
Figure 1. The first purpose of selecting a divider tap is to generate a square wave output signal on the
SQW pin. The RS0–RS3 bits in Register A establish the square wave output frequency. These
frequencies are listed in Table 1. The SQW frequency selection shares its 1-of-15 selector with the
periodic interrupt generator. Once the frequency is selected, the output of the SQW pin can be turned on
and off under program control with the square wave enable bit (SQWE).

PERIODIC INTERRUPT SELECTION

The periodic interrupt will cause the

IRQ

pin to go to an active state from once every 500ms to once

every 122

µ

s. This function is separate from the alarm interrupt which can be output from once per second

to once per day. The periodic interrupt rate is selected using the same Register A bits which select the
square wave frequency (see Table 1). Changing the Register A bits affects both the square wave
frequency and the periodic interrupt output. However, each function has a separate enable bit in Register
B. The SQWE bit controls the square wave output. Similarly, the periodic interrupt is enabled by the PIE
bit in Register B. The periodic interrupt can be used with software counters to measure inputs, create
output intervals, or await the next needed software function.

PERIODIC INTERRUPT RATE AND SQUARE WAVE OUTPUT FREQUENCY
Table 2

EXT. REG. B

SELECT BITS REGISTER A

E32K

RS3

RS2

RS1

RS0

t

PI

PERIODIC INTERRUPT

RATE

SQW OUTPUT

FREQUENCY

0

0

0

0

0

None

None

0

0

0

0

1

3.90625 ms

256 Hz

0

0

0

1

0

7.8125 ms

128 Hz

0

0

0

1

1

122.070 µs

8.192 kHz

0

0

1

0

0

244.141 µs

4.096 kHz

0

0

1

0

1

488.281 µs

2.048 kHz

0

0

1

1

0

976.5625 µs

1.024 kHz

0

0

1

1

1

1.953125 ms

512 Hz

0

1

0

0

0

3.90625 ms

256 Hz

0

1

0

0

1

7.8125 ms

128 Hz

0

1

0

1

0

15.625 ms

64 Hz

0

1

0

1

1

31.25 ms

32 Hz

0

1

1

0

0

62.5 ms

16 Hz

0

1

1

0

1

125 ms

8 Hz

0

1

1

1

0

250 ms

4 Hz

0

1

1

1

1

500 ms

2 Hz

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