DS12887
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cycle is terminated and the bus returns to a high-impedance state as DS transitions low in the case of
Motorola timing or as RD transitions high in the case of Intel timing.
AS (Address Strobe Input) – A positive-going address-strobe pulse serves to demultiplex the bus. The
falling edge of AS/ALE causes the address to be latched within the DS12887. The next rising edge that
occurs on the AS bus clears the address regardless of whether CS is asserted. Access commands should
be sent in pairs.
DS (Data Strobe or Read Input) – The DS/ RD pin has two modes of operation depending on the level
of the MOT pin. When the MOT pin is connected to V
CC
, Motorola bus timing is selected. In this mode,
DS is a positive pulse during the latter portion of the bus cycle and is called Data Strobe. During read
cycles, DS signifies the time that the DS12887 is to drive the bidirectional bus. In write cycles the trailing
edge of DS causes the DS12887 to latch the written data. When the MOT pin is connected to GND, Intel
bus timing is selected. In this mode the DS pin is called Read ( RD ). RD identifies the time period when
the DS12887 drives the bus with read data. The RD signal is the same definition as the output-enable
( OE ) signal on a typical memory.
R/ W (Read/Write Input) – The R/ W pin also has two modes of operation. When the MOT pin is
connected to V
CC
for Motorola timing, R/ W is at a level that indicates whether the current cycle is a read
or write. A read cycle is indicated with a high level on R/ W while DS is high. A write cycle is indicated
when R/ W is low during DS.
When the MOT pin is connected to GND for Intel timing, the R/ W signal is an active-low signal called
WR. In this mode, the R/ W pin has the same meaning as the write-enable signal ( WE ) on generic RAMs.
CS (Chip-Select Input) – The chip select signal must be asserted low for a bus cycle in the DS12887 to
be accessed. CS must be kept in the active state during DS and AS for Motorola timing and during RD
and WR for Intel timing. Bus cycles that take place without asserting CS latch addresses but no access
occur. When V
CC
is below 4.25V, the DS12887 internally inhibits access cycles by internally disabling
the CS input. This action protects both the RTC data and RAM data during power outages.
IRQ (Interrupt Request Output) – The IRQ pin is an active-low output of the DS12887 that can be
used as an interrupt input to a processor. The
IRQ output remains low as long as the status bit causing the
interrupt is present and the corresponding interrupt-enable bit is set. To clear the IRQ pin, the processor
program normally reads the C register. The
RESET pin also clears pending interrupts.
When no interrupt conditions are present, the
IRQ level is in the high-impedance state. Multiple
interrupting devices can be connected to an IRQ bus. The IRQ bus is an open drain output and requires an
external pullup resistor.
RESET (Reset Input) – The RESET pin has no affect on the clock, calendar, or RAM. On power-up, the
RESET pin can be held low for a time to allow the power supply to stabilize. The amount of time that
RESET is held low is dependent on the application. However, if RESET is used on power-up, the time
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