Reducing current consumption at 32 kHz with ST62

INTRODUCTION

In many cases a 32kHz crystal is chosen for the oscillator of the ST62 microcontroller in order

to achieve the minimum current consumption in the application.

This note provides a technique for minimising the current consumption when using a crystal

oscillator at this frequency.

The note should be read in conjunction with Application Note AN670 - Oscillator Selection for

the ST62.

1 CIRCUIT MODIFICATION

A standard oscillator circuit is shown in Figure 1. This circuit is suitable for higher frequency

oscillations up to the maximum oscillation frequency of the ST62.

The current consumption of the oscillator of the ST62 can be reduced effectively when a

resistor is added in the circuit as shown in Figure 2.

.

t

i

u

c

r

i

C

d

e

i

f

i

d

o

M

.

2

e

r

u

g

i

F

.

t

i

u

c

r

i

C

d

r

a

d

n

a

t

S

.

1

e

r

u

g

i

F

Cout

Cin

Vin

Vout

ST62

OSCin

Q

R

OSCout

The Resistor (R) limits the power dissipation through the crystal, resulting in a lower load for

the inverter circuit and a minimum power consumption.

Note that the resistor adds a phase shift to the Capacitor and oscillator network. This shift must

not be excessive such that it affects the loop functionality.

August 2008

Rev 2

1/3

AN673

Application note