12.5PF crystal oscillator 32.768KHz external matching capacitor is appropriate?

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(32.768KHz crystal oscillator specifications)

 

12.5PF crystal oscillator 32.768KHz external capacitance should be matched? The instructions are as follows:

 

For example, according to the data sheet of the STM32L011 chip, it can be seen that the chip can be connected to an external 32768Hz passive crystal oscillator as the RTC clock signal source, and the specific electrical parameters of the crystal oscillator are as follows:

• Nominal frequency: 32.768KHz

• Load capacitance: 12.5PF

• Adjust the frequency difference: ±20PPM

Since there is a certain amount of stray capacitance on the circuit board (generally calculated according to 3~5PF), we recommend two external capacitors of 15~18PF respectively. Because the design of different circuit boards is different, there are also cases where the crystal oscillator is in the best working state under the condition of external capacitance = 22PF. Therefore, the actual capacitance matching scheme is best determined according to the measured frequency offset degree of the crystal oscillator.

From a technical point of view, the larger the load capacitance, the more difficult it is for the crystal to vibrate. Conversely, the smaller the load capacitance, the easier it is for the crystal oscillator to vibrate. However, it is important to note here that external capacitors play two important roles: one is to improve the stability of the crystal frequency output, and the other is to improve the accuracy of the crystal frequency output. The specific performance is as follows: fine-tuning the external capacitor upwards, the stability of the output frequency of the crystal oscillator increases, and the frequency decreases. If you fine-tune the external capacitor downward, the stability of the output frequency of the crystal oscillator will decrease and the frequency will rise. Therefore, the external capacitor is not as big as possible, or as small as possible, but rather to achieve the best match.

(Jingcheng Technology passive chip crystal oscillator SMD3215 32.768KHz electrical parameters)

From the above, it can be seen that when some engineers match two 27PF external capacitors for the 32768Hz passive crystal oscillator with load capacitance CL=12.5PF, it is easy to have the problem of difficult or non-vibration of crystal oscillator.

Summary:

• Load capacitance (CL) refers to the electrical parameters of the passive crystal oscillator (crystal resonator) itself, and is not an external shunt capacitor (external capacitor/ground capacitor).

• If the load capacitance of the passive crystal oscillator CL=12.5PF, considering the distributed stray capacitance of 3~5PF, the external capacitance of 15~22PF is recommended.

• When the external capacitor increases, the output frequency of the crystal oscillator decreases, and conversely, when the external capacitor decreases, the output frequency of the crystal oscillator increases. When the external capacitance is too large, it is easy to cause the crystal to not vibrate.

Further reading:

Description of Jingcheng Technology's 32.768KHz passive crystal oscillator load capacitance and matching capacitor:

If CL=12.5pF, C1=C2=15pF~22pF is recommended;

If CL=9pF, C1=C2=12pF~15pF is recommended;

If CL=7pF, C1=C2=10pF~12pF is recommended;

If CL=6pF, C1=C2=8pF~9pF is recommended.