CDP1879 Intersil Corporation, CDP1879 Datasheet - Page 6

• Manufacturer Part Number: CDP1879
• Description: CMOS Real-Time Clock
• Manufacturer: Intersil Corporation

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.
A2
0
0
0
0
1
1
1
1
1
1
0
0
1
1
1
ADDRESS
A1
1
1
1
1
0
0
0
0
1
1
1
1
0
1
1
AO
0
0
1
1
0
0
1
1
0
0
0
1
0
1
1
CDP1879, CDP1879C-1
REGISTER TRUTH TABLE
TPB/WR
X
X
X
X
X
X
X
X
X
-
-
-
-
-
-
ACTIVE SIGNAL
4-109
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.
RD
X
X
X
X
X
X
-
-
-
-
-
-
-
-
-
BIT 3 CONTROL
REGISTER
0
0
0
0
0
0
0
0
0
0
1
1
1
-
-
Read Seconds Counter
Read Minutes Counter
Read Hours Counter
Read Date Counter
Read Month Counter
Write Seconds Counter
Write Minutes Counter
Write Hours Counter
Write Date Counter
Write Month Counter
Write Seconds Alarm Latch
Write Minutes Alarm Latch
Write Hours Alarm Latch
Write Control Register
Read Int. Status Register
REGISTER OPERATION