ST AN670 Application note

October 2008 Rev 2 1/12

AN670

Application note

Oscillator selection for ST62

Introduction

The purpose of this note is to give indications on how to choose a resonator or a quartz

crystal in order to achieve reliable oscillation with the ST62 Microcontroller. This document

provides first the major resonator parameters useful for a design. It then proposes

measurement methods to ensure a safe oscillation.

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Contents AN670

2/12

Contents

1 Oscillation frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Oscillation conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1 Barkhausen criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2 Start-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.3 Measurement of the loop gain (open loop) . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.4 Frequency stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.5 Start-up time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Appendix A Test of a CSA Murata crystal resonator with an ST6210xx . . . . . . . 8

A.1 Choice of the network capacitances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

A.2 Pseudo closed loop measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

A.3 Start-up time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

A.4 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Appendix B Calculation of the resonant frequency of ceramic resonator . . . . . 9

B.1 Equivalent circuit at the resonance frequency. . . . . . . . . . . . . . . . . . . . . . . 9

B.2 Transformation for simple calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

B.3 Resonant frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

B.4 Note. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

AN670 Oscillation frequency

3/12

1 Oscillation frequency

The resonator can be modelised by a serial/parallel oscillator circuit as described in

Figure 1.

The additional capacitances C

ext

are usually connected to the oscillator pins in order to

define a stable oscillating frequency. The value of these capacitances is usually given by the

manufacturer of the resonator.

The oscillation frequency is the resonant frequency of the equivalent circuit given in

Figure 2. The resonator is inductive in the oscillation frequency range.

Figure 1.

Resonator model Figure 2. Equivalent circuit

C

1

L

R

Co

C

1

L

C

ext

R

Co

Oscillation conditions AN670

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2 Oscillation conditions

The proposed method is based on the Barkhausen criteria. This leads to a safe result

providing that the oscillator fulfills these criteria. Three points have to be analysed: oscillator

start-up, frequency stability and the start-up time.

2.1 Barkhausen criteria

An oscillator can be modelized as defined in Figure 3. B is the resonator gain and G the

amplifier/inverter gain. The value of BxG defines the oscillator behaviour:

● BxG >> 1: square waveform, start-up OK

● BxG > 1: waveform with harmonic distortion, start-up OK

● BxG = 1: sine waveform, start-up critical

● BxG < 1: no oscillation

Figure 3. Oscillator model

2.2 Start-up

The oscillator can start if the gain BxG is above 1. The amplifier gain must compensate for

the resonator circuit attenuation and provide a sufficent gain margin (>3 dB).

In addition, the resonator circuit B must introduce a 180 ° phase delay if the G amplifier is an

inverter and no rotation if it is a non inverting amplifier.

With classical circuits such as a Pierce type oscillator (Figure 4.), the 180 ° phase rotation is

due to capacitances (C

out

and C

in

).

G

B

Input

Output

G: Inverter/Amplifier

B: Resonator circuit

Figure 4. Pierce type oscillator Figure 5. Equivalent schematic at the

resonant frequency

Vout

Vin

Cout

Cin

Cout

Cin

+

-

-

+

LR

Co