All crystal have a frequency of series resonant (Fs, at which the impedance of the crystal is minimum)
If a load capacitance (CL) is connected in series with the crystal, the oscillation frequency of the crystal will increase slightly. This frequency is called the frequency of parallel resonant or load frequency(FL).
Customers need to specify the value of the load capacitance (CL) when placing an order for a parallel resonant crystal.

Generally, the IC datasheet will recommend Crystal specifications and matching the Cin/Cout capacitance.
Cin/Cout is more accurate on board testing.
When the Load value is large, the relative frequency adjustable gain TS value is small.
On board, the frequency changes slightly due to spurious effects.
When the Load value is small, the relative frequency adjustable gain TS value is large.
On board, the frequency changes greatly due to spurious effects.
According to the application of the customer, you can choose whether the TS needs to be larger or smaller to determine the Load value.

After actual matching test, the best external capacitor can be matched. Make the frequency accurate on the board.
In addition, the Negative Resistance (-Rx) and Power drive consuming can be confirmed through matching test, reducing defects caused by poor matching after mass production.

Negative resistance refers to the impedance characteristic value of the oscillation circuit at the oscillation frequency when looking from the two terminals of the quartz crystal resonator to the oscillation circuit.
The oscillation circuit must provide sufficient amplification gain value to compensate for the mechanical energy loss of the quartz crystal resonator during resonance. Negative resistance is not a product parameter of the quartz oscillator, but it is an important characteristic parameter of the oscillation circuit. From the perspective of the resonator, it is the “negative resistance” of the oscillation circuit. Negative resistance is an important parameter for whether the oscillation circuit can sustain stable oscillation. Negative resistance is an indicator to judge the quality of oscillation amplitude. If the negative resistance of oscillation amplitude is not good enough, the amplitude maybe not oscillate due to aging, temperature, and voltage changes.

a. Too large a Drive Level will cause the circuit (or crystal) to age faster and consume more power. It will affect the electrical properties of Crystal and damage the electrical properties, the chip maybe break in serious cases .
b. If necessary, a current limiting resistor can be added to the Xtal output

a. Reduce the ground capacitance C1 // C2 value at both ends of the crystal
b. The current limiting resistor value Rd (at the Xtal output end), when increasing the resistance, the oscillation characteristics of the entire circuit must be considered, and the upper limit should not exceed 1KΩ.
If you want to increase or change the current limiting resistor, you should obtain the minimum Vpp specification required by the Chipset or inform the customer to obtain their consent and confirmation to avoid increasing the current limiting resistor and causing the Vpp to be too low and unable to drive the Chipset!

Crystal Start-up Time of Each Chipset manufacturer has its own definition requirements, Some ICs will require Crystal start-up time to comply with system operation.
By adjusting the XTAL C0 value or adjusting the external capacitance, the crystal start oscillation time can be effectively improved.
a. The same external capacitance, the XTAL with a larger C0 value has a shorter Start-up time.
b. When the C0 value is the same, If the smaller the external capacitance value then the Start-up time is shorter.