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PICList Thread
'Please Help - Osc.Problem'
1999\08\25@042342 by John Bes

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Hi all,
I think I'm having a real serious problem now ;)

I build a programmer and a separate test board for the 16c84.
Programming went well, but testing doesn't. The program in the Pic is the
famous flashing led.

The problem seems to be the crystal. It doesn't oscillate very well.

I have been working on this problem for 2 weeks now, and I get kind of
desperate ;(
Please let me now if you have a probable solution!

Here are the facts:

- my scope shows a mildly distorted sine wave between osc1 and osc2 (pin
15/16).
- when I connect the scope -> it works
The scope has R=1M and C=33pF -> when I replace the scope with a R (1m)
between pin 15 and 16 and parallel to that, a C (33pF)
no result.
- when I touch the GND side of the caps with my fingers, sometimes the led
flashes up.
- The R (10 Ohm) is implemented to reduce the current through the crystal.
replacing it by a wire doesn't make a lot of diffenrence, it seems.
- I tried several crystals (4 MHz) -> No go.

- connection:

       GND
      __|__
    _|      |_
    _       _ 2x C=47pF
     |       |
     |_|X|_|
     |      |_
     |     |  | R=10 Ohm
     |     |_|
     |16   |15
  _ |____|______
  | Pic 16c84    |
  ---------------------

Thanx,
John

1999\08\25@085223 by Dan Creagan

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I'm assuming you are just starting with programming, if so then a common
mistake is to forget to set the watch dog timer fuse to OFF.  That will
cause problems if you don't periodically reset the WDT in your code - and
most of the common LED flash programs don't do that.

Be sure you set the watch dog timer off when you program your chip. The
setting will be in the configuration/fuses menu in your programming
software.

If you are familiar with this problem and have allowed for it, please
disregard all that preceded 8)


Dan

{Original Message removed}

1999\08\25@092933 by King, Jonathan

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>>The problem seems to be the crystal. It doesn't oscillate very well.

<snip>

>>- when I touch the GND side of the caps with my fingers, sometimes the led
flashes up.

I had the same symptom once before.  I did not have the configuration word
in the hex file and I failed to notice that the PICSTART+ oscillator
configuration box defaulted to RC instead of HS.
It was a little confusing at first.  I had one prototype (programmed
correctly)  that worked and about 15 first run units that did not work with
no apparent physical difference. :)>

Hope this helps,
Jonathan

P.S. I've lurked long enough... I suppose it's finally time to attempt to
contribute to this incredibly useful list.

1999\08\25@132921 by Erik Reikes

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At 10:18 AM 8/25/99 +0200, John Bes wrote:

>- connection:
>
>        GND
>       __|__
>     _|      |_
>     _       _ 2x C=47pF
>      |       |
>      |_|X|_|
>      |      |_
>      |     |  | R=10 Ohm
>      |     |_|
>      |16   |15
>   _ |____|______
>   | Pic 16c84    |
>   ---------------------
>

I don't know the specifics of the specs for your xtal, but what I used for
my 4Mhz crystal was : 10pF for the two load caps and no series resistor.

It seems to me with such big load caps and the additional load of the
resistor that maybe its having trouble starting out.  I don't know dick
about crystal design so I just guessed with my values.  I seem to remember
somewhere reading that the load caps should be 2x the capacitance of your
crystal, so mine should probably each be 20pF's.  I also read that the load
caps weren't too critical as far as value was concerned as they don't add
much in the "filter effect" of the extra components.  It seems to me that a
series resistance would change the freq. response of the system quite a
bit.  I'd first try just chucking the resistor.

Ohh yeah, also double and triple check that your MC reset line is being
pulled high.  This will stop you dead in the water if its floating.

Good luck.

The way I figured mine out was to look at the spots for components on my
proto board and to read the Mchip app note on this stuff.  Its on their web
page in the app note section I believe.

-E

1999\08\25@161056 by Richard Prosser

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Try adding a high value resistor (1-10MEG) across the xtal. This tends to
linearise the amplifier slightly, allowing a noise build up that starts
oscillation. (or something). We do it anyway and the accepted reason is that
it speeds the start-up of the xtal.

