4-115

Design Considerations for Stable Crystal Oscillation

1. Stray capacitances should be minimized for best oscilla-

tor performance. Circuit board traces should be kept to a

maximum of 1 inch, and there should be no parallel

traces.

2. A signal line or power source line must not cross or go

near the oscillator circuit line.

3. It is advisable to put a 0.1

Standby (Timekeeping) Voltage Operation

When any one of the four specified crystals is used with the

on-board oscillator, the Real-Time Clock can operate at a

(timekeeping only, no device READ/WRITE accesses) down

To achieve this low voltage operation, an external 32kHz clock

source must be supplied at the XTAL input (see Figure 7). The

standby requirements for CHIP SELECT/DESELECT are

listed in Table 4, and Figure 8 indicates the timing waveforms.

Figure 9 illustrates the typical timekeeping curve over the full

temperature range.

TABLE 3. TYPICAL OSCILLATOR CIRCUIT PARAMETERS FOR SUGGESTED OSCILLATOR CIRCUIT, SEE FIGURE 6

PARAMETER

4.197MHz

2.097MHz

1.049MHz

32768Hz (NOTE)

UNITS

22

22

22

22

M

Ω

39

39

39

39

pF

5555

pF

-

-

-

200

k

Ω

-

-

-

91

pF

Crystal Impedance

73

200

200

50K (max.)

Ω

NOTE:

CDP1879C-1 only.

FIGURE 5. CONNECTIONS FOR AN EXTERNAL FREQUENCY

SOURCE APPLIED TO REAL-TIME CLOCK

CDP1879

PIN 23

XTAL

EXTERNAL

FREQUENCY

SOURCE

PIN 22

XTAL

FIGURE 6. SUGGESTED OSCILLATOR CIRCUIT APPLIED TO

REAL-TIME CLOCK (SEE TABLE 3)

CDP1879

PIN 23

XTAL

PIN 22

PARALLEL

RESONANT

CRYSTAL

XTAL

200K

22 MEG.

1

2

3

14

4

+3V

24

23

22

1/3 CD54/74HC04

CDP1879

FIGURE 7. TYPICAL EXTERNAL CLOCK-SOURCE CIRCUIT

CDP1879, CDP1879C-1