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PICList Thread
'Crystal/Capacitor Selection for the hobbiest'
2000\05\03@004142 by Mike Knoll

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I am attempting to select a crystal for a 16F84, running at 4Mhz.  I am
just getting started with PICs.  The PIC datasheet for the 16C84 has a
table instructing me to use two 15pF, one on each pin of the cyrstal,
connected to ground.  But, from ECS's website(http://www.ecsxtal.com), thier 4Mhz
crystal(ECS-40-20-1)) has a load capacitance of 20pF, using the formula on
thier application notes:

      C1 * C2
CL =  ---------  + 5pF
      C1 + C2

Instructs me to use 30pF caps.  (Setting CL=20pF, C1=C2)

Who do I believe?  I've found information out there too general, or too
technical.  The data sheet for the 16F84 seems too general, and I can't
understand too far into MicroChip's app note on crystals.  An index
of commonly avaliable crystals and caps to use would work great.

Also, if series crystals don't need capacitors, why not use them to reduce
parts count?

Also, all instructions I've seen to prevent from over driving the crystal
include using an oscilliscope to measure current.  An oscilliscope is far
from in my budget.  Is there any way to prevent overdriving my crystal
without using an oscilliscope?

Mike

2000\05\03@095803 by David VanHorn

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>Also, if series crystals don't need capacitors, why not use them to reduce
>parts count?

Because this is a paralell circuit!

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2000\05\03@164052 by Mark Willis

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Mike Knoll wrote:
{Quote hidden}

The crystal's data sheet is THE thing to listen to, the MicroChip data
sheets tell you what to look at when it doesn't work - as Dave says, you
do want to use a Parallel crystal.  The rock you're specifying is one,
designed to use 20pF Cap's, should work fine.

Ceramic Resonators can reduce parts count if they're accurate enough for
your job - ECS makes a 0.5% 4MHz ZTT-4.00MG part, cheaper than the
crystal as well.  Murata is another good name here.

Low Bandwidth oscilloscope's are cheaper than higher quality 'scopes -
Check out local HamFest's and swap meets, also local University surplus
sales, maybe bring a test oscillator (9V batt., 78L05 and a few crystal
oscillators would do - or a Pic would do) and look at the signal you get
on the scope's display - check focus (don't want a truly gassy CRT) and
general usability.  Maybe ask for help from a local PICLister or Ham.
They go for $50ish sometimes.  If all you can afford is a 500kHz
bandwidth scope, you can at least see some signals instead of none.  Or,
save up like the rest of us usually have to do, for that pricey 'scope.

 Mark

2000\05\03@172450 by David VanHorn

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>They go for $50ish sometimes.  If all you can afford is a 500kHz
>bandwidth scope, you can at least see some signals instead of none.  Or,
>save up like the rest of us usually have to do, for that pricey 'scope.

I don't know where you are, but the dayton hamfest is coming up around the
20th this month.  Loads of good test equipment there.

You could do worse to start off, than a Tek 7000 series mainframe with vert
amps and timebase plugins. This would probably set you back $300-ish, but
you'd be able to get 100 MHz bandwidth.  I use the 7000 mainframe to hold a
pair of spectrum analyzers.

I wish they still made the same style of scope, I like the "what test
instrument do you want to make today?" approach.
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2000\05\03@203605 by Sean Breheny

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I second this opinion. I bought a 7704 with a 7A26 and 7B53 plugins almost
two years ago. It gives >200MHz BW (100MHz triggering) at about $350. What
a workhorse! It will show things that digital scopes won't, at a small
fraction of the price (as long as you don't need the storage ability).

Sean

At 04:13 PM 5/3/00 -0700, you wrote:
>You could do worse to start off, than a Tek 7000 series mainframe with vert
>amps and timebase plugins. This would probably set you back $300-ish, but
>you'd be able to get 100 MHz bandwidth.  I use the 7000 mainframe to hold a
>pair of spectrum analyzers.
>

|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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2000\05\04@191312 by Brian Kraut

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I spoke to one of the engineers at Fox crystals one time about what caps to
use for 20pf crystals and they told me to use 27pf caps.  I have never had
problems with the 27pf.

Mike Knoll wrote:

{Quote hidden}

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