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'crystals and osc freq'
1996\06\18@034401 by David E. Queen

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I got my interface test unit built and running.

To my surprise the times were not as expected. 241, 482, and 964 milliseconds
not 250, 500, and 1sec. I had double checked in MPLAB and I am using the
right number of cycles. My problem is my crystal is 4% high in frequency.
About 4.16 to 4.18 depending on where I try to measure it.

I am using a 16c84 rated at 10mhz, a 4.000mhz crystal and 2 22pf caps from
Digikey. I set the oscillator bits to HS. The unit was built on one of MEL's
proto boards.

What are the key factors I need to look at when I need the oscillator to be
within 0.1% without tweaking the frequency?

I really thought I could just grab a 4mhz rock and a couple caps ant it would
be at least as close as I need. Should I consider using a canned dip oscillator?


Another question is can you use a solder less breadboard (I just got one of the
E.N.D. unit's) and a 20mhz clock on a 16c65 windowed part. I have concerns that
the oscillator will go nuts with all the stray coupling, and I will need 0.1%
accuracy on that unit too.

1996\06\18@035901 by Steve Hardy

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> From: "David E. Queen" <spam_OUTdqueenTakeThisOuTspamnnl.net>
>
> I got my interface test unit built and running.
>
> To my surprise the times were not as expected. 241, 482, and 964 milliseconds
> not 250, 500, and 1sec. I had double checked in MPLAB and I am using the
> right number of cycles. My problem is my crystal is 4% high in frequency.
> About 4.16 to 4.18 depending on where I try to measure it.

4% is awfully bad for an xtal.  They are usually spec'd to within 100ppm
(0.01%) if correct loading is used.  Try a different xtal.  Note that there
are 2 basic types of xtal: series and parallel resonant.  I can't remember
which, but one should be in an inverting loop and the other in a non-inverting
loop.  Don't know which type the PIC is expecting.

>
> I am using a 16c84 rated at 10mhz, a 4.000mhz crystal and 2 22pf caps from
> Digikey. I set the oscillator bits to HS. The unit was built on one of MEL's
> proto boards.
>
> What are the key factors I need to look at when I need the oscillator to be
> within 0.1% without tweaking the frequency?

If used correctly, the xtal will be within 0.01%.  Try adding a series resistor
since the xtal may be being grossly overdriven.  Perhaps the seal is broken
(although I would expect moisture on the quartz would slow it down not
speed it up as you state).

>
> I really thought I could just grab a 4mhz rock and a couple caps ant it would
> be at least as close as I need. Should I consider using a canned dip
oscillator?
>

That would certainly work.

>
> Another question is can you use a solder less breadboard (I just got one of
the
> E.N.D. unit's) and a 20mhz clock on a 16c65 windowed part. I have concerns
that
> the oscillator will go nuts with all the stray coupling, and I will need 0.1%
> accuracy on that unit too.
>

I have had no problems with 4MHz logic (was a Z80 system) done up on a
breadboard.  Using a frequency meter and changing loading capacitance by
a few 10's of pF produced only a few 100Hz difference at 4MHz.

Regards,
SJH
Canberra, Australia

1996\06\18@043459 by David E. Queen

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At 06:00 PM 6/18/96 EST, you wrote:

>4% is awfully bad for an xtal.  They are usually spec'd to within 100ppm
>(0.01%) if correct loading is used.  Try a different xtal.  Note that there
>are 2 basic types of xtal: series and parallel resonant.  I can't remember
>which, but one should be in an inverting loop and the other in a non-inverting
>loop.  Don't know which type the PIC is expecting.
>
>>
That's where  I think I shot my self in the foot.
I order the crystals and by mistake, I just double checked,
got series cut for the 4 mhz ones.  Thanks for a quick answer!


{Quote hidden}

I believe my 10mhz crystals are the right cut and I will redo the delay loops
and try it out again
{Quote hidden}

Thanks again for the reply.

I am sorry I wasted the list's time on this, but a quick look in the book
showed
nice charts for RC oscillators and resonators, but only a more careful reading
spotted the series cut warning. Its near the start of section 13 of the
16c6x book.

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