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CDP1879

CMOS Real-Time Clock

Intersil 

CDP1879, CDP1879C-1

FREEZE

CIRCUIT

AM - PM

AND

HOUR LOGIC

CALENDAR

LOGIC

XTAL

OSCILLATOR

XTAL

PRESCALE

SECOND

MINUTE

HOUR

DAY

MONTH

PRESCALE CLOCK

SELECT SELECT

CLOCK OUT

INT

RESET

VDD

CLOCK AND

INT. LOGIC

VSS

CONTROL

REGISTER

INT. STATUS

REGISTER

8-BIT DATA BUS

COMPARATOR

SECOND

LATCH

MINUTE

LATCH

HOUR

LATCH

I/O

INTERFACE

DB0-DB7

A0

A1

A2

TPA

IO/MEM

TPB/WR

RD

CS

POWER DOWN

ADDRESS DECODE

AND

CONTROL LOGIC

FIGURE 3. FUNCTIONAL DIAGRAM - ALARM CIRCUIT, CLOCK OUTPUT, INTERRUPT, AND STATUS REGISTERS HIGHLIGHTED

Operational Sequence

Power is applied and the real-time clock is reset. This sets

the interrupt output pin high. After the CS pin is set high and

with address 7 on the address input lines, the control register

is loaded via the data bus to configure the clock.

With selective addressing, the seconds through month

counters are then written to and loaded to set the current time.

The real-time clock will now hold the current “wall clock” time,

with an accuracy determined by the crystal or external clock

used. If the alarm function is desired, the control register is

accessed and loaded again. This new byte will allow subse-

quent time data to be entered into the seconds, minutes and

hours alarm latches. This sequence is also used when select-

ing one of the 15 available clock-out signals.

If the alarm function was selected, the interrupt output pin

will be set low when the values in the seconds, minutes and

hour alarm latches match those in the seconds, minutes and

hour counters.

If one of the 15 sub second-to-day clock outputs is selected

by the byte in the control register, the clock output pin tog-

gles at that frequency (50% duty cycle) The interrupt output

will also be set low on the first clock out negative transition.

The interrupt source (alarm or clock out) can be determined

by reading the interrupt status register. The clock output can

be deselected by placing zero in the upper nibble of the con-

trol register if the alarm function is selected as the only inter-

rupt source.

Counters

The counter section consists of an on-board oscillator, a

prescaler and 5 counters that hold the time of day/calendar

information (see Figure 2).

1 of 4 possible external crystals determine the frequency of the

on-board oscillator (32,768Hz, 1.048576MHz, 2.097152MHz,

4.194304MHz). The oscillator output is divided down by a pres-

4-111

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