Application of crystal oscillator circuit

Almost all digital time circuits are used in crystals, the following is a brief description of the crystal working principle and application considerations.

How does the chip produce the clock? The PIN definition has xtal_in and xtal_out, in fact the internal can be equivalent to an inverter analysis. The inverter is further equivalent and can be converted into an oscillating three-point in the analog circuit.

The vibration condition and the balance can refer to the university's analog electronic textbook. As for the amplitude after the vibration, I find a lot of books are not detailed analysis, are a stroke or slightly.

For crystal selection, "Load capacitance", "Operating temperature range" and "frequency accuracy" are generally considered. In the process, the higher the frequency of the crystals, the smaller the package. In industrial-grade applications, the "aging" should also be considered.

Load Capacitance cl = c1//c2 + CIN. CIN for chip internal capacitance and PCB traces, should refer to the chip specification to estimate CIN confirm C1 and C2, general situation c1=c2.

In layout, the crystal should be careful to avoid placing the temperature changes in the position, avoid frequency offset, while away from the source of interference. In a 4pin package, it is best to ground the housing angle to prevent EMI problems.

The crystal waveform of a SOC is measured below.

Xtal_in waveforms such as:

Xtal_out waveforms such as:

It can be seen that the waveform amplitude of xtal_in is small and not distorted. The waveform amplitude of the xtal_out is large and distorted.

The reason is that the inverter/transistor is a non-linear device amplification distortion, xtal_in after the crystal and capacitor feedback frequency selection to obtain an undistorted waveform.

Assuming that circuit a needs to use the frequency of crystal resonance, the xtal_in/xtal_out can be used as the clock input of circuit a.

1: When circuit A's clock pin input impedance is large, can be directly connected to xtal_in output to a, but the conservative approach is to add a buffer on the xtal_in;

2: When the clock of circuit A does not require a strict sine wave, it can be directly connected to the Xtal_out output to a.

Application of crystal oscillator circuit