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External crystal load capacitance

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I'm attempting to compute values for the load caps I need.  I understand the equation is C1 = C2 = 2(CL - Cstray)...but I'm having a problem figuring out exactly what to use for Cstray.  Looking at the data sheet for my mcu (FR5739), it says that the integrated effective load capacitance is 1 pF, but there is a little note that says "Include parasitic bond and package capacitance(approximately 2pF per pin)".


Does this imply that Cstray is 5pF (2* 2pF + 1pF)?




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Hi @@mbeals!


The effective load capacitance (1pf) value is really the impact on the effective load, the impact that is coming from the 2pF that is on each pin - so the two notes are really describing the same thing. Basically, there's 2pF on each pin, which is 1pF when taken as a whole for the crystal (it's how the value comes out depending on which side of the equation you put it on - on the side where you are plugging in per-pin values, or the side where you're getting your effective load capacitance).


If you haven't seen it yet, a good resource is the MSP430 32kHz crystal oscillators app note: www.ti.com/lit/pdf/slaa322

Section 2.1 includes an example of how to calculate your required capacitance. If you use this equation, the Cparasitic is referring to the per pin (2pF) value, so that's what you'd plug in.



Crystal requires 12 pF load.
Parasitic capacitance per pin is 2 pF.
CL1 = (2 x CLoad)

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