DS1497 datasheet(8/19 Pages) DALLAS

8 / 19 page

DS1495/DS1497

020894 8/19

OSCILLATOR CONTROL BITS

When the DS1495/DS1497 is shipped from the factory,

the internal oscillator is turned off. This feature prevents

the lithium battery 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 oscilla-

tor 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 Fig-

ure 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 2.

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 500 ms to once every

122

µs. This function is separate from the alarm inter-

rupt 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 fre-

quency (see Table 1). Changing the Register A bits af-

fects 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 inter-

rupt can be used with software counters to measure in-

puts, create output intervals, or await the next needed

software function.

PERIODIC INTERRUPT RATE AND SQUARE WAVE OUTPUT FREQUENCY Table 2

SELECT BITS REGISTER A

tPI PERIODIC

SQW OUTPUT

RS3

RS2

RS1

RS0

tPI PERIODIC

INTERRUPT RATE

SQW OUTPUT

FREQUENCY

None

3.90625 ms

256 Hz

7.8125 ms

128 Hz

122.070

µs

8.192 KHz

244.141

µs

4.096 KHz

488.281

µs

2.048 KHz

976.5625

µs

1.024 KHz

1.953125 ms

512 Hz

3.90625 ms

256 Hz

7.8125 ms

128 Hz

15.625 ms

64 Hz

31.25 ms

32 Hz

62.5 ms

16 Hz

125 ms

8 Hz

250 ms

4 Hz

500 ms

2 Hz

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