First, a general understanding of what Crystal:
Oscillator crystal resonator is generally known, is an electromechanical device, power loss is small quartz crystals by precision grinding and cutting the upper electrode plated solder tails made. This crystal has a very important feature, if you give him energized, he will have a mechanical oscillation, on the contrary, if you give him a mechanical force, he will produce electricity, this feature is called electromechanical effects. They have a very important feature, the oscillation frequency is closely related to their shape, material, and other cutting direction. Since quartz is very stable chemical properties, the thermal expansion coefficient is very small, the oscillation frequency is very stable, since the control can be done very precise geometry, and therefore, its resonance frequency is also very accurate.
A quartz oscillator crystal is short, English called the Crystal, a clock circuit which is the most important member, its role is to provide a reference frequency to each part of the graphics card, network card, motherboard and other parts, it is like a scale, the operating frequency instability will cause the device to work-related frequency instability, naturally prone to problems. Due to the continuous improvement of manufacturing processes, are now crystal frequency deviation, temperature stability, aging rate, tightness and other important technical indicators are good, has not prone to failure, but the selection can still look at the quality of the crystal.
Crystal Oscillator Function
Crystal role of specific applications, the microcontroller clock source can be divided into two categories: Clock Source mechanical resonant devices, such as crystals, ceramic resonators; the RC (resistance, capacitance) oscillator. One is a Pierce oscillator arranged for the crystal and ceramic resonators. Another simple discrete RC oscillator. Typically provide very high initial accuracy and low temperature coefficient based on a crystal oscillator and a ceramic resonant tank. RC oscillator fast startup, relatively low cost, but generally suffer from poor accuracy over temperature and operating voltage range, varies in the range of 5-50% of the nominal output frequency. However, its performance is affected by environmental conditions and the circuit element selection. Need seriously oscillator circuit board layout and component selection. In use, the ceramic resonators and their respective load capacitance must be optimized according to a particular logic family. Crystal oscillator having a high Q value is not sensitive to the selection of the amplifier, but it is susceptible to frequency drift overdrive (even damage).
Environmental factors affecting the oscillator work are: electromagnetic interference (EMI), mechanical vibration and shock, humidity and temperature. These factors will increase the output frequency changes, increased instability, and in some cases, can also cause the oscillator to stop. Most of the above problems can be avoided by using the oscillator module. The module has its own oscillator to provide low impedance square wave output, and to ensure the operation under certain conditions. The two most common types are integrated RC oscillator and the crystal oscillator module (silicon oscillators). Crystal modules provide the same discrete crystal accuracy. Silicon oscillator accuracy than discrete RC oscillator is high, can be provided in most cases with ceramic resonator reasonable precision.
Consideration of oscillator selection power. Discrete power oscillator is primarily determined by the capacitance value of the internal supply current and a feedback amplifier circuit. CMOS power amplifier proportional to the operating frequency, power dissipation can be expressed as the capacitance value. For example, the capacitance value of the power dissipation of the inverter gate circuit HC04 is 90pF. When working in 4MHz, 5V power supply current of 1.8mA equivalent. 20pF load capacitance coupled with the crystal, the entire supply current of 2.2mA. Usually ceramic resonator having a large load capacitance, accordingly requires more current. In contrast, crystal module generally requires a supply current of 10mA ~ 60mA. Silicon oscillator supply current depends on its type and function, and can range from a low of a few microamps to milliamps programmable devices (fixed) devices. A low-power silicon oscillator, such as the MAX7375, only work when a current of 2mA than 4MHz. Optimization clock source in a particular application needs to consider the following factors: accuracy, cost, power consumption, and environmental requirements.
Crystal Oscillator Applications
1, common crystal oscillator used in various circuits, oscillation frequency.
2, clock pulses of quartz crystal resonators, and other elements with a pulse signal to generate a standard, widely used in digital circuits.
3, the microprocessor with a quartz crystal resonator.
4, CTVVTR using quartz crystal resonators.
5, a watch crystal oscillator.
Why crystal to increase capacitance
1, in order to satisfy the resonance conditions. Specifically that: the load capacitance of the crystal element is the total effective capacitance across the outside across the crystal in the circuit. It refers to the normal shock crystal capacitance required. General external capacitor, are provided so that both ends of the oscillator equivalent capacitive load capacitance is equal or close. Not all crystal oscillation circuit are required matching capacitance. Whether the oscillation circuit is determined by the form, it requires the use of an equivalent model of a crystal analysis.
2, the ground: two ground next capacitors crystal capacitance of the capacitive voltage division is actually three-point circuit, dividing points is a ground point. I.e. the Ground dividing point as a reference point, the input and output pins of the oscillation is inverted, but both ends of the parallel resonant circuit of view i.e. quartz, form a positive feedback circuit continues to oscillate in order to ensure
Of course, you can also interpret it this way:
Crystal nominal value when there is a test condition "load capacitance", to meet this condition at work, the oscillation frequency was consistent with the nominal value, that is, only connect the appropriate crystal capacitance to meet the start-up requirements, crystal to work properly.
Crystal needs with how much capacitance
This is to be determined according to the crystal size and circuit elements, with a 16MHZ crystal resonator, its load capacitance values may differ, as 10PF, 20PF load capacitance is determined in its production process, can not be change. You should be able to get accurate specifications when buying crystal.
When the crystal oscillator used in the circuit should satisfy CL = C + CS.
CL is the load capacitance of the crystal Specifications and, C is the capacitance of an external circuit (typically obtained from the serial-parallel relationship by means of two capacitors), CS is the distributed capacitance of the circuit, and this circuit design factors, the distribution of components the relevant value is indeterminate, usually 3 to 5PF.
So the above formula can probably calculate the capacitance values should be used, and this capacitance value can make crystal work at its nominal frequency nearby.
Such as: C51 microcontroller, 11M crystal with 22pF capacitor is, in fact, no need to size the capacity of more than accurate, with 33pF also entirely possible, a few pF to tens of pF you can.