DS1385 Datasheet, PDF (7/20 Pages) Dallas Semiconductor – RAMified Real Time Clock 4K x 8

DS1385/DS1387
The alternative flag bit usage method is with fully en-
abled interrupts. When an interrupt flag bit is set and the
corresponding interrupt enable bit is also set, the IRQ
pin is asserted low. IRQ is asserted as long as at least
one of the three interrupt sources has its flag and enable
bits both set. The IRQF bit in Register C is a one when-
ever the IRQ pin is being driven low. Determination that
the RTC initiated an interrupt is accomplished by read-
ing Register C. A logic one in bit–7 (IRQF bit) indicates
that one or more interrupts have been initiated. The act
of reading Register C clears all active flag bits and the
IRQF bit.
OSCILLATOR CONTROL BITS
When the DS1385/DS1387 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 6 through 4 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 6 through 4 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 RS3–RS0 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 RATE AND SQUARE WAVE OUTPUT FREQUENCY Table 2
SELECT BITS REGISTER A
RS3
RS2
RS1
RS0
t_PI PERIODIC
INTERRUPT RATE
SQW OUTPUT
FREQUENCY
0
0
0
0
None
None
0
0
0
1
3.90625 ms
256 Hz
0
0
1
0
7.8125 ms
128 Hz
0
0
1
1
122.070 µs
8.192 KHz
0
1
0
0
244.141 µs
4.096 KHz
0
1
0
1
488.281 µs
2.048 KHz
0
1
1
0
976.5625 µs
1.024 KHz
0
1
1
1
1.953125 ms
512 Hz
1
0
0
0
3.90625 ms
256 Hz
1
0
0
1
7.8125 ms
128 Hz
1
0
1
0
15.625 ms
64 Hz
1
0
1
1
31.25 ms
32 Hz
1
1
0
0
62.5 ms
16 Hz
1
1
0
1
125 ms
8 Hz
1
1
1
0
250 ms
4 Hz
1
1
1
1
500 ms
2 Hz
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