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'USART and LED dimming'
1996\06\28@091604 by Harrison Cooper

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Ya know, electrons are funny folks.  They come from all
sorts of places.  I was debugging a circuit that the test
tech just could not get to behave.  The thing would trigger
and run a timer, and as the timer ran the one of the LED's
would start to dim...hmmm, kinda like an RC circuit.  To make
a long story short, check to be sure you really do have Vss and
Vdd on the chip. It could be drawing power thru the serial link,
or who knows where else.  When you measure the supply, are you
really measuring on the chip, or just at the rails on the proto
board.  Speaking from experiance, its sometimes the real obvious
that you overlook (you just assume power is there).  It turned out
that the package driving the LEDs did not have Vdd connected (busted
trace _under_ the part).

1996\06\28@100358 by Scott Newell

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>Ya know, electrons are funny folks.  They come from all
>sorts of places.  I was debugging a circuit that the test
>tech just could not get to behave.  The thing would trigger
>and run a timer, and as the timer ran the one of the LED's
>would start to dim...hmmm, kinda like an RC circuit.  To make
>a long story short, check to be sure you really do have Vss and
>Vdd on the chip. It could be drawing power thru the serial link,
>or who knows where else.  When you measure the supply, are you
>really measuring on the chip, or just at the rails on the proto
>board.  Speaking from experiance, its sometimes the real obvious
>that you overlook (you just assume power is there).  It turned out
>that the package driving the LEDs did not have Vdd connected (busted
>trace _under_ the part).

Rechecked the supply, everything's ok at the rails and at the chip.

Thanks,
newell

1996\06\28@113613 by Harrison Cooper

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OK, voltage is OK, and remains stable assuming that you have
a hefty enough supply to keep the voltage at different loads.
Are you sinking or sourcing the LED's ?  If you have a scope,
can you look at the pins for the LED's and determine if the
dimming is caused by a current or strobe problem.  If its
current (meaning that the output does not oscillate), then
try buffering - you also mentioned that you are NOT putting
a series resistor.  You might also break the power circuit
and look at the current going to the PIC.  When you start
to exercise the part, such as during RS232 period, you might
be getting current spikes that may also be corrupting the
data.  Also check the supplies at the translator chip, or
even disconnect the translator and see how the LED's react.
Somehow, it needs to be narrowed down to hardware or software.

1996\06\28@122754 by Brian Read

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Hey Newell, do you have VCC and VSS tied to BOTH of
the VCC and VSS pins? This has burnt more than one
PIC'er. The internals are split up between the power
pins, so BOTH of the VCC and the VSS pins MUST be connected
to their respective rails.

Just a thought,
Brian

1996\06\28@165256 by Reginald Neale

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As others have mentioned, omitting series resistors for LED's is bad practice.
Just because the average supply voltage looks OK doesn't necessarily mean
much. Combination of LED's could be sucking the supply down for just a
microsecond at some particularly critical time. Make sure you've got
adequate filtering and bypassing right at the chip. Easy to tell if this is
part of the problem. Grab a suitable cap and just hold the leads on the
supply pins while it's operating.




.....................Reg Neale.....................
"Ignorance is a renewable resource"   P.J. O'Rourke

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