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'[PIC] Help troubleshooting a F84 cct with dmm'
2003\03\11@150857 by Des Bromilow

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>>> "Des Bromilow" <spam_OUTDes.BromilowTakeThisOuTspamcitec.com.au> 11/03/03 8:48:14 pm >>>
G'Day,
My circuit with a 16F84a operating off a 4MHz crystal (with 2 x 33pF caps) doesn't seem to work on the most simple of programming tests.
I suspect my Crystal may be dead.. but I don't have a CRO to check, all I have to test with is a Digital multimeter (DMM)
I would expect to see a 2.5 VDC voltage on OSC1, OSC2 due to the 5VDC clock, and it's duty cycle.
IS that correct? (that an operating Crystal will appear to a DMM as 2.5 VDC?)

Any assistance in this arena appreciated,

Des Bromilow
Brisbane
OZ

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2003\03\11@154007 by Jinx

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A DMM will likely collapse the oscillator and you'll read
nothing. Even a x1 scope probe can do that. IMHO your
best bet is parts substitution, a thorough circuit check and
a little LED code

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2003\03\11@154337 by Des Bromilow

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Thanks!!!,
That's what I'm trying to do, but the LED code isn't even lighting up, hence why I suspected the clock.

I'll desolder the Crystal and try another.

Any tips for soldering Crystals?
Des

>>> EraseMEjoecolquittspam_OUTspamTakeThisOuTCLEAR.NET.NZ 12/03/03 6:40:07 am >>>
A DMM will likely collapse the oscillator and you'll read
nothing. Even a x1 scope probe can do that. IMHO your
best bet is parts substitution, a thorough circuit check and
a little LED code

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2003\03\11@154338 by Jai Dhar

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If possible, try and obtain a 16f628 - they are cheaper than the f84 and offer
more features... one important one pertaining to your application: The
internal 4MHz oscillator. You will save yourself a lot of time and trouble by
switching to this chip. That is, of course, if it is a viable option.

Quoting Jinx <KILLspamjoecolquittKILLspamspamCLEAR.NET.NZ>:

{Quote hidden}

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2003\03\11@155013 by Gavin Jackson
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What have you done with the MCLR pin?
This should be tried to VCC (+5V) through a resistor?

Gavin Jackson, R&D Software Engineer, Auckland, New Zealand

> {Original Message removed}

2003\03\11@155224 by Jinx

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> Any tips for soldering Crystals?

Don't dither (ie don't overheat), and ground the case. Mount
xtal and caps as close as possible to the PIC and add a couple
of decoupling caps, something like a 10uF+10n, also as close
as possible to the PIC

Osc problems do happen so you could be right. I'd still be
tempted to do a methodical pin-by-pin circuit check. How
often do "obvious" errors magically appear ? 8-)

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2003\03\11@160852 by Dwayne Reid

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At 06:10 AM 3/12/03 +1000, Des Bromilow wrote:
> >>> "Des Bromilow" <RemoveMEDes.BromilowspamTakeThisOuTcitec.com.au> 11/03/03 8:48:14 pm >>>
>G'Day,
>My circuit with a 16F84a operating off a 4MHz crystal (with 2 x 33pF caps)
>doesn't seem to work on the most simple of programming tests.
>I suspect my Crystal may be dead.. but I don't have a CRO to check, all I
>have to test with is a Digital multimeter (DMM)
>I would expect to see a 2.5 VDC voltage on OSC1, OSC2 due to the 5VDC
>clock, and it's duty cycle.

You can check the OSC2 pin with a multi-meter if you wrap one end of a 100K
resistor on the positive test lead and use the free end of the resistor to
measure the DC voltage.  It should be about 1/2 the supply voltage.

The 100K resistor isolates the capacitance of the DMM and test leads from
the crystal and allows it to continue oscillating (if it is).

This same trick does not work on the OSC1 pin - even 100K load is usually
too much.

If it looks like an oscillator problem, try removing the crystal components
and configuring the PIC to use external RC oscillator mode.  This requires
connecting a resistor from Vdd to OSC1 and a small capacitor from OSC1 to
ground.  See the data sheet for the details.

The accuracy is horrific but you can at least get a LED flashing.  Then
work back to using the crystal.

One final thought - just for the heck of it, try changing from XT mode to
HS mode before giving up on your current crystal configuration.

dwayne

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2003\03\11@162550 by Des Bromilow

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Thanks!!!

FWIW, I am using it in HS mode.. that is correct isn't it? or should I be in XT mode?

