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)

(V)

LIMITS

UNITS

CDP1879

CDP1879C-1

MIN

MAX

MIN

MAX

Chip Deselect to Standby

(Timekeeping) Voltage Time

5

10

2.5, 3

2.5, 3

2

1

-

-

2

-

-

-

µs

Recovery to Normal

Operation Time

5

10

2.5, 3

2.5, 3

2

1

-

-

2

-

-

-

NOTE:

FIGURE 8. STANDBY (TIMEKEEPING) VOLTAGE AND TIMING

WAVEFORMS

FIGURE 9. TYPICAL STANDBY (TIMEKEEPING) VOLTAGE vs

FULL TEMPERATURE RANGE

STANDBY

VOLTAGE MODE

CS

5

4

3

2

1

0

-40

-20

0

20

40

60

80

100

EXTERNAL CLOCK SOURCE OF 32kHz

TYPICAL STANDBY (TIMEKEEPING) VOLTAGE

CDP1879, CDP1879C-1