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I am designing a very low power portable device that has an RTC and I cannot choose a crystal for it. Most crystals on the market have 12.5pF load capacitance, however there are also 6 or 4pF crystals which are made in much smaller volume for some reason, and this made me wonder why there's less demand for them. The obvious tradeoff for decreased consumption in lower-capacitance crystals is increased sensitivity to stray capacitance and reduced noise immunity but how significant these trade-offs are in practice, given the PCB will not get covered in dirt/dust and such? Are there any solid reasons to use a 12.5pF vs 6pF crystal?

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  • \$\begingroup\$ Capacitance is harder to control the smaller it gets \$\endgroup\$
    – Voltage Spike
    Commented Mar 18 at 4:50

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Once standards are established they can be quite “sticky”, take the example of supply voltages where 1960s 5V is still with us, even though internally many chips use much lower voltages.

Unless you are in a position to influence de facto standards (meaning a reliable customer requiring enormous quantities) the market makes the decisions for you. You can live with a couple hundred nA more consumption and a cheap and easily available (in a variety of packages) crystal or specify something that’s less available and likely costlier. Others will make similar decisions.

Lower capacitance is easier to put on a chip, so that may have some influence. Another factor is that you may wish to have a trimcap to adjust the frequency for accurate timekeeping (some more modern RTC hardware provides digital corrections that achieve a similar effect even with a slightly inaccurate clock). That may require a larger load capacitance to get adequate adjustment range.

Of course the crystal manufacturers can make the crystals tuned for whatever load capacitance the market demands. Probably even custom one-offs, at relatively high cost.

In this particular case, 6pF seems relatively available, but 5, 4, 3 and even 1.2pF have been made in quantity. Sometimes specifying a part that is single-sourced and requires a factory order of a certain (large) size every single time is the way to go, but it is rare.

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Few things to take into account:

  1. Higher CL usually means longer startup times.
  2. It is easier to find a 22pF capacitor (+1 or 2 pf for parasitic capacitance) and get 12.5pF CL. 10pF and 8pF tolerances are usually measured in pF, not %, and designing a circuit with 0.5 to 1pF parasitic capacitance is more difficult.
  3. 12.5pF Crystals are more common than the alternatives.
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