I'm using a dsPIC30F4011 in my project and I want to put a 6MHz XTAL between OSC1 and OSC2 pins. The circuitry goes like this:


I want to know the values of C1 and C2 in the circuit, and I read somewhere that the formula is:



The XTAL I've bought came with this datasheet. There it says:


The thing is that the exact load capacitance is not specified, it depends on which XTAL I buy. But in the one I've bought there is no serial code or something that could tell me the specific I need. Although, in the datasheet appears an almost specific "effective series resistance" (100 Ohm max for 6MHz).

So, is there a way to get the specific load capacitance of a crystal resonator knowing its effective series resistance?

Edit (Thanks to SamGibson): Is there a way, method or circuit to get the load capacitance of an XTAL?


2 Answers 2


Load capacitance and crystal ESR are unrelated, these are two different characteristics and design parameters.

The load capacitance is the necessary parallel load for a crystal to produce its specified resonant frequency. This value is designed in by manufacturer. By virtue of Pierce oscillator schematics, the load capacitance is split into two in-series caps, so the C1/C2 values must be a doubles of the required load, and must be corrected for parasitic capacitance of traces, pads, and IC pins. If the effective load capacitor deviates from specifications, the oscillator frequency will deviate slightly.

The ESR is a measure of crystal quality, and depends on particular crystal geometry and materials used for fabrication, metal coating, and wire bond support. The ESR value is important for starting and maintaining circuit oscillations. If the ESR is too high, the circuit will fail to start. The criteria for safe stable operations is that the crystal ESR should be 5 times smaller than so-called "negative resistance" of the circuit. The necessary margins can be easily determined by putting a non-inductive variable resistor in-series with crystal, and finding at what value the oclillations stops. This value of extra resistor should be about 5x of the expected ESR of crystal.

For more explanations and details, please consult with corresponding application notes.

ADDITION: if you don't have technical specifications for the particular crystal, you need to characterize the crystal. If it oscillates in your circuit, it is relatively easy - you need to measure the resulting frequency at the OUTPUT of xtal circuit using high-impedance low-capacitive probe (<1pF). Then compare the frequency with nominal. It will be a fairly delicate measurements, since crystal pullability is usually just ~50 ppm for 10 pF of load mismatch. For more hints check these posts, 1, 2, and 3.

If it doesn't oscillate, reduce caps until it does, or you need to get a special test board with high-quality driver to do the job.


On page 2 of your crystal datasheet is the decoder ring for the crystal part number. One of those part number elements is the specific load capacitance of the specific part. So you will have to be sure to order the specific part number with the specific load capacitance that you need.

It would be nice if they gave you a table of the valid part numbers - maybe they have another document for that.

  • \$\begingroup\$ yup, but in the local electronics store they call them XTAL-6 or whatever, and they do not give you any manufacturer part number. so it won't be useful. "But in the one I've bought there is no serial code..." \$\endgroup\$
    – Iaka Noe
    Sep 11, 2017 at 22:03
  • \$\begingroup\$ @IakaNoe - "they do not give you any manufacturer part number" - So are you saying that the datasheet you linked is not the one for your crystal, because there is no manufacturer part number on your crystal? And you cannot provide the datasheet for your crystal. Is that all correct? \$\endgroup\$
    – SamGibson
    Sep 11, 2017 at 22:23
  • \$\begingroup\$ @SamGibson sorry if I wasn't self-explanatory. they give me the linked datasheet, that is like universal for that model. but I don't know the specific part number of my specific xtal, to get from it the CL as said in the datasheet page 2 \$\endgroup\$
    – Iaka Noe
    Sep 11, 2017 at 22:26
  • \$\begingroup\$ @IakaNoe - Thanks for explaining. "they give me the linked datasheet, that is like universal for that model" The crystals from that datasheet would have a specific part number marked on them, as shown on page 2. Since your crystal has no part number, how do you know that the datasheet applies to your crystal? Despite what you have been told, I see no reason to believe that it is the correct datasheet for your crystal, since your crystal has no part number on it. :-( Therefore IMHO your question becomes: How do you find the correct load capacitance for a crystal without a datasheet? :-( \$\endgroup\$
    – SamGibson
    Sep 11, 2017 at 22:38
  • \$\begingroup\$ @SamGibson i was just about to edit and add that question \$\endgroup\$
    – Iaka Noe
    Sep 11, 2017 at 22:39

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