6

CDP6872

Typical values of the capacitors in Figure 2 are shown below.

Some trial and error may be required before the best combi-

nation is determined. The values listed are total capacitance

including parasitic or other sources. Remember that in the

10kHz to 100kHz frequency range setting the CDP6872

switches in two internal 15pF capacitors.

Crystal Pullability

Figure 4 shows the basic equivalent circuit for a crystal and

its loading circuit.

FIGURE 4.

Where:

If loading capacitance is connected to a Series Mode Crys-

tal, the new Parallel Mode frequency of resonance may be

calculated with the following equation:

Where:

In a similar way, the Series Mode resonant frequency may

be calculated from a Parallel Mode crystal and then you may

calculate how much the frequency will “pull” with a new load.

CRYSTAL

FREQUENCY

LOAD CAPS

C1, C2

TRIMMER CAP

C3

32kHz

33pF

5-50pF

1MHz

33pF

5-50pF

2MHz

25pF

5-50pF

4MHz

22pF

5-100pF

2

OSC IN

3

OSC OUT

C

CL

1

1

C

1

-------

1

C

2

-------

+





-----------------------------

Equivalent Crystal Load

==

F

P

F

S

1

C

M

2C

0

C

CL

+





-------------------------------------

+

=

Layout Considerations

Due to the extremely low current (and therefore high imped-

ance) the circuit board layout of the CDP6872 must be given

special attention. Stray capacitance should be minimized.

Keep the oscillator traces on a single layer of the PCB. Avoid

putting a ground plane above or below this layer. The traces

between the crystal, the capacitors, and the OSC pins

should be as short as possible. Completely surround the

coupling with any digital signals. The final assembly must be

free from contaminants such as solder flux, moisture, or any

other potential source of leakage. A good solder mask will

help keep the traces free of moisture and contamination over

time.

Further Reading

Al Little “HA7210 Low Power Oscillator: Micropower Clock

Oscillator and Op Amps Provide System Shutdown for

Battery Circuits”. Intersil Application Note AN9317.

Robert Rood “Improving Start-Up Time at 32KHz for the

HA7210 Low Power Crystal Oscillator”. Intersil Application

Note AN9334.

S. S. Eaton “Timekeeping Advances Through COS/MOS

Technology”. Intersil Application Note ICAN-6086.

E. A. Vittoz et. al. “High-Performance Crystal Oscillator cir-

cuits: Theory and Application”. IEEE Journal of Solid-State

Circuits, Vol. 23, No3, June 1988, pp774-783.

M. A. Unkrich et. al. “Conditions for Start-Up in Crystal Oscil-

lators”. IEEE Journal of Solid-State Circuits, Vol. 17, No1,

Feb. 1982, pp87-90.

Marvin E. Frerking “Crystal Oscillator Design and Tempera-

ture Compensation”. New York: Van Nostrand-Reinhold,

1978. Pierce Oscillators Discussed pp56-75.