5

Crystal Selection

For general purpose applications, a Parallel Mode Crystal is

a good choice for use with the HA7210. 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

capacitance and appear as a near resistive impedance when

oscillating. 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 HA7210 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,

automatically 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 HA7210 in the lowest range, the effective

loading capacitance 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

manufactured for a different load will be “pulled” from its

nominal frequency (see Crystal Pullability).

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

tuning 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

frequency 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

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

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

preferred in applications where cost or space is critical. Note

that in both cases the crystal loading capacitors are

as an AC ground. The Simplified Block Diagram shows that

ground available.

Typical values of the capacitors in Figure 2 are shown

below. Some trial and error may be required before the best

combination is determined. The values listed are total

capacitance including parasitic or other sources. Remember

that in the 10kHz to 100kHz frequency range setting the

HA7210 switches in two internal 15pF capacitors.

HA7210

+5V

XTAL

2

OSC IN

3

OSC OUT

1

FIGURE 2.

+

-

CRYSTAL

FREQUENCY

LOAD CAPS

TRIMMER CAP

32kHz

33pF

5pF to 50pF

1MHz

33pF

5pF to 50pF

2MHz

25pF

5pF to 50pF

4MHz

22pF

5pF to 100pF

HA7210

+

-

+5V

XTAL

2

OSC IN

3

OSC OUT

1

FIGURE 3.

HA7210