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I'm trying to make a pcb. I've seen this reference board from Figure 34. XTAL positioning of ST an5165 where they didn't add any capacitors to the HSE nx2016 crystal. Why? I tried to look on datasheets, but I didn't understand.

enter image description here

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  • \$\begingroup\$ Did u look at section 3.4 (External Crystal) of AN5165? \$\endgroup\$
    – Rodo
    Commented Jun 13 at 17:33
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    \$\begingroup\$ "Do all crystals need capacitors"? If you generalised the question like that, the short answer to that is yes, one way or another. Does STM32WBxx implementation need external capacitors, the answer to that is no based on section 3.4 of AN5165. \$\endgroup\$
    – hcheung
    Commented Jun 14 at 1:28

3 Answers 3

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Do all crystals need capacitors?

and

I tried to look on datasheets, but I didn't understand

You really must read the data sheet more carefully: -

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No. Not all crystal circuits need capacitors.

That's because different circuits are different. And crystals too.

Chips may have internal capacitors so you don't need external capacitors if you select a correct crystal. This is the case here. With another crystal, it may not work, so the crystal needs to be selected very carefully to be compatible with the chip.

Also sometimes different resonators such as ceramic resonators are used in place of crystals, and there are three-pin ceramic resonators that include the required capacitors for correct operation.

There are also crystals that are intended for "series" resonance circuit, and they do not use capacitors in a series resonance circuit. The load capacitance of a crystal is specified when you use the crystal in standard "Pierce" oscillator circuit. So you need to know which crystal is correct for your circuit.

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Some Microcontroller families have built in capacitors in their crystal oscillator circuits, either fixed or programmable values, to avoid the need for external capacitors. E.g. from the TI MSP430™ 32-kHz Crystal Oscillators application report:

All MSP430 MCU 32-kHz oscillators have built-in load capacitors, CL1 and CL2. In some MSP430 MCU versions, these load capacitors are fixed; in other MSP430 MCU versions, the internal load capacitor values can be programmed or external capacitors can be used. For details, see the data sheets and MSP430 MCU family user's guides. The various MSP430 MCU families have the following load capacitor configuration:

  • MSP430x1xx: 6 pF (fixed effective capacitance with 12 pF per pin), external capacitors are not recommended
  • MSP430F2xx: 0 pF to 12.5 pF (programmable effective capacitance), external capacitors are possible
  • MSP430F4xx: 0 pF to 10 pF (programmable effective capacitance), external capacitors are possible

Such manufacturer application reports may give guidance about PCB layout and component selection to give stable crystal oscillator operation.

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