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'[PIC:] Interesting (and scary) observation about a'
2004\01\20@162135 by 8859-9?B?1m1lciBZYWxo/Q==?=

I have a board with 16F628A running with internal osc. Also, a 32KHz xtal is
connected for basic time keeping.  It sleeps all the time and wakes up about
every 500ms and does some stuff for short time then sleeps again.  During
the awake time I turn on a led and turn it off just before going to sleep.

I have discovered that the chip does not wake up from sleep (the led does
not blink) for a while (4 seconds to 20 seconds or so) under 2 seperate
conditions:

1. If I touch (with my bare finger) to the cap that is connected to the RB7
pin of the pic (where the xtal's one pin is connected) the unit does not
wake up

Or

2. If I "breathe" on the same cap above.

Note that I do not know the exact word for what I am doing so I said
breathing.  Let's say that you want to see if your breath smeels, and you
"breathe" to your hand and smeel it.  That is what I do to the cap (except I
don't smell it afterwards).  When I did this, I was actually trying the get
something off of the pcb, and was not testing my breath.  I also did this
test right after brushing so probably this is not because of my breath! :-)

I have put this board to a test in a fridge with -16 deg C and +85 deg C
(+/- 5 deg C).  And the board still works.  Also, this behaviour has been
observed with many of the same boards and many of different kind of board.

The cap is 22pF.

Why does this happen?  Bad board design?
Has anybody else experienced this before?
How would I know this wouldn't happen after the unit is shipped?
Is this a humidity problem?  Would this problem occur again after it rains
on the unit (in IP68 box) and then there is sun right after the rain?

P.s. The unit will be in an IP68 enclosure and the problem occurred without
the enclosure, no way to test it inside the enclosure.

I am sorry for the lengthy message but I am really curious as to what is
going on here and to find out if I can take any precautions.

Best regards,

Omer

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2004\01\20@165226 by

picon face
Omer Yalhi wrote :

> 1. If I touch (with my bare finger) to the cap that is
> connected to the RB7
> pin of the pic (where the xtal's one pin is connected) the
> unit does not wake up

These "clock-crystals" are made to run in a very low-energy,
low-power environment. That also makes them sensitive. To
intruduce the (rellativily) large capacitance
in your finger/hand, probably upsets the crystal a lot.

> The cap is 22pF.

And your fingers may add several 100's of pF's to that.

>
> Why does this happen?  Bad board design?

Don't have to be. Just keep your hands in your pockets :-)

Note also that most PIC data sheets has some examples of
board design (often together with the timer1 desription)
that might help in making the tmr1 osc more stable.

Regards
Jan-Erik.

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2004\01\20@173403 by Olin Lathrop

face picon face
Vmer Yalh} wrote:
> 1. If I touch (with my bare finger) to the cap that is connected to
> the RB7 pin of the pic (where the xtal's one pin is connected) the
> unit does not wake up

The power level of some 32Khz crystals is deliberately very low because they
are meant to run a few years in a wrist watch on a single tiny battery.  As
a result, the signals are high impedence and easily disturbed.  The best
answer to this is "don't do that".

> 2. If I "breathe" on the same cap above.
>
> ...
>
> I also did this test right after brushing so probably
> this is not because of my breath! :-)

Doh, there goes my first thought.

> The cap is 22pF.
>
> Why does this happen?  Bad board design?

Probably not.

Is the crystal specified with such a large value?  22pF is usually about
right for "normal" crystals, but these low power watch crystals often want
half that or less.  The extra capacitance may be attenuating the signal to
where it barely oscillates.  The extra leakage from the humidity may be just
enough to kill it.  Try removing the cap from the oscillator in pin side of
the crystal and see if breathing on it still kills it.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2004\01\20@181156 by Jinx

face picon face
> 1. If I touch (with my bare finger) to the cap that is connected to the
RB7
> pin of the pic (where the xtal's one pin is connected) the unit does not
> wake up

Consider this, from The Simpsons -

Ralph Wiggum - "The doctor said I wouldn't get so many nose-bleeds
if I kept my finger out of there"

> 2. If I "breathe" on the same cap above.

Hmm, I don't know if capacitors can register disgust. Perhaps it doesn't
like the look of your tonsils. Do you trim your nose hairs ?

> I have put this board to a test in a fridge with -16 deg C and +85 deg C
> (+/- 5 deg C).  And the board still works.  Also, this behaviour has been
> observed with many of the same boards and many of different kind of
> board.

The probem is the water vapour in your breath

> Is this a humidity problem?  Would this problem occur again after it rains
> on the unit (in IP68 box) and then there is sun right after the rain?

Possible, but unlikely. A conformal coating would prevent humidity
affecting the circuit. You could do it yourself with a can of polyurethane.
Then you can huff on it till the cows come home or you fall over

You should also check that someone's fingers haven't been trapped
in the box to avoid problem 1

You could ground the case of the crystal. Be quick and careful, particularly
with the small cylindrical 32k types, or tie it down with a loop of grounded
bare wire. In a sealed box though this shouldn't be necessary

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2004\01\20@183932 by Bob Axtell

face picon face
GREAT Comment!

Ömer Yalhý wrote:

{Quote hidden}

I'll bet that you don't have a good GND return path for the GND of the crystal capacitors. You have to be very careful about layouts. Your finger offers a different (better?) gnd that the circuit offers.

But, having said that, it is also an issue when moisture gets into a
system. The impedance is so high, that almost anything can affect it.

{Quote hidden}

Theoretically 22pF should be OK, but I always make it 33pF; it's slower to start up, but not as noise sensitive.

>
> Why does this happen?  Bad board design?

Sometimes, i.e. the GND issue above.

> Has anybody else experienced this before?

Yes. It was a layout problem (the GND return of the crystal caps ran around the board before it found its way to the PIC GND pin.

> How would I know this wouldn't happen after the unit is shipped?
> Is this a humidity problem?  Would this problem occur again after it rains
> on the unit (in IP68 box) and then there is sun right after the rain?

Yes, in some cases I spray the oscillator section of the board with a protective plastic coating. NOTE: make sure the can is grounded before you spray.


> P.s. The unit will be in an IP68 enclosure and the problem occurred without
> the enclosure, no way to test it inside the enclosure.

You might want to consider a sealed case, with a packet of moisture absorber. I like the clay type best, very easy to recycle.

> I am sorry for the lengthy message but I am really curious as to what is
> going on here and to find out if I can take any precautions.

--Bob
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               Bob Axtell
       PIC Hardware & Firmware Dev
         http://beam.to/baxtell
             1-520-219-2363

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