The function and working principle of "crystal oscillator" in computer motherboard

The author:admin Number of visits: Date:2020-12-28 13:50

Computer motherboard typically have two or three semiconductor, many people asked about the mainboard crystals exactly is what effect, above the others in baidu know questions on this subject, I also had to answer, but still often see similar questions, especially a cylindrical crystals, someone focus mentioned what do you use this kind of crystals, I have to answer this kind of crystals is 32.768 KHZ, industry is referred to as "for the wafer table or inside crystal, a tuning fork at the same time, I have marked when it comes to this is the clock crystals, is used to control applications such as time, but still a lot of people look very fuzzy, very don't understand, simple to introduce below.Computers have a timing circuit, and although the word clock is commonly used to refer to these devices, they are not really clocks in the usual sense, and timers are perhaps more appropriate.A computer timer is usually a finely machined quartz crystal that oscillates within its tension limits at a frequency that depends on how the crystal itself is cut and how much tension it is subjected to.There are two registers associated with each quartz crystal oscillator, a counter and a hold register.Each oscillation of the quartz crystal subtracted 1 from the counter.When the counter decrements to 0, an interrupt is generated and the counter reloads the initial value from the hold counter.This approach makes it possible to program a timer to produce 60 interrupts per second (or any other desired frequency).Each interrupt is called a clocktick.The crystal oscillator can be electrically equivalent to a two-terminal network in which a capacitor and a resistor are connected in parallel and a capacitor is connected in series. In electrical engineering, this network has two resonant points, which are divided by the high and low frequencies, among which the lower frequency is series resonance and the higher frequency is parallel resonance.Due to the characteristics of the crystal itself, the distance between the two frequencies is quite close. Within this very narrow frequency range, the crystal oscillator is equivalent to an inductor. Therefore, as long as the two ends of the crystal oscillator are connected in parallel with appropriate capacitors, it will form a parallel resonant circuit.This parallel resonant circuit can be added to a negative feedback circuit to form a sinusoidal oscillator circuit, because the crystal oscillator equivalent to the inductor frequency range is very narrow, so even if the parameters of other components change greatly, the oscillator frequency will not change very much.There is an important parameter of crystal oscillator, that is the load capacitance value. The nominal resonant frequency of crystal oscillator can be obtained by choosing the parallel capacitance equal to the load capacitance value.General vibration crystal oscillation circuit are in an inverting amplifier (note the amplifier is not inverter) at both ends of the crystals, another two capacitance receives the ends of the crystals, respectively each capacitance on the other side of the receiving, the capacity of the two capacitors in series value should be equal to the load capacitance, please pay attention to general IC pins have the equivalent input capacitance, this can't be ignored.Generally, the load capacitance of the crystal oscillator is 15P or 12.5P. If the equivalent input capacitance of the component pin is considered, the oscillator circuit with two 22P capacitors forming the crystal oscillator is a better choice.


Crystal oscillator plays a specific role in the application, the clock source of the microcontroller can be divided into two categories: clock source based on mechanical resonant devices, such as crystal oscillator, ceramic resonator channel;RC(resistor, capacitor) oscillator.One is the Pierce oscillator configuration for crystal oscillators and ceramic resonator channels.The other is a simple discrete RC oscillator.Oscillators based on crystal oscillators and ceramic resonator channels usually provide very high initial accuracy and low temperature coefficients.RC crystal oscillators can start quickly and cost less, but are generally less accurate over the entire range of temperature and operating supply voltage, varying between 5% and 50% of the nominal output frequency.However, its performance is affected by the environmental conditions and the selection of circuit components.The selection of components and the layout of the circuit board of the oscillator should be taken seriously.When in use, the ceramic resonator slots and the corresponding load capacitance must be optimized for a specific logic series.Crystal oscillators with high Q values are not sensitive to the selection of amplifiers, but can easily cause frequency drift (or even damage) when overdriven.The environmental factors affecting the work of the oscillator are: electromagnetic interference (EMI), mechanical vibration and impact, humidity and temperature.These factors can increase the output frequency variation, increase instability, and in some cases, cause the oscillator to stop.Most of these problems can be avoided by using an oscillator module.These modules come with an oscillator, provide a low impedance square wave output, and can be guaranteed to operate under certain conditions.The two most commonly used types are crystal modules and integrated RC oscillators (silicon oscillators).The crystal oscillator module provides the same precision as the discrete crystal oscillator.The silicon oscillator has a higher accuracy than the vertical RC oscillator and in most cases can provide the same accuracy as the ceramic resonator channel.Power consumption also needs to be considered when choosing an oscillator.The power consumption of the discrete oscillator is mainly determined by the supply current of the feedback amplifier and the capacitance value inside the circuit.The power consumption of CMOS amplifier is proportional to the operating frequency, which can be expressed as the value of power dissipation capacitance.For example, the HC04 inverter gate circuit has a power dissipation capacitance of 90pF.When working under 4MHz, 5V power supply, it is equivalent to 1.8mA power supply current.Combined with a 20pF crystal load capacitor, the entire supply current is 2.2mA.Ceramic resonant channels generally have larger load capacitance and correspondingly require more current.In contrast, crystal oscillator modules typically require a power supply current of 10mA to 60mA.Depending on the type and function of a silicon oscillator, the supply current can range from a few microamperes for low-frequency (fixed) devices to a few milliamperes for programmable devices.A low-power silicon oscillator, such as the MAX7375, operates at 4MHz with less than 2mA of current.Optimizing a clock source for a specific application requires a combination of the following factors: accuracy, cost, power consumption, and environmental requirements.Electronic products under the current development trend, the vast majority of products will choose to use semiconductor vibration series of products, and choose ceramic crystals products are less and less, because there are a few points, one is the common plug-in semiconductor prices is not as high as he used to be, to the impact of the low price and high precision ceramic crystals in the market, the second stage is the degree of preciseness and semiconductor vibration is better than ceramic crystal vibration scope of heat-resistant, precision ceramic crystal vibration often can not reach the expected effect, all subsequent market ceramic crystals will be gradually phased out.