Mike Knoll wrote:
> 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(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

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