What factors are related to the oscillator startup time?

In order to build a complete passive crystal oscillator circuit, the main parameters needed to be considered are: package size, frequency tolerance, frequency stability, load capacitance, external capacitance, operating temperature, drive level and insulation impedance.

In the crystal oscillator circuit, the most easily to be confused by the preliminary engineers are load capacitance and external capacitance. The load capacitance of the passive crystal oscillator is the inherent parameter of the crystal oscillator itself, which cannot be changed after the type selection is determined, such as 9PF, 12PF, 12.5pF, 20pF, etc. The external capacitance refers to the capacitance in the external circuit of the passive crystal oscillator. 

By adjusting the value of the external capacitance, the actual output frequency of the crystal oscillator can be fine tuned. The unit of capacitance is PF.

There is a certain relationship between the oscillator startup time of crystal oscillator and the value of external capacitance. The principle is that the larger the external capacitance is, the more current is consumed. That is, less drive level will be worked on the crystal oscillator itself, and the rising time of crystal oscillators will be longer.

In some digital circuit design, after the circuit board is powered on, the system needs to capture the clock signal of crystal oscillator to complete the diagnosis instruction. In this case, the oscillator startup time of the crystal oscillator should be prior to the system capturing the clock signal, otherwise the system will fail to run. 

Generally, the rising time for crystal oscillator is within 10ms.