8M Mini 24MHz crystal load capacitance

[adevries]

Hello,

I'm trying to choose a 24MHz crystal to use with the 8M Mini, but I am having difficultly determining which crystals would be acceptable to use. While the datasheet recommends a crystal with a typical Cload of 12pF, the eval kit uses a crystal with a Cload of 9pF. If I am interested in using a crystal with a Cload that isn't 12pF, how can I go about determining if it will work with the 8M Mini?

Additionally, when adding load capacitance, it's important to match the load capacitance listed on the crystal datasheet, correct?

So, if the crystal CL = 9pF, then I would pick C1 and C2 such that this equation:

 is satisfied, right? In this case, Cs is stray capacitance, Ci is the input capacitance, and Co is the output capacitance.

If so, does anyone know what the pin capacitance is for pins 24M_XTALI and 24M_XTALO? 

Thanks

[igorpadykov]

Hi  adevries

right, datasheet recommends Cload of 12pF as  typical, however other also
can be used. Examples of CL calculation can be found on

Crystal Oscillator Load Cap Selection - Tech Tips - Engineering and Component Solution Forum - TechF... 

http://notes-application.abcelectronique.com/179/179-48215.pdf 

crystal drive level calculator in EB830

https://www.nxp.com/docs/en/engineering-bulletin/EB830.pdf 

Pin capacitance can be found in ibis model.

Best regards
igor
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[adevries]

Hi Igor,

Thanks for the information. I understand how to calculate what capacitor to connect to the crystal, but I don't understand how to choose the proper Cload value for a crystal.

Table 19 in the 8M Mini datasheet recommends a typical Cload of 12pF, so obviously crystals that meet the other requirements and have a Cload of 12pF should work. The eval board uses a crystal with a Cload of 9pF, so that would imply crystals with a Cload of 9pF should work. But how large or small can the crystal Cload be before it will not work anymore? Can my crystal have a Cload of 100pF? or 1000pF? Why or why not?

Is there really no minimum or maximum value for the crystal's Cload?

P.S. Due to a security update in Adobe Acrobat Reader, macro-enabled excel sheets embedded in PDF's cannot be opened anymore. So I cannot access the crystal drive level calculator in EB830

[igorpadykov]

Hi adevries

>But how large or small can the crystal Cload be before it will not work anymore?

>Can my crystal have a Cload of 100pF? or 1000pF? Why or why not?

to answer this question one needs to read how works Pierce oscillator - this is described

in common physics documentation, like https://www.electronics-tutorials.ws/oscillator/crystal.html 

ftp://download.epson-europe.com/pub/electronics-de/quartz/applnotes/oscillation%20circuit%20design%20guide.pdf

and educational books for electronics.

Best regards
igor

[adevries · Accepted solution]

Hi Igor,

Thanks for linking to those resources. I read the documents you linked and did some additional research, and this is the conclusion I've reached:

According to this document by silabs (Link), "Crystals with small load capacitance would typically start faster than crystals requiring a large CL. Large load capacitors also increase power consumption". This statement is supported by the power calculator you provided.

I can calculate the drive level of the crystal using the formula from Chrontel application note AN-06:

But what is the voltage level that drives the crystal? Is it a 1.8V square wave at 50% duty cycle? Or something else?

So, I believe I can pick a crystal with any load capacitance value as long as its maximum drive level isn't exceeded and I choose the appropriate capacitors to accompany it. Would  agree with that statement?

Also, the power calculator was designed to be used with the i.MX 6 series. Will it calculate an accurate drive level for the 8M Mini as well?

Thanks,

adevries

[igorpadykov]

Hi adevries

>I believe I can pick a crystal with any load capacitance value as long as its maximum drive level isn't exceeded

>and I choose the appropriate capacitors to accompany it. Would  agree with that statement?

yes

>power calculator was designed to be used with the i.MX 6 series. Will it calculate an accurate

>drive level for the 8M Mini as well?

yes

Best regards
igor