Question regarding an external xtal… - Electronic Circuits Projects Diagrams Free

mbu · 11th March 2007, 07:20 AM

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 · 11th March 2007, 07:26 AM

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

gramo · 11th March 2007, 07:32 AM

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

Heres a list of different ones available through Futurlec

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

gramo · 11th March 2007, 07:32 AM

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

mbu · 11th March 2007, 07:47 AM

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?

http://foxonline.com/LFSERIESxytal49u.htm

Sceadwian · 11th March 2007, 09:40 AM

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** · 11th March 2007, 09:59 AM

Quote:

Originally Posted by 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.

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 · 11th March 2007, 10:08 AM

Thanks everyone for the help!

gramo · 11th March 2007, 10:26 AM

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

Hero999 · 11th March 2007, 11:00 AM

Why do people use ceramic resonators then?

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

gramo · 11th March 2007, 11:09 AM

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

Found a bit of info on the two here

http://www.maxim-ic.com/appnotes.cfm/an_pk/2154

This site has some info on ceramic resonators as well

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

A quote from the above site

Quote:

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** · 11th March 2007, 11:20 AM

Quote:

Originally Posted by Hero999

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!.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Title	Starter	Forum	Replies	Latest
Basic Transmission question	CamNuge	General Electronics Chat	2	8th March 2006 11:08 PM
Question on 3-phase power	Kurupt	General Electronics Chat	1	8th January 2005 07:12 AM
High speed data logging question	cubdh23	Micro Controllers	3	19th August 2004 12:54 PM
Data Logging question with high speed!!!!	cubdh23	General Electronics Chat	4	16th August 2004 07:33 PM
PBX question	waqar	Electronic Projects Design/Ideas/Reviews	0	30th March 2003 07:21 PM

11th March 2007, 07:26 AM	(permalink)
gramo	They should be both 22pF, its in the PIC's datasheet __________________ Spency. PIC Micro's - Your mind is the limit PIC's and interfacing with other devices - a PIC Basic Guide @ digital-diy.net

11th March 2007, 07:32 AM	(permalink)
gramo	For better acuracy and sability, you could use ceramic resonators (they have the capictors built-in) Heres a list of different ones available through Futurlec http://www.futurlec.com/ICCrystalsResonators.shtml __________________ Spency. PIC Micro's - Your mind is the limit PIC's and interfacing with other devices - a PIC Basic Guide @ digital-diy.net

11th March 2007, 07:47 AM	(permalink)
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? http://foxonline.com/LFSERIESxytal49u.htm

11th March 2007, 09:40 AM	(permalink)
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. __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har." Last edited by Sceadwian; 11th March 2007 at 09:49 AM.

11th March 2007, 10:08 AM	(permalink)
mbu	Thanks everyone for the help!

11th March 2007, 10:26 AM	(permalink)
gramo	Sorry guys, I've been mislead by someone else then __________________ Spency. PIC Micro's - Your mind is the limit PIC's and interfacing with other devices - a PIC Basic Guide @ digital-diy.net

11th March 2007, 11:00 AM	(permalink)
Hero999	Why do people use ceramic resonators then? They certainly aren't much cheaper than crystals so why bother? __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez And http://www.silicontronics.com, same screen name as here.