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AP242005 07.99
Crystal Oscillators of the
C500 / C166 Microcontroller Family
6
Start-up- and Oscillation Reliability
Most problems concerning the oscillator in a microcontroller system occur during the oscillation
start-up time. During start-up time the drive level of the oscillation is very small and is increased up
to the maximum. During that time the resistance of the crystal can reach high values because
crystals show resistance dips depending on the drive level and the temperature. This effect is called
drive level dependence (DLD). The DLD of a quartz crystal depends on the quality and can alter
during production and during the life time of the crystal. If the resistance dips of the crystal increase
in a range where the amplification of the oscillator is lower than one, than the oscillation cannot start.
Therefore
it is strongly recommended to check the start-up and oscillation reliability
.
This test is done with the negative resistance method.
For further details please refer to the following IEC standards:
IEC 122-2-1: Quartz crystal units for microprocessor clock supply
IEC 444-6:
Measurement of drive level dependence (DLD)
6.1
Principle of the Negative Resistance Method
The oscillator can be divided into the on-chip oscillator-inverter and the external circuitry. The
oscillator circuitry can be simplified as shown in figure 5. The load capacitance C
L
contains C
X1
, C
X2
and the stray capacitance C
S
. The amplification ability of the oscillator-inverter is replaced with a
negative resistance -R
INV
and the quartz crystal is replaced by the load resonance resistance R
L
(effective resistance) and the effective reactance L
Q
.
Figure 5 :
Equivalent Circuit for Negative Resistance Methode
C
S
C
X1
C
X2
Q
R
fint
C
L
-
R
INV
L
Q
R
L
Equivalent Circuit of Oscillator Circuit
Oscillator Circuit
C
L
R
Q
R
Q
Microcontroller
XTAL1
XTAL2