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Crystal with "load capacitance" or not?

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I wish to purchase a crystal for PIC18F4431. I've two options as follows:

[ a ] XT9M-NL-A-NA-10M

[ b ] XT9M-18-A-NA-10M

These devices are manufactured by Vishay and the datasheet can be downloaded here: http://www.vishay.com/doc?35012

The designation of the part# is shown in attached image.

I'm confused with the "load capacitance". Should I purchase Part [ a ] or Part [ b ]?

If I use Part [ a ], does the oscillation circuit need C1 and C2 as shown in Figure 2-1?



For reference to others: The PIC data sheets recommends a value of between 27pf and 22pf.

I'd go with the 18pF as specified by Vishay - this is how I've loaded crystals on PICs in the past and not had problems. Use the circuit as per PIC data sheet, that is two Cap's but no resistor, I've never needed it.

Would always check your osc on a scope and enable osc start up timer in the PIC (delays reset line while it becomes stable).

Also connect nice and close to chip with a good ground, not just a bit of wire or thin track, something solid.

Thermal Runaway:

--- Quote from: monpjc on March 27, 2011, 04:13:19 pm --- Use the circuit as per PIC data sheet, that is two Cap's but no resistor, I've never needed it.

--- End quote ---

@op The external feedback resistor monpjc talks about is used to aid oscillator startup.  It plays no further part once the crystal is up and running.  I normally do include it in my designs, but to be honest I have to agree with monpjc that I've never actually come across a situation where the crystal would not oscillate when the resistor is missing.  So, from a practical point of view, I can't disagree with his advice to omit it.

Here's a very good application note about the use of crystals by Microchip:


According to the datasheet, the load capacitance of part [ a ] XT9M-NL-A-NA-10M is not specified. Does it mean C1 and C2 are not required? Kindly advise. Thanks.

Series resonant crystals don't need load capacitance, but I don't think they'll work with the PIC18 crystal circuit, you would probably need to build an external oscillator circuit (I think the PIC16 datasheet contained an example) to use it. This webpage explains the difference and links to various other pages. Microchip AN826 explains it in more detail.


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