Question regarding an external xtal…

[mbu]

Question regarding an external xtal…

The attachment contains part of a schematic that contains a MCU that has an external xtal. If someone has the time, can you explain how the values of the two parallel capacitors are determined?

Thanks

 

[gramo]

They should be both 22pF, its in the PIC's datasheet

 

[gramo]

For better acuracy and sability, you could use ceramic resonators (they have the capictors built-in)

**broken link removed**



Heres a list of different ones available through Futurlec

https://www.futurlec.com/ICCrystalsResonators.shtml

 

[gramo]

The middle Pin is the Earth for the capacitors that are internally connected

 

[mbu]

gramo,

Thanks for the help!

Will a ceramic resonator be accurate enough - I'm using it for an electric motor tachometer? I would like the tachometer to be as accurate as I can make it.

I have a FOXLF100-20 oscillator on hand – will it be okay?

**broken link removed**

https://foxonline.com/LFSERIESxytal49u.htm

 

[Sceadwian]

Gramo, ceramic resonators are less accurate and stable than crystals, not more. The crystal will work just fine. If you need to fine tune the exact frequency of the crystal you can use trimmer caps, but calibrating the whole setup can be a bit finacky.

 

[Nigel Goodwin]

Gramo, ceramic resonators are less accurate and stable than crystals, not more. The crystal will work just fine. If you need to fine tune the exact frequency of the crystal you can use trimmer caps, but calibrating the whole setup can be a bit finacky.

Yes, crystals are FAR more accurate and stable than ceramic resonators, the capacitors used on the crystal are very non-critical - anything from 10pF to 47pF usually works fine.

 

[mbu]

Thanks everyone for the help!

 

[gramo]

Sorry guys, I've been mislead by someone else then

 

[Hero999]

Why do people use ceramic resonators then?

They certainly aren't much cheaper than crystals so why bother?

 

[gramo]

Might just be ease of use.. 3 components in one.

Found a bit of info on the two here

**broken link removed**


This site has some info on ceramic resonators as well

https://hem.passagen.se/communication/ceramic.html

A quote from the above site

The ceramic resonator is often used instead of crystal because of it's lower price. The frequency accuracy is not as good as a crystal but in many application the accuracy is not critical.

 

[Nigel Goodwin]

Why do people use ceramic resonators then?

They certainly aren't much cheaper than crystals so why bother?

But they are cheaper (certainly in production quantities) - and for many applications the extra accuracy of a crystal isn't required. Personally I've never bought one, but I have used them occasionally, taking them out of old TV's - it saves you looking out a couple of small capacitors!.