DS1287 datasheet(4/19 Pages) DALLAS

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DS12887

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occurs on the AS bus will clear 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 VCC, 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 VCC for Motorola timing, R/ W is at a level which 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.

(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 which take place without asserting CS will latch addresses but no

access will occur. When VCC is below 4.25 volts, the DS12887 internally inhibits access cycles by

internally disabling the CS input. This action protects both the real time clock 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 effect on the clock, calendar, or RAM. On power–up the

RESET

pin can be held low for a time in order 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 RESET is low should exceed 200 ms to make sure that the internal timer that controls the DS12887

on power-up has timed out. When RESET is low and VCC is above 4.25 volts, the following occurs:

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