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'[PIC] TMR2IF dissapearing on 16F628 ?!'
2005\05\04@023227 by Aadu Adok

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hello,

I'm experimenting with 16F628 @ 4mhz. First program is rather simple.
All io pins are defined as outputs.
TMR2IF emerges every 1ms and is handled in ISR (actually toggles the
LED).

So - my LED is blinking and all seems to be fine. BUT - when I take my
breadboard and hold it on my hand, placing my finger near the PIC (from
downside) - the blinking stops!!

There is no galvanical contact - there is plastic between my finger and
the PIC.

Something weird is going on, and I would like to know what is it ;)

thanks,
Aadu Adok

2005\05\04@040825 by Shawn Tan Ser Ngiap

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On Wednesday 04 May 2005 07:32 am, Aadu Adok wrote:
> So - my LED is blinking and all seems to be fine. BUT - when I take my
> breadboard and hold it on my hand, placing my finger near the PIC (from
> downside) - the blinking stops!!

Maybe there are some loose connections... Have you tried migrating your PIC to
another part of the board/another board altogether?? Or maybe it's a loose
power supply cable..

> There is no galvanical contact - there is plastic between my finger and
> the PIC.
> Something weird is going on, and I would like to know what is it ;)

Maybe it's magic... d: Or maybe you're radioactive!!! d:

cheers..

--
with metta,
Shawn Tan

2005\05\04@055045 by ThePicMan

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At 09.32 2005.05.04 +0300, you wrote:
>hello,
>
>I'm experimenting with 16F628 @ 4mhz. First program is rather simple.
>All io pins are defined as outputs.
>TMR2IF emerges every 1ms and is handled in ISR (actually toggles the
>LED).
>
>So - my LED is blinking and all seems to be fine. BUT - when I take my
>breadboard and hold it on my hand, placing my finger near the PIC (from
>downside) - the blinking stops!!
>
>There is no galvanical contact - there is plastic between my finger and
>the PIC.
>
>Something weird is going on, and I would like to know what is it ;)

Try to remake all the solder joints, I can't think of anything else.


>thanks,
>Aadu Adok
>
>-

2005\05\04@060823 by Russell McMahon

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> On Wednesday 04 May 2005 07:32 am, Aadu Adok wrote:
>> So - my LED is blinking and all seems to be fine. BUT - when I take
>> my
>> breadboard and hold it on my hand, placing my finger near the PIC
>> (from
>> downside) - the blinking stops!!

>> There is no galvanical contact - there is plastic between my finger
>> and
>> the PIC.
>> Something weird is going on, and I would like to know what is it ;)

It sounds like the capacitance of your hand is affecting the circuit.
Some key areas to look at.

1.    Is the clock stopping? Have you got recommended capacitors to
ground on each leg if using a crystal. Or any at all? Is clock set to
right mode for the crystal or resonator you are using if using such.

2.    Are all unused pins set to outputs OR set as inputs and
connected to Vdd or ground (either via a resistor (recommended) or
directly. A floating input pin can cause almost anything to happen.
Enabling internal pullups where available is good enough. Setting as
outputs (if the cct allows you to do this safely) is even better.

Report back on the above for more ideas :-)



       RM

2005\05\04@071021 by Aadu Adok

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> It sounds like the capacitance of your hand is affecting the circuit.
> Some key areas to look at.
>
> 1.    Is the clock stopping? Have you got recommended capacitors to
> ground on each leg if using a crystal. Or any at all? Is clock set to
> right mode for the crystal or resonator you are using if using such.

Yes, I have 18pf caps with 4mhz crystal. Also tried with 20mhz - same
result.

> 2.    Are all unused pins set to outputs OR set as inputs and

All unused pins (as the one driving the LED) are defined as outputs.

> Report back on the above for more ideas :-)

Aadu Adok

2005\05\04@071437 by Aadu Adok

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> Try to remake all the solder joints, I can't think of anything else.

Whole thing currently resides on breadboard. No soldering involved. I'm
100% sure that the problem is NOT in 'bad contact'.
I think that Russell pointed into right direction - hand capacitance
comes into play.

I have quite long (1 meter) power (5 volts) wires to breadboard. Maybe
this has something to do with it?

Aadu

2005\05\04@072615 by olin_piclist

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Aadu Adok wrote:
> So - my LED is blinking and all seems to be fine. BUT - when I take my
> breadboard and hold it on my hand, placing my finger near the PIC (from
> downside) - the blinking stops!!

Sounds like a floating input.  In particular check MCLR and PGM.

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2005\05\05@063753 by Howard Winter

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Aadu,

On Wed, 4 May 2005 14:14:31 +0300, Aadu Adok wrote:

> > Try to remake all the solder joints, I can't think of anything else.
>
> Whole thing currently resides on breadboard. No soldering involved. I'm
> 100% sure that the problem is NOT in 'bad contact'.
> I think that Russell pointed into right direction - hand capacitance
> comes into play.

Indeed, and the capacitance of the connection-strips in your solderless breadboard will be an issue too - they
are significant lumps of metal on the scale we're talking about (pF) and you may find that the addition of
hand capacitance to that is stopping the oscillator.  You could try switching to internal oscillation as a
test.

> I have quite long (1 meter) power (5 volts) wires to breadboard. Maybe
> this has something to do with it?

Have you got any capacitors across the power supply at the PIC?  You want at least a 0.1uF ceramic between 0V
and 5V as close to the PIC as you can get it for decoupling, and say 100uF or so electrolytic across the power
strips to give a local reservoir in case the cable-length is an issue.

Cheers,


Howard Winter
St.Albans, England


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