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'[PIC] What are the symptoms of oscillation?]'
2006\04\09@135936 by Bill Kuncicky

picon face
Jan-Erik Soderholm wrote:


>
> > What is your oscillator config ?
> Any 100 nF cap near the PIC ?
> Any I/O pins configurated as inputs but left unconnected ?
> How is MCLR connected ?
> Anything else you haven't told us ?


.
1.  The oscillator config is a 10 MHz resonator, between OSC1 and OSC2.
Center pin grounded, and of course the resonator has built-in 22 pf
capacitors to ground.

2.  There were not any 100 nf caps from Vdd to Vss.  I have now put them in,
as suggested by Olin Lathrop.

3.  All unused I/O pins are configured as output.

4.  MCLR is tied through a 10K resistor to +5 volts.

5.  I have told you everything that I think is significant, but of course
Murphey's Law says that I have forgotten to tell you something crucial.  :-)

I'm going to let it run awhile with the 100 nf capacitors put in and see if
it fails again.

(Since no one told me whether or not this sounded like oscillation, I guess
it must have).

Bill

2006\04\09@141042 by Phillip

picon face
Hi Bill
When it was oscillating what did the voltage look like?
I'm not follow your trouble too closely but I know some regulators will
oscillate if the cap[s] on the output have too little ESR. (I can't see how
it would make you device hot but maybe if it was not regulating the voltage
could be going too high....)
It is something to check
When tantalum was difficult to get a few years back (due to a civil war in
Africa I think) I would use a low ESR ceramic cap with a 1 ohm in series.





Phillip
Things should be as simple as possible but no simpler



Phillip Coiner
CTO, GPS Source, Inc.


Your source for quality GNSS Networking Solutions and Design Services, Now!

{Original Message removed}

2006\04\09@141825 by Spehro Pefhany

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At 01:59 PM 4/9/2006 -0400, you wrote:


>(Since no one told me whether or not this sounded like oscillation, I guess
>it must have).

Bzzzt. Nope. It sounds like latch-up or an intermittent short of an output
to supply rail.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2006\04\09@155545 by Jan-Erik Soderholm

face picon face
Bill Kuncicky wrote :

> 5.  I have told you everything that I think is significant,
> but of course Murphey's Law says that I have forgotten
> to tell you something crucial.  :-)

Yep, right, you never know :-)

One thing, doesn't one usualy uses both a smaller (say 100nF)
ceramic cap on the output side of the 7805 (to prevent HF
osccillation) at the same time as a larger (10-22 uF)
elyt for smoothing ?

I usualy do something like that, but it might also
depend on the brand of the 7805.

Jan-Erik.



2006\04\09@185428 by Bob Axtell

face picon face
Bill Kuncicky wrote:

{Quote hidden}

but these caps are only useful to the resonator tank, and will have no
bearing on any
other oscillation in the system.

>2.  There were not any 100 nf caps from Vdd to Vss.  I have now put them in,
>as suggested by Olin Lathrop.
>
>  
>
PICs (nor any other uP) are not designed to run properly without  bypass
caps.  People leave
them out to their peril. Olin called it right; this is the fix.


>3.  All unused I/O pins are configured as output.
>
>4.  MCLR is tied through a 10K resistor to +5 volts.
>  
>
22K is ok, too, if you need to.

>5.  I have told you everything that I think is significant, but of course
>Murphey's Law says that I have forgotten to tell you something crucial.  :-)
>  
>
Murphy was an optomist <g>

>I'm going to let it run awhile with the 100 nf capacitors put in and see if
>it fails again.
>
>(Since no one told me whether or not this sounded like oscillation, I guess
>it must have).
>  
>
Absolutely classic symptoms. We've all been there.

>Bill
>  
>


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