|Crystals and Resonators|
Types of crystals and resonators
Many digital components, microcontrollers and CPUs in particular, require some sort of timing signal or clock in order to operate. This signal must be stable, predictable and accurate in order for the dependant circuit to operate as expected.
Things like RC (resistor-capacitor) and astable 555 circuits may be sufficient for clock frequencies running into a few hundred or thousand cycles per second, but with the speeds of modern CPUs and micros running into millions and billions of clock cycles per second these methods become woefully inadequate. RC oscillators are susceptible to random error due to component tolerances, not to mention frequency drift with temperature changes due to the tendency of certain capacitor dielectrics to change their capacitance with temperature. RC circuits also produce a frequency that is a function of the supply voltage, which is obviously not desirable. While the 555 timer is somewhat better, having an output frequency independent of the supply when configured as an oscillator, but it is simply not fast enough for most modern applications. (the CMOS TLC555 is rated for operation up to 2MHz)
Types of crystals and resonators:
So what can be used instead? The two most common approaches are quartz crystals and ceramic resonators. Quartz crystals are known for high accuracy and stability with low frequency drift with temperature, while ceramic resonators are still quite accurate and stable they tend to suffer from frequency drift a little more with changes in temperature.
The frequency of quartz crystals can be accurately determined by trimming the crystal to a precise thickness during manufacture. Speeds range from few hundred Hz to tens of MHz, with common values being powers of two (such as 32768Hz, often used in real-time clocks and watches because 32KHz can be repeatedly divided by two to obtain 1Hz) or of 10. A quartz crystal and ceramic resonator are pictured below.
The use of crystals and resonators is quite simple, or at least for micros and CPUs which are designed to use such devices as a clock source. Usually, two pins are provided on the average microcontroller for connection of an external oscillator called something like "X1" and "X2", "XIN" and "XOUT", or "CLKIN" and "CLKOUT". All that is required is that the crystal be connected between the two pins and each pin be coupled to ground with a small ceramic capacitor, as in the schematic below.
Correct selection of capacitors is important for stability. Larger values (33p, 50p or more) give better stability but cause longer oscillator startup times which can lead to other problems. Usually the datasheet for the component concerned will contain some useful information for capacitor selection.
Using ceramic resonators is even easier because they have suitable capacitors built into the package. (thus the third lead, for the ground connection) In case you couldn't guess, a schematic is below.
Unless you want to build your own crystal driving circuits, (something which I must confess almost complete ignorance of) that's about all there is to using crystals or ceramic resonators as a clock source!
If you have any comments or questions please don't hesitate to contact me.
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