4-109

General Operation

The real-time clock contains seconds, minutes, and hours,

date and month counters that hold time of day/calendar

information (see Figure 2). The frequency of an intrinsic

oscillator is divided down to supply a once-a-second signal

to the counter series string. The counters are separately

addressable and can be written to or read from.

The real-time clock contains seconds, minutes and hour

write-only alarm latches that store the alarm time (see Fig-

ure 3). When the value of the alarm latches and counters are

equal, the interrupt output is activated. The interrupt output

can also be activated by a clock output transition. The clock

output is derived from the prescaler and counters and can

be one of 15 square-wave signals. The value in the read-

only interrupt status register identifies the interrupt source.

Operational control of the real-time clock is determined by

the byte in a write-only control register. The 8-bit value in this

register determines the correct divisor for the prescaler, a

data direction and alarm enable bit, clock output select, and

start/stop control (see Figure 4).

Data transfer and addressing are accomplished in two

modes of operation, memory mapping and I/O mapping

using the CDP1800-series microprocessors. The mode is

selected by the level on an input pin. (IO/MEM). Memory

mapping implies use of the address lines as chip selects and

address inputs during linear selection or partial or full decod-

ing methods. I/O mapping with the CDP1800-series micro-

processors involves use of the N line outputs in conjunction

with input and output instructions to transfer data to and from

memory.

REGISTER TRUTH TABLE

ADDRESS

ACTIVE SIGNAL

BIT 3 CONTROL

REGISTER

REGISTER OPERATION

A2

A1

AO

TPB/WR

RD

0

1

0

X

-

0

Write Seconds Counter

0

1

0

-

X

0

Read Seconds Counter

0

1

1

X

-

0

Write Minutes Counter

0

1

1

-

X

0

Read Minutes Counter

1

0

0

X

-

0

Write Hours Counter

1

0

0

-

X

0

Read Hours Counter

1

0

1

X

-

0

Write Date Counter

1

0

1

-

X

0

Read Date Counter

1

1

0

X

-

0

Write Month Counter

1

1

0

-

X

0

Read Month Counter

0

1

0

X

-

1

Write Seconds Alarm Latch

0

1

1

X

-

1

Write Minutes Alarm Latch

1

0

0

X

-

1

Write Hours Alarm Latch

1

1

1

X

-

-

Write Control Register

1

1

1

-

X

-

Read Int. Status Register

CDP1879, CDP1879C-1