Thanks,
Des

>>> RemoveMEdwaynerEraseMEspamEraseMEPLANET.EON.NET 12/03/03 7:07:31 am >>>
At 06:10 AM 3/12/03 +1000, Des Bromilow wrote:
> >>> "Des Bromilow" <RemoveMEDes.Bromilowspam_OUTspamKILLspamcitec.com.au> 11/03/03 8:48:14 pm >>>
>G'Day,
>My circuit with a 16F84a operating off a 4MHz crystal (with 2 x 33pF caps)
>doesn't seem to work on the most simple of programming tests.
>I suspect my Crystal may be dead.. but I don't have a CRO to check, all I
>have to test with is a Digital multimeter (DMM)
>I would expect to see a 2.5 VDC voltage on OSC1, OSC2 due to the 5VDC
>clock, and it's duty cycle.

You can check the OSC2 pin with a multi-meter if you wrap one end of a 100K
resistor on the positive test lead and use the free end of the resistor to
measure the DC voltage.  It should be about 1/2 the supply voltage.

The 100K resistor isolates the capacitance of the DMM and test leads from
the crystal and allows it to continue oscillating (if it is).

This same trick does not work on the OSC1 pin - even 100K load is usually
too much.

If it looks like an oscillator problem, try removing the crystal components
and configuring the PIC to use external RC oscillator mode.  This requires
connecting a resistor from Vdd to OSC1 and a small capacitor from OSC1 to
ground.  See the data sheet for the details.

The accuracy is horrific but you can at least get a LED flashing.  Then
work back to using the crystal.

One final thought - just for the heck of it, try changing from XT mode to
HS mode before giving up on your current crystal configuration.

dwayne

--
Dwayne Reid   <RemoveMEdwaynerTakeThisOuTspamspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 19 years of Engineering Innovation (1984 - 2003)
 .-.   .-.   .-.   .-.   .-.   .-.   .-.   .-.   .-.   .-
    `-'   `-'   `-'   `-'   `-'   `-'   `-'   `-'   `-'
Do NOT send unsolicited commercial email to this email address.
This message neither grants consent to receive unsolicited
commercial email nor is intended to solicit commercial email.

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2003\03\11@171029 by Herbert Graf

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> FWIW, I am using it in HS mode.. that is correct isn't it? or
> should I be in XT mode?

       It SHOULD be XT for a 4MHz crystal, I'm not sure what would happen in HS
mode, you might damage the crystal, or result in no oscillation. I'm not
familiar enough to know, it's possible it would just work, with higher power
consumption.
       Please though, READ THE DATASHEET, this is VERY CLEAR in the datasheet and
if you haven't perused the datasheet properly you are wasting our time.
Sorry to be so harsh, but a PIC is something you NEED to consult the
datasheet for. I've been working with PICs on a hobby level for a few years
and STILL have to regularly consult the datasheet. TTYL

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2003\03\11@184449 by Des Bromilow

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Thanks!!.. I was under the inpression that the XT setting was for crystals lower than 4MHz, and HS for the higher than 4MHz...Goes to show I obviously didn't read far enough into it to pick the difference.

Thanks, again,
Des

>>> spamBeGonemailinglistSTOPspamspamEraseMEFARCITE.NET 12/03/03 8:10:36 am >>>
> FWIW, I am using it in HS mode.. that is correct isn't it? or
> should I be in XT mode?

       It SHOULD be XT for a 4MHz crystal, I'm not sure what would happen in HS
mode, you might damage the crystal, or result in no oscillation. I'm not
familiar enough to know, it's possible it would just work, with higher power
consumption.
       Please though, READ THE DATASHEET, this is VERY CLEAR in the datasheet and
if you haven't perused the datasheet properly you are wasting our time.
Sorry to be so harsh, but a PIC is something you NEED to consult the
datasheet for. I've been working with PICs on a hobby level for a few years
and STILL have to regularly consult the datasheet. TTYL

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2003\03\12@025944 by hael Rigby-Jones

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{Quote hidden}

It will almost certainly still work with the HS setting.  Long term
reliability may be slightly compromised, and the oscillator may not be
exactly on frequency if it is being overdriven.  FWIW I've had to use the HS
setting when using certain 4MHz ceramic resonators with built in caps that
simply wouldn't start reliably using the XT fuse setting.

Mike


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2003\03\12@101535 by Wouter van Ooijen

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>         It SHOULD be XT for a 4MHz crystal

IIRC XT for up to 4 MHz, HS for above, so for 4 MHz both should be
valid. RTFM!

Wouter van Ooijen

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2003\03\12@115716 by Ian McLean

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IME, absolutely proven in practise !  Because I only use 4MHz and 20MHz
crystals in all my designs, I only use the HS fuse setting to help prevent
mistakes and confusion when I port code.  HS works fine for 4MHz crystals,
and has never failed on me.  XT however will not work for 20Mhz crystals.

Rgs
Ian.

{Original Message removed}

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