Also - 47pF caps is a bit strong - try 22 or 10pf instead.

Richard

> {Original Message removed}

1999\08\25@171939 by paulb

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Erik Reikes wrote:

> It seems to me with such big load caps and the additional load of the
> resistor that maybe its having trouble starting out.

 10 ohms as specified is negligible.  Recall the DC impedance of the
OSC2 line is already well over 50 ohms.

> I seem to remember somewhere reading that the load caps should be 2x
> the capacitance of your crystal, so mine should probably each be
> 20pF's.

 Twice the specified load capacity of the crystal (because they are
effectively in series) minus the capacity of the PIC pins (which are in
parallel with the load caps).  *This parameter isn't specified*!  The
latest revision appears to be DS30430C on the website, and unless
they've altered it without changing the revision code, this is stamped
"Preliminary" on the specs sections.  Will this persist while the 16F84
becomes a "legacy" device like the 16C84 whose datasheet *has* been
removed from the website?

 Assume about 10 pF (may be excessive) for the pin capacitance.  If
crystal loading was 16 pF, twice this would be 32 pF, drop 10 and you
get 22 pF.

Richard Prosser wrote:

> Try adding a high value resistor (1-10MEG) across the xtal. This tends
> to linearise the amplifier slightly, allowing a noise build up that
> starts oscillation. (or something).

 But this is supposed to be provided in the chip.  Indeed, it is
purported to be the *only* difference between LP, XT and HS modes, so
the same effect would/ should result from selecting HS mode.  Some have
actually suggested this in the past!

> We do it anyway and the accepted reason is that it speeds the start-up
> of the xtal.

 It reduces the "Q".  This is the same thing as speeding start-up.

> Also - 47pF caps is a bit strong - try 22 or 10pf instead.

 Sounds good.
--
 Cheers,
       Paul B.

1999\08\25@205425 by William K. Borsum

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At 10:18 AM 8/25/99 +0200, you wrote:
>Hi all,
>I think I'm having a real serious problem now ;)
>
>I build a programmer and a separate test board for the 16c84.
>Programming went well, but testing doesn't. The program in the Pic is the
>famous flashing led.
>
>The problem seems to be the crystal. It doesn't oscillate very well.
>
>I have been working on this problem for 2 weeks now, and I get kind of
>desperate ;(
>Please let me now if you have a probable solution!

Check the arkives.  I know I put in a looong discussion thread on this
subject.
With all the responses, ALL of the gory details were there.

Boils down to this:
       Get the values for the caps from the data book for the pic you are using
.
       Be sure you are using the right type of crystal (Parallel loaded).
       If you add a resistor in series as shown--be sure it is CARBON COMPOSITI
ON.
       Carbon comp generates the noise needed to kick the crystal into
oscillations--specially for watch crystals.
       Check your solder joints for opens, shorts, crystallization, etc.
       be sure your caps are the right value!!!

Scope probe will work on OSC out, but not on OSC IN.  Loads the line too much.

Pray.

kelly



William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems
<spam_OUTborsumTakeThisOuTspamdascor.com> & <http://www.dascor.com>

1999\08\26@002812 by kypros.vassiliou

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John Bes wrote:
{Quote hidden}

Hi John,
I had a similar, may be the same, problem when I first programed a
PIC16C84. What I did finaly is to configure my programmer (Newfound
Electronics programmer) for XT or HS oscilator acording to the oscilator
type I used. I noticed that my PIC worked in RC (and Crystal of course)
mode when programed in XT or HS mode but did not work the oposite.
I hope it helps.
Regards
Kypros

1999\08\26@144448 by John Bes

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First of all: thanx for all your advice!

I have been trying all your suggestions, but it still didn't work.
Then, when I was about to throw everything out the window, I suddenly saw a
little led flash!
I really don't know, what did the trick......but I guess it was the default
RC mode and maybe the load of the capacitors.
Maybe  tomorrow I will replace the caps to the ones I used before (47pF) and
see if it still works.

The rest of the evening I'm gonna sit on the couch with my flashing led and
let my roommate make fun of me :)

Thanx again,
I'm happy!

John

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