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'PIC16F877 unreliable start up'
1999\09\23@010204 by Myke Predko

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Hi Folks,

I've been playing around with a sample 16F877 and I am having problems with
not getting a reliable start up.  I'm trying to run it at 4 MHz (using a
crystal and two 15 pF caps).  The operation of the oscillator seems to be
hit or miss (mostly miss).  I've played around with different parts (I've
tried a ceramic resonator, different values for the capacitors) and still
nothing that is reliable.  I have not tried an in-line resistor.

Has anybody else seen this type of problem and if you did, what did you do
to fix it?

Thanx,

myke

1999\09\23@093010 by Barry King

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> I'm trying to run it at 4 MHz (using a
> crystal and two 15 pF caps).  The operation of the oscillator seems to be
> hit or miss (mostly miss).  I've played around with different parts (I've
> tried a ceramic resonator, different values for the capacitors) and still
> nothing that is reliable.  I have not tried an in-line resistor.
>
> Has anybody else seen this type of problem and if you did, what did you do
> to fix it?

Yes.  Exactly the same symptom I had when I was *sure* the config
bits were set for XT oscillator, but they were REALLY set for LP
oscillator.  This is a new part for you?  Even money the config is
mis-programmed as LP or RC or something, not XT.

I hope I'm right, 'cause its easy to fix by re-setting the config bits
somewhere in the tool chain.  In my case, I had an old #define that
was setting the __config bits incorrectly for the part I was using.

------------
Barry King, KA1NLH
Engineering Manager
NRG Systems "Measuring the Wind's Energy"
Hinesburg, Vermont, USA
http://www.nrgsystems.com

1999\09\23@100749 by wzab

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On Thu, Sep 23, 1999 at 12:59:52AM -0400, Myke Predko wrote:
> Hi Folks,
>
> I've been playing around with a sample 16F877 and I am having problems with
> not getting a reliable start up.  I'm trying to run it at 4 MHz (using a
> crystal and two 15 pF caps).  The operation of the oscillator seems to be
> hit or miss (mostly miss).  I've played around with different parts (I've
> tried a ceramic resonator, different values for the capacitors) and still
> nothing that is reliable.  I have not tried an in-line resistor.
>
> Has anybody else seen this type of problem and if you did, what did you do
> to fix it?
I'm not sure if entering the programming mode may affect the oscillator, but
is the RB3 pin floating? If yes, tie it to the ground.
--
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                             Wojciech M. Zabolotny
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1999\09\23@102847 by Matt Bonner

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Myke Predko wrote:
>
> I've been playing around with a sample 16F877 and I am having problems with
> not getting a reliable start up.  I'm trying to run it at 4 MHz (using a
> crystal and two 15 pF caps).  The operation of the oscillator seems to be
> hit or miss (mostly miss).  I've played around with different parts (I've
> tried a ceramic resonator, different values for the capacitors) and still
> nothing that is reliable.  I have not tried an in-line resistor.
>
> Has anybody else seen this type of problem and if you did, what did you do
> to fix it?

I plugged my 877 sample directly into my 16C74A based product and have
no startup problems.  3.6864 MHz crystal, 15pF caps, no resistors.

I suspect that Barry is correct.

--Matt

1999\09\23@125713 by Tom Handley

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  Myke, I'm probably not of much help other than to suggest the obvious
which you already know:

  1. What's MCLR tied to?
  2. Any noise on the supply rails?
  3. Is this on a plug-in breadboard? That adds capacitance.
  4. Is it a parallel resonance crystal?
  5. What Oscillator mode are you using?
  6. Is LVP enabled? If so, what's on RB3?

  I recently upgraded a 16C77 prototype to a 16F877 (using ES samples)
and I've had no problems running at 20MHz with 18pf caps at 5Vdd. I'm using
a microEngineering Labs PicProto64 board.

  - Tom

At 12:59 AM 9/23/99 -0400, Myke Predko wrote:
{Quote hidden}

------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

1999\09\23@134309 by Bob Drzyzgula

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Myke,

Also, probably obvious as well, but did you try
unbalanced caps?

--Bob

On Thu, Sep 23, 1999 at 09:42:09AM -0700, Tom Handley wrote:
{Quote hidden}

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Bob Drzyzgula                             It's not a problem
.....bobKILLspamspam@spam@drzyzgula.org                until something bad happens
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============================================================

1999\09\23@212617 by Myke Predko

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Hi Wojciech,

Thanx for the pointer - you helped me find the problem.  In my application
(when it did work), I wrote some parameter data to PortB - when I checked it
with my Logic Probe, I couldn't understand why it wasn't being driven (I
cleared TRISB).

I should have looked more closely at the documentation.

While it would never boot with RB3 tied to ground, I added the parameter:

"_LVP_OFF"

To the "__CONFIG" statement and the 16F877 runs fine now.

I think this little hint will end up on my web page.

Thanx to everyone else who replied.

myke

{Original Message removed}

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