Rainbow Electronics DS1687 - Manual (Page 16)

DS1685/DS1687
16 of 38
When using the flag bits with fully enabled interrupts, the IRQ line is driven low when an interrupt flag
bit is set and its corresponding enable bit is also set. IRQ is held low as long as at least one of the six
possible interrupt sources has its flag and enable bits both set. The IRQF bit in Register C is a 1 whenever
the
IRQ pin is being driven low as a result of one of the six possible active sources. Therefore,
determination that the DS1685/DS1687 initiated an interrupt is accomplished by reading Register C and
finding IRQF = 1. IRQF remains set until all enabled interrupt flag bits are cleared to 0.
SQUARE-WAVE OUTPUT SELECTION
The SQW pin can be programmed to output a variety of frequencies divided down from the 32.768kHz
crystal tied to X1 and X2. The square-wave output is enabled and disabled by the SQWE bit in Register B
or the E32K bit in extended register 4Bh. If the square wave is enabled (SQWE = 1 or E32K = 1), then
the output frequency is determined by the settings of the E32K bit in Extended Register 4Bh and by the
RS3–0 bits in Register A. If E32K = 1, then a 32.768kHz square wave is output on the SQW pin
regardless of the settings of RS3–0 and SQWE.
If E32K = 0, then the square-wave output frequency is determined by the RS3–0 bits. These bits control a
1-of-15 decoder, which selects one of 13 taps that divide the 32.768kHz frequency. The RS3–0 bits
establish the SQW output frequency as shown in Table 2. In addition, RS3–0 bits control the periodic
interrupt selection as described below.
If E32K = 1 and the auxiliary-battery enable bit (ABE, bank 1; register 04BH) is enabled, and voltage is
applied to V
BAUX
, then the 32kHz square-wave output signal is output on the SQW pin in the absence of
V
CC
. This facility is provided to clock external power management circuitry. If any of the above
requirements are not met, no square-wave output signal is generated on the SQW pin in the absence of
V
CC
.
A pattern of 01X in the DV2, DV1, and DV0 bits respectively turns the oscillator on and enables the
countdown chain. Note that this is different than the DS1287, which required a pattern of 010 in these
bits. DV0 is now a “don’t care” because it is used for selection between register banks 0 and 1.
A pattern of 11X turns the oscillator on, but the oscillator’s countdown chain is held in reset, as it was in
the DS1287. Any other bit combination for DV2 and DV1 keeps the oscillator off.
Oscillator Control Bits
When the DS1687 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 01X in bits 4 through
6 of Register A turns the oscillator on and enables the countdown chain. A pattern of 11X turns 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.
PERIODIC INTERRUPT SELECTION
The periodic interrupt causes the IRQ pin to go to an active state from once every 500ms to once every
122 ms. 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 RS3–0 bits in Register A, which select
the square-wave frequency (Table 2). Changing the 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 and
E32K bits control the square-wave output. Similarly, the periodic interrupt is enabled by the PIE bit in