Crystal oscillator function
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The role of the crystal oscillator in the application of the specific, the microcontroller's clock source can be divided into two categories: based on mechanical resonator clock source, such as crystal, ceramic resonator tank; RC (resistor, capacitor) oscillator. One is the Pierce oscillator configuration for crystal and ceramic resonator tanks. The other is a simple discrete RC oscillator. Oscillators based on crystal and ceramic resonant tanks usually provide very high initial accuracy and a low temperature coefficient. The RC oscillator can be started quickly and at a low cost, but is usually less accurate over the entire temperature and operating supply voltage range, and can vary from 5% to 50% of the nominal output frequency. However, its performance is affected by the environmental conditions and the choice of circuit components. The component selection and board layout of the oscillator circuit need to be taken seriously. When used, the ceramic resonant tank and the corresponding load capacitance must be optimized for the particular logic series. 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) when overdriven. The environmental factors that affect the operation of the oscillator include electromagnetic interference (EMI), mechanical vibration and shock, humidity, and temperature. These factors increase the change in output frequency, increase instability, and, in some cases, cause the oscillator to stop. Most of the above problems can be avoided by using an oscillator module. These modules come with an oscillator, provide a low-resistance square-wave output, and are guaranteed to operate under certain conditions. The two most commonly used types are the crystal oscillator module and the integrated RC oscillator (silicon oscillator). The crystal module provides the same accuracy as a discrete crystal. The accuracy of the silicon oscillator is higher than that of the discrete RC oscillator, and in most cases it can provide the same accuracy as the ceramic resonant tank.
Power consumption also needs to be considered when selecting an oscillator. The power consumption of the discrete oscillator is mainly determined by the power supply current of the feedback amplifier and the internal capacitance of the circuit. The power consumption of a CMOS amplifier is proportional to the operating frequency and can be expressed as a power dissipation capacitor value. For example, the power dissipation capacitance of the HC04 inverter gate is 90 pF. When operating at a 4MHz, 5V supply, it is equivalent to a 1.8mA supply current. Together with a 20-pF crystal load capacitor, the entire supply current is 2.2mA. Ceramic resonant tanks generally have larger load capacitances and accordingly require more current. In contrast, a crystal oscillator module generally requires a supply current of 10 mA to 60 mA. The supply current of the silicon oscillator depends on its type and function, ranging from a few microamperes of a low-frequency (fixed) device to several milliamps of a programmable device. A low-power silicon oscillator, such as the MAX7375, requires less than 2mA of current when operating at 4MHz. Optimizing the clock source for a specific application requires a combination of the following factors: accuracy, cost, power consumption, and environmental requirements.
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