Why the crystal oscillator circuit with 22pf or 30pf capacitor instead of anything else.
In fact, the oscillation circuit of the microcontroller and other IC real name "three-point capacitive oscillating circuit", as shown
Y1 is a crystalline , three-point corresponding to the inside of the inductors , C1, and C2 is the capacitance of the NAND gate 5404 and the NPN R1 implement a transistor , then analyze this circuit.
5404 will need a resistor , or it is turned off in the saturation region rather than the enlarged area, Rl corresponds to the bias of the transistor, so that the enlarged region is 5404, then 5404 is an inverter, to achieve the effect of the NPN transistor, NPN transistor emitter electrodes when the inverter is also a connection in common.
We know the conditions to be a sinusoidal oscillation circuit is oscillating, the system magnification is greater than 1, this is easy to achieve, phase 360 degrees to meet with the same crystal oscillator frequency to a small oscillation is amplified. The next major phase explain this problem:
Since 5404 is an inverter, that is to achieve a 180 ° phase shift, then we need to C1, C2 and Y1 can achieve 180 ° phase shift, just when C1, C2, Y1 form a resonance, the phase shift can be realized 180 this we can solve the equation, etc., to do Y1 as an inductor. Capacitance and inductance characteristics can also be used, such as the capacitor voltage behind current 90 °, the inductor voltage leads the current by 90 ° analysis, are possible. When increases C1, C2 the amplitude emphasizing end, when lowering the C2, the amplitude is also enhanced. Sometimes C1, C2 can not weld start-up, this is not to say there is no C1, C2, but because the distributed capacitance caused by the chip pin, because originally the C1, C2 do not need a lot, so this is very important. Analysis of these two capacitors Next the oscillation stability.
Because the voltage feedback 5404 is to rely on, assuming C2 C2 is too large, the feedback voltage is too low, this is not stable, assuming that C2 is too small, the feedback voltage is too high, the stored energy is too small, vulnerable to external interference, radiation will affect the outside world . C1 is contrary to the effect of C2. Because when we layout, assuming double-sided , relatively thick, the distributed capacitance is not very big, it is assumed in the high-density multilayer boards , the need to consider the distributed capacitance.
Some of the projects for industrial, passive crystal method is not recommended to start-up, but directly connected to the active crystal. Is mainly due to the need to play a passive crystal oscillator reason, and industrial projects require better stability, it will be directly active crystal. Have the higher crystal frequency stability is not high, it is less demanding in the case of speed would use a lower frequency crystal oscillator.