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Grogster

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Joined: 31/12/2012
Location: New Zealand
Posts: 9078

Posted: 01:52am 31 Oct 2014

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Hi folks.

I'm about to buy in some stock of 8MHz crystals for use with various forms of the VT100 terminal of Geoff's, but I note that in Geoff's circuit, he has used 27pF loading caps for the crystal oscillator, but just about every data sheet for 8MHz crystals that I look at specifies a load capacitance of 18pF.

Just how critical are the load capacitors?

Perhaps the 18pF is the minimum recommended, and so using 27pF is just Geoff being a little conservative?

It is common to see crystal oscillator circuits with anything from 18pF - 30pF or so.
22pF is a common choice for many circuits.

So, can anyone help clarify this a little more for me?

Smoke makes things work. When the smoke gets out, it stops!

Geoffg

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Joined: 06/06/2011
Location: Australia
Posts: 3167

Posted: 03:33am 31 Oct 2014

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The actual value depends on the crystal but unfortunately the manufacturer's recommended value is hard to find when you are buying from ordinary retailers.

The value will be somewhere in the range of 12 to 32 pF. Too low and the oscillator may not start, too high and the oscillator will overload causing instability. Temperature, PCB layout and voltage variations help to confuse the picture.

In the end I picked a value that was not too small and not too large but hopefully just about right (to paraphrase Goldilocks).

If you really want to get into the details read this:
http://www.t-es-t.hu/download/microchip/an849a.pdf
or this
http://ww1.microchip.com/downloads/en/AppNotes/00826a.pdf

In practice any value in the range of 18 to 30 pF seems to be OK.

Geoff Edited by Geoffg 2014-11-01

Geoff Graham - http://geoffg.net

MicroBlocks

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Joined: 12/05/2012
Location: Thailand
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Posted: 05:49am 31 Oct 2014

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I have used 8Mhz crystals in combination with 22pf and that works.
Which crystal package are you looking for, i might have some for a good price.

Microblocks. Build with logic.

hitsware
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Joined: 23/11/2012
Location: United States
Posts: 535

Posted: 10:13am 31 Oct 2014

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> I have used 8Mhz crystals in combination with 22pf and that works.

Me too on VT-100. 22pf seems most common and available.

Grogster

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Joined: 31/12/2012
Location: New Zealand
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Posted: 01:34pm 31 Oct 2014

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Thanks for the replies, guys.(hey - that rhymes!)

I am using 27pF SMD caps for my VT100 creations, and I have not had any issues at all, but I was just curious about the difference between MOST datasheets saying 18pF for 8MHz crystals, vs the 27pF that Geoff decided to use.

As I say - no problems at all with 27pF, I was just curious.

@ TZA - Element 14 can sell me 8MHz HC49 crystals for 49c each.(US 35c or so)

Smoke makes things work. When the smoke gets out, it stops!

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