Crystal is a circuit element commonly used in the clock, full name is called the crystal oscillator, crystal oscillator in the microcontroller the role of the system is very large, is a combination of MCU's internal circuitry, resulting in the need microcontroller clock frequency, single-chip implementation of all directives are built on this basis, the crystal clock frequency to provide higher speed and that the sooner SCM.
Crystal with a can into electrical energy and mechanical energy between the crystal in the resonant mode of operation in order to provide stable and accurate single frequency oscillation. In normal working conditions, the ordinary crystal oscillator frequency absolute accuracy of up to 50 millionths. High-level precision. Some crystal by the applied voltage can also be adjusted within a certain range of frequencies, known as voltage-controlled oscillator (VCO).
Crystal's role is to provide the basic system clock signal. Usually a system share a single crystal, easy to synchronize the various parts. Some of the fundamental frequency communication systems and RF using a different crystal and electronic means to adjust the frequency to keep pace.
Crystal is usually used in conjunction with the phase-locked loop circuit to provide the required system clock frequency. If different sub-systems need a different clock signal frequencies can be used with the same crystal connected to a different phase-locked loop to provide.
Here I will introduce the specific role of crystal, as well as the principle of crystal commonly used in Figure 1a, three-terminal type capacitor (Colpitts) to exchange the equivalent oscillation circuit; the actual exchange of crystal equivalent circuit shown in Figure 1b, which Cv is used to adjust the oscillation frequency, tends to be used with different varactor reverse bias voltage to achieve, which is the role of voltage-controlled mechanism; the crystal equivalent circuit instead of crystals shown in Figure 1c. Where Co, C1, L1, RR is the crystal equivalent circuit.