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Introduction
1.12 Crystal Oscillator Consideration
Samsung ASIC
1-47
STDM110
1.12.2.3 Oscillation Frequency
The oscillation frequency is determined 99.5% by the crystal and up to about
0.5% by the circuit external to the crystal.
The on-chip amplifier has little effect on the frequency, which is as it should be,
since the amplifier parameterizes temperature and process dependent.
The influence of the on-chip amplifier on the frequency is by means of its input
and output (pin-to-ground) capacitances, which parallel C1 and C2, and the
PADA-to-PADY (pin-to-pin) capacitance, which parallels the crystal. The input
and pin-to-pin capacitances are about 7pF each.
Internal phase deviations capacitance of 25 to 30pF. These deviations from the
ideal have less effect in the positive reactance oscillator (with the inverting
amplifier) than in a comparable series resonant oscillator (with the non-inverting
amplifier) for two reasons: first, the effect of the output capacitor; second, the
positive reactance oscillator is less sensitive, frequency-wise, to such phase
errors.
1.12.2.4 C1 / C2 Selection
Optimal values for the capacitors C1 and C2 depend on whether a quartz crystal
or ceramic resonator is being used, and also on application-specific
requirements on start-up time and frequency tolerance.
Start-up time is sometimes more critical in microcontroller systems than
frequency stability, because of various reset and initialization requirements.
Less commonly, accuracy of the oscillator frequency is also critical, for example,
when the oscillator is being used as a time base. As a general rule, fast start-up
and stable frequency tend to pull the oscillator design in opposite directions.
Considerations of both start-up time and frequency stability over temperature
suggest that C1 and C2 should be about equal and at least 15pF. (But they don’t
have to be either.)
Increasing the value of these capacitances above some 40 or 50pF improves
frequency stability. It also tends to increase the start-up time. These is a
maximum value (several hundred pH, depending on the value of R1 of the quartz
or ceramic resonator) above which the oscillator won’t start up at all.
If the on-chip amplifier is a simple inverter, the user can select values for C1 and
C2 between some 15 and 50pF, depending on whether start-up time or
frequency stability is the more critical parameter in a specific application.
1.12.2.5 Rf / Rx Selection
A CMOS inverter might work better in this application since a large Rf (1mega-
ohm) can be used to hold the inverter in its linear region.
Logic gates tend to have a fairly low output resistance, which testabilizes the
oscillator. For that reason a resistor Rx (several k-ohm) is often added to the
feedback network, as shown in Figure 1-19.
At higher frequencies a 20 or 30pF capacitor is sometimes used in the Rx
position, to compensate for some of the internal propagation delay.