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description of crystal used
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Cryatals

12 MHz Crystal

A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. This frequency is commonly used to keep track of time (as in quartz wristwatches), to provide a stable clock signal for digital integrated circuits,The most important property of an oscillator is its frequency : the rate at which the output repeats. This is measured in Hertz (Hz for short). One Hertz is one repetition (aka cycle) per second. One MegaHertz (MHz) is one million repetitions per second. One of the problems in designing a high quality oscillator is maintaining the output frequency at the value required. One method is to control it by a quartz crystal; Crystals are mechanical oscillator and depending on the mass they will oscillate at a specific frequency.this is cut so that it vibrates mechanically at the design frequency, and is coupled to the electronics by the piezo-electric effect.

A 12 MHz crystal oscillator is an electronic circuit, whose output frequency is controlled by a quartz crystal to repeat 12 million times per second. When exited a crystal will oscillate at precise frequency due to its mechanical make up. An electronic oscillator external influences can make it to shift frequency very easily.Standard frequency crystals - use these crystals to provide a clock input to your microprocessor. Rated at 20pF capacitance and +/- 50ppm stability. Low profile HC49/US Package.12MHz is good for use with devices that need USB.
A quartz crystal provides both series and parallel resonance. The series resonance is a few kilohertz lower than the parallel one. Crystals below 300 MHz are generally operated between series and parallel resonance, which means that the crystal appears as an inductive reactance in operation. Any additional circuit capacitance will thus pull the frequency down. For a parallel resonance crystal to operate at its specified frequency, the electronic circuit has to provide a total parallel capacitance as specified by the crystal manufacturer.
A quartz crystal can be modeled as an electrical network with a low impedance (series) and a high impedance (parallel) resonance point spaced closely together. Mathematically (using the Laplace transform) the impedance of this network can be written as:

or,

where s is the complex frequency (s = jω), ωs is the series resonant frequency in radians per second and ωp is the parallel resonant frequency in radians per second.
Adding additional capacitance across a crystal will cause the parallel resonance to shift downward. This can be used to adjust the frequency at which a crystal oscillates. Crystal manufacturers normally cut and trim their crystals to have a specified resonance frequency with a known 'load' capacitance added to the crystal