Now that technology is developing rapidly, this article gives you an in-depth explanation of the working principle of Jing Zhen and hopes to help everyone.
Crystal oscillator
Computers have a timing circuit. Although the word "clock" is generally used to refer to these devices, they are not actually clocks in the usual sense. It may be more appropriate to call them timers. The computer's timer is usually a precision-machined quartz crystal that oscillates at a certain frequency within its tension limit, depending on how the crystal itself is cut and the amount of tension it receives. There are two registers associated with each quartz crystal, a counter and a holding register. Each oscillation of the quartz crystal decrements the counter by one. When the counter is decremented 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 generate 60 interrupts per second (or to generate an interrupt at any other desired frequency). Each interrupt is called a clock tick.
The crystal oscillator can be electrically equivalent to a capacitor and a resistor in parallel and then connected in series with a capacitor. The electrical network has two resonance points. The frequency is high and low, and the lower frequency is series resonance. The frequency is parallel resonance. Due to the characteristics of the crystal itself, the distance between the two frequencies is quite close. In this extremely narrow frequency range, the crystal oscillator is equivalent to an inductor, so as long as the crystal oscillator is connected in parallel with a suitable capacitor, it will form a parallel resonant circuit. . The parallel resonant circuit is added to a negative feedback circuit to form a sine wave oscillating circuit. Since the crystal oscillator is equivalent to a narrow frequency range of the inductor, even if the parameters of other components vary greatly, the frequency of the oscillator will not be a big change. The crystal oscillator has an important parameter, that is, the load capacitance value. Selecting the parallel capacitance equal to the load capacitance value can obtain the nominal resonant frequency of the crystal oscillator. The general crystal oscillator circuit is connected to the crystal oscillator at both ends of an inverting amplifier (note that the amplifier is not an inverter), and then two capacitors are respectively connected to the two ends of the crystal oscillator, and the other end of each capacitor is connected. Ground, the capacitance value of these two capacitors in series should be equal to the load capacitance. Please note that the pins of the general IC have equivalent input capacitance, which cannot be ignored. The load capacitance of a typical crystal oscillator is 15p or 12.5p. If you consider the equivalent input capacitance of the component pins, it is better to have two 22p capacitors to form the crystal oscillator.
Crystal-function
The role of the crystal oscillator in the application, the clock source of the microcontroller can be divided into two categories: based on the mechanical resonant device clock source, such as crystal oscillator, ceramic resonant tank circuit; RC (resistance, capacitor) oscillator. One is the Pierce oscillator configuration for crystal and ceramic resonant tanks. The other is a simple discrete RC oscillator. Oscillator based on crystal and ceramic resonant tanks typically provides very high initial accuracy and a low temperature coefficient. The RC oscillator can be started quickly and at a lower cost, but is typically less accurate over the entire temperature and operating supply voltage range and will vary from 5% to 50% of the nominal output frequency. However, its performance is affected by environmental conditions and circuit component selection. The component selection and board layout of the oscillator circuit must be taken seriously. When in use, the ceramic resonant tank and the corresponding load capacitance must be optimized for a specific logic family. A crystal with a high Q value is not sensitive to the choice of amplifier, but it is prone to frequency drift (and possibly even damage) during overdrive. Environmental factors that affect the operation of the oscillator are: electromagnetic interference (EMI), mechanical shock and shock, humidity and temperature. These factors increase the output frequency variation, increase instability, and in some cases, cause the oscillator to stop. Most of the above problems can be avoided by using the oscillator module. These modules come with an oscillator, provide a low-impedance square wave output, and are guaranteed to operate under certain conditions. The two most common types are crystal modules and integrated RC oscillators (silicon oscillators). The crystal module provides the same accuracy as a discrete crystal. Silicon oscillators are more accurate than discrete RC oscillators and, in most cases, provide comparable accuracy to ceramic resonant tanks.
Power consumption also needs to be considered when selecting 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 CMOS amplifier power consumption is proportional to the operating frequency and can be expressed as the power dissipation capacitor value. For example, the power dissipation capacitor value of the HC04 inverter gate is 90pF. When operating at 4MHz, 5V power supply, it is equivalent to 1.8mA supply current. Coupled with a 20pF crystal load capacitor, the entire supply current is 2.2mA. Ceramic resonant tanks typically have large load capacitances and correspondingly require more current. In contrast, crystal modules typically require a supply current of 10 mA to 60 mA. The power supply current of a silicon oscillator depends on its type and function, ranging from a few microamps of low frequency (fixed) devices to a few milliamps of programmable devices. A low-power silicon oscillator, such as the MAX7375, requires less than 2mA to operate at 4MHz. Optimizing the clock source for a specific application requires a combination of factors such as accuracy, cost, power consumption, and environmental requirements.
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