參數(shù)資料
型號(hào)： DS17887-5
英文描述： Real-Time Clock
中文描述： 實(shí)時(shí)時(shí)鐘
文件頁(yè)數(shù)： 13/38頁(yè)
文件大小： 515K
代理商： DS17887-5
DS17885/DS17887
13 of 38
1 whenever the
IRQ
pin is being driven low as a result of one of the six possible active sources.
Therefore, determination that the DS17885/DS17887 initiated an interrupt is accomplished by reading
Register C and finding IRQF=1. IRQF will remain 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 32.768 kHz
crystal tied to X1 and X2. The square wave output is enabled and disabled via 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 will be 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 32.768 kHz square wave will be 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 thirteen taps that divide the 32.768 kHz 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 32 kHz square wave output signal will be 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 will be generated on the SQW pin in the absence
of V
CC
.
A pattern of 01X in the DV2, DV1, and DV0, bits respectively, will turn the oscillator on and enable 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 will turn the oscillator on, but the oscillator’s countdown chain will be held in reset, as
it was in the DS1287. Any other bit combination for DV2 and DV1 will keep the oscillator off.
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 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 (see 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 Register B. The periodic interrupt can be used with software counters to measure inputs,
create output intervals, or await the next needed software function.
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