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Rainbow Electronics DS12887 User Manual

Page 5

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DS12887

5 of 19

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