AN673 Datasheet, PDF (1/3 Pages) Silicon Laboratories – PRECISION32™ SOFTWARE DEVELOPMENT KIT

AN673
Application note
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.
Figure 1. Standard Circuit.
Figure 2. Modified Circuit.
Vin
Cin
Vout
Cout
ST62
OSCin
C2
R _OSCout
Q
C1
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