
4-116
Applications
A typical application for this real-time clock is as a wake-up
control to a CPU to reduce total system power in intermit-
tent-use systems. A hookup diagram illustrating this feature
is shown in Figure 10. In this configuration, the alarm and
power-down features of the CDP1879 are utilized in the con-
trol of the sleep and wake-up states of the CPU. A typical
shut-down/start-up sequence for this system could proceed
as follows:
1. The CPU has finished a current task and will be inactive
for the next six hours.
2. The CPU loads the CDP1879 alarm registers with the
desired wake-up time.
3. The CDP1800 Q output is set high, which stops the CPU
oscillator (as an alternative, in an NMOS system, power
to all components except the clock chip could be shut off).
4. This Q output signal is received by the CDP1879 as a
power-down signal.
5. The CDP1879 three-states the interrupt output pin.
6. The CDP1879 eventually times out, and sets an alarm by
driving the INT output low.
7. The alarm signal resets the CPU (to avoid oscillator start-
up problems) and flags the processor for a warm-start
routine.
8. The CPU, once into its normal software sequence, writes
to the CDP1879 control register to reset the interrupt
request.
STANDBY (TIMEKEEPING) CHARACTERISTICS AT FULL TEMPERATURE RANGE
PARAMETER
V
DD
(V)
V
STBY
(V)
LIMITS
UNITS
CDP1879
CDP1879C-1
MIN
MAX
MIN
MAX
Chip Deselect to Standby
(Timekeeping) Voltage Time
t
CSTBY
5
10
2.5, 3
2.5, 3
2
1
-
-
2
-
-
-
μ
s
Recovery to Normal
Operation Time
t
RC
5
10
2.5, 3
2.5, 3
2
1
-
-
2
-
-
-
NOTE:
1. t
R
, t
F
≥
1
μ
s
FIGURE 8. STANDBY (TIMEKEEPING) VOLTAGE AND TIMING
WAVEFORMS
FIGURE 9. TYPICAL STANDBY (TIMEKEEPING) VOLTAGE vs
FULL TEMPERATURE RANGE
0.95 V
DD
STANDBY
VOLTAGE MODE
V
STBY
0.95 V
DD
t
STBY
V
IH
V
IL
t
R (NOTE 1)
t
F (NOTE 1)
t
RC
V
IH
V
IL
V
DD
CS
5
4
3
2
1
0-40
-20
0
20
40
60
80
100
EXTERNAL CLOCK SOURCE OF 32kHz
TYPICAL STANDBY (TIMEKEEPING) VOLTAGE
3V (-40
C
≤
T
≤
+85
C)
2.5V (-0
o
C
≤
T
≤
+70
o
C
S
(
S
D
)
FULL TEMPERATURE RANGE -
o
C
CDP1879, CDP1879C-1