5

CDP6872

Crystal Selection

For general purpose applications, a Parallel Mode Crystal is

a good choice for use with the CDP6872. However for

applications where a precision frequency is required, the

designer needs to consider other factors.

Crystals are available in two types or modes of oscillation,

Series and Parallel. Series Mode crystals are manufactured

to operate at a specified frequency with zero load capaci-

tance and appear as a near resistive impedance when oscil-

lating. Parallel Mode crystals are manufactured to operate

with a specific capacitive load in series, causing the crystal

to operate at a more inductive impedance to cancel the load

capacitor. Loading a crystal with a different capacitance will

“pull” the frequency off its value.

The CDP6872 has 4 operating frequency ranges. The higher

three ranges do not add any loading capacitance to the

oscillator circuit. The lowest range, 10kHz to 100kHz, auto-

matically switches in two 15pF capacitors onto OSC IN and

OSC OUT to eliminate potential start-up problems. These

capacitors create an effective crystal loading capacitor equal

to the series combination of these two capacitors. For the

CDP6872, in the lowest range, the effective loading capaci-

tance is 7.5pF. Therefore the choice for a crystal, in this

range, should be a Parallel Mode crystal that requires a

7.5pF load.

In the higher 3 frequency ranges, the capacitance on OSC

IN and OSC OUT will be determined by package and layout

parasitics, typically 4 to 5pF. Ideally the choice for crystal

should be a Parallel Mode set for 2.5pF load. A crystal man-

ufactured for a different load will be “pulled” from its nominal

frequency (see Crystal Pullability).

FIGURE 2.

CDP6872

+

-

+5V

VREG

XTAL

2

OSC IN

3

OSC OUT

1

Frequency Fine Tuning

Two Methods will be discussed for fine adjustment of the

crystal frequency. The first and preferred method (Figure 2),

provides better frequency accuracy and oscillator stability

than method two (Figure 3). Method one also eliminates

start-up problems sometimes encountered with 32kHz tun-

ing fork crystals.

For best oscillator performance, two conditions must be met:

the capacitive load must be matched to both the inverter and

crystal to provide ideal conditions for oscillation, and the fre-

quency of the oscillator must be adjustable to the desired

frequency. In Method two these two goals can be at odds

with each other; either the oscillator is trimmed to frequency

by de-tuning the load circuit, or stability is increased at the

expense of absolute frequency accuracy.

Method one allows these two conditions to be met indepen-

frequency at which the circuit oscillates without appreciably

changing the load (and thus the stability) of the system.

type of crystal being used, it could be replaced with a fixed

capacitor. For the most precise control over oscillator fre-

This three capacitor tuning method will be more accurate

and stable than method two and is recommended for 32kHz

tuning fork crystals; without it they may leap into an overtone

mode when power is initially applied.

Method two has been used for many years and may be pre-

ferred in applications where cost or space is critical. Note

that in both cases the crystal loading capacitors are con-

AC ground. The Simplified Block Diagram shows that the

ground available.

FIGURE 3.

CDP6872

+

-

+5V

VREG

XTAL

2

OSC IN

3

OSC OUT

1