DS12C887
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 122µs. 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
tPI PERIODIC INTERRUPT
RATE
SQW OUTPUT
FREQUENCY
E32K
RS3
0
RS2
0
RS1
0
RS0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
None
None
0
0
0
1
3.90625 ms
7.8125 ms
122.070 µs
244.141 µs
488.281 µs
976.5625 µs
1.953125 ms
3.90625 ms
7.8125 ms
15.625 ms
31.25 ms
62.5 ms
256 Hz
128 Hz
8.192 kHz
4.096 kHz
2.048 kHz
1.024 kHz
512 Hz
256 Hz
128 Hz
64 Hz
0
0
1
0
0
0
1
1
0
1
0
0
0
1
0
1
0
1
1
0
0
1
1
1
1
0
0
0
1
0
0
1
1
0
1
0
1
0
1
1
32 Hz
16 Hz
8 Hz
4 Hz
1
1
0
0
1
1
0
1
125 ms
250 ms
500 ms
1
1
1
0
1
1
1
1
2 Hz
11 of 19
DALLAS [ DALLAS SEMICONDUCTOR ]
