DS17885/DS17887

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possible interrupt sources has its flag and enable bits both set. The IRQF bit in Register C is a 1 whenever

determination that the DS17885/DS17887 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.

OSCILLATOR CONTROL BITS

When the DS17887 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 enable the countdown chain. Not 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 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

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 through 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 that 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

requirements are not met, no square-wave output signal is generated on the SQW pin in the absence of

PERIODIC INTERRUPT SELECTION

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 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 PIE bit in Register B enables the periodic

interrupt. The periodic interrupt can be used with software counters to measure inputs, create output

intervals, or await the next needed software function.