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'[PIC]: Re: Did I burn it?'
2006\05\19@122845 by Spehro Pefhany

picon face
At 09:17 AM 5/19/2006 -0700, you wrote:
>Hi,
>
>I have a 18F452 in a home made board. This pic has the main responsibility
>of generating PWM signals to control RC servos.
>A while ago I was doing some tests with AND gates, but not directly
>related to the pic. Then I removed the AND gate, and now the PWM signal is
>wicked.
>The timing is correct (1ms pulse on a 20ms pulsetrain), but the amplitude
>is wrong. Before I had 0-5V, and now I'm getting 0-1V. I tried both with
>the servo connected to it and with nothing connected to the port.
>
>I didn't measure the other outputs (LEDs), but since they are as bright as
>before, I assumed they are ok. Is it possible to damage only one (in this
>case two) ports? Any other suggestions?

If you're looking at it on a 20MHz  or better oscilloscope and the signal
is going cleanly from 0V to ~1V with nothing connected to it, then you've
hosed the output.

If you're reading the average voltage on a DVM, it could be just about
anything. If you don't have any better tools and the chip is soldered in,
reprogram it to set the port high and read the voltages. If you've got a
socket, swap chips with a known-good chip.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->>Test equipment, parts OLED displys http://search.ebay.com/_W0QQsassZspeff


2006\05\19@130432 by Padu

face picon face
From: "Spehro Pefhany" <.....speffKILLspamspam@spam@interlog.com>
> If you're looking at it on a 20MHz  or better oscilloscope and the signal
> is going cleanly from 0V to ~1V with nothing connected to it, then you've
> hosed the output.
>
> If you're reading the average voltage on a DVM, it could be just about
> anything. If you don't have any better tools and the chip is soldered in,
> reprogram it to set the port high and read the voltages. If you've got a
> socket, swap chips with a known-good chip.
>

Sorry, I should've said. I'm looking at it with a 100MHz oscope, and the
chip (PLCC package) is soldered to the board. I don't have a desoldering
tool for this type of chip, so removing it from the board I believe will be
very difficult. I have lots of spare ports, maybe I could cut the trace that
is connected to the damaged port and re-wire it to another pin using hookup
wire?

Cheers

Padu

2006\05\19@144621 by Robert Rolf

picon face
Padu wrote:

> From: "Spehro Pefhany" <speffspamKILLspaminterlog.com>
>
>>If you're looking at it on a 20MHz  or better oscilloscope and the signal
>>is going cleanly from 0V to ~1V with nothing connected to it, then you've
>>hosed the output.
>>
>>If you're reading the average voltage on a DVM, it could be just about
>>anything. If you don't have any better tools and the chip is soldered in,
>>reprogram it to set the port high and read the voltages. If you've got a
>>socket, swap chips with a known-good chip.
>>
> Sorry, I should've said. I'm looking at it with a 100MHz oscope, and the
> chip (PLCC package) is soldered to the board. I don't have a desoldering
> tool for this type of chip, so removing it from the board I believe will be
> very difficult.

Not really.
Get a paint stripper hot air gun. Make a air directing funnel out of
aluminum foil, or plumbing parts, to get a smaller diameter hot air blast.
Carefully bring the running stripper up to the chip on the board (board held up side
down). The solder will melt on all pins very quickly and the chip will
drop off the board. (you may need to tap it).
 Be VERY careful to come in quickly and pull away
quickly so as to not overheat the other parts and traces.
Should take all of 5 seconds with a preheated stripper.

> I have lots of spare ports, maybe I could cut the trace that
> is connected to the damaged port and re-wire it to another pin using hookup
> wire?

Cutting the trace would also let you confirm that it is the PIC that
is fried, and not the logic gate that it connects to.
If the logic is fried, it could be clamping the PIC pin to 1V (about
a diode drop at 20mA)

Robert


2006\05\19@154209 by Padu

face picon face
----- Original Message -----
From: "Robert Rolf" <.....Robert.RolfKILLspamspam.....ualberta.ca>
> Not really.
> Get a paint stripper hot air gun. Make a air directing funnel out of
> aluminum foil, or plumbing parts, to get a smaller diameter hot air blast.
> Carefully bring the running stripper up to the chip on the board (board
> held up side
> down). The solder will melt on all pins very quickly and the chip will
> drop off the board. (you may need to tap it).
>  Be VERY careful to come in quickly and pull away
> quickly so as to not overheat the other parts and traces.
> Should take all of 5 seconds with a preheated stripper.

Do you think it would work with a hot air gun (the ones used for shrinking
plastic)?
I do have one of these, I'll try on a trash board first. Otherwise I'll buy
one at home depot.

{Quote hidden}

So if I read correctly, you'd recommend replacing the chip either way right?

Cheers

Padu

2006\05\19@234010 by Robert Rolf

picon face
Padu wrote:

> ----- Original Message -----
> From: "Robert Rolf" <EraseMERobert.Rolfspam_OUTspamTakeThisOuTualberta.ca>
>
>>Not really.
>>Get a paint stripper hot air gun. Make a air directing funnel out of
>>aluminum foil, or plumbing parts, to get a smaller diameter hot air blast.
>>Carefully bring the running stripper up to the chip on the board (board
>>held up side
>>down). The solder will melt on all pins very quickly and the chip will
>>drop off the board. (you may need to tap it).
>> Be VERY careful to come in quickly and pull away
>>quickly so as to not overheat the other parts and traces.
>>Should take all of 5 seconds with a preheated stripper.
>
>
> Do you think it would work with a hot air gun (the ones used for shrinking
> plastic)?

Yes. They probably get hot enough.
Try it on coil of solder on your bench and see what it takes to
melt it. Needs to be FAST. e.g. less than 10 seconds.
Generally if you can see the heating coils turning red, it's
probably hot enough.
You can crudely control temperature by blocking the air inlet
to make the flow slower and hotter.
You can also you a suitably rated (1800W) light dimmer (commercial
grade) depending on what the hot air gun pulls for current.

{Quote hidden}

No. If it turns out that it is your logic chip that is loading the PIC,
you only need to replace the logic. Had this happen to me so just
passing on what I've learned.


Robert


2006\05\20@161513 by Florian Voelzke

picon face
Robert Rolf wrote:
{Quote hidden}

Or better yet: Get a a hot air gun (just like the paint stripper one,
only better) with air flow and temperature control.

We use one at our company as the main SMD desoldering tool in hardware
development all the time. Both air flow and temperature are digitally
controlled, so overheating is quite impossible. Well, as long as you
don't get impatient and choose the temperature too high... Our failure
rate is so low I can not remember the last time anyone lost a chip due
to the air gun.

Florian

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