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'[PIC]: Reading Outputs as Inputs'
2001\01\17@173040 by Shawn Yates

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> Hello all,
>
>       I remember from the tech data that if you read an output pin it will
> read as the physical state of the pin rather than the output buffer.  SO,
> if I were to set a pin as an output and set it, then pull it low, I would
> read a low.  I am trying to reuse an existing board without any
> 'engineering modifications' (read as CLUDGE).  Can I set a pin as an
> output, drive it high and short it to ground with a jumper without
> damaging the processor?  Current overhead is not a big deal, I just dont
> want to cook my processor.
>
> TIA
>
> Shawn

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2001\01\18@073830 by Simon Nield

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shawn:
>Can I set a pin as an
> output, drive it high and short it to ground with a jumper without
> damaging the processor?

I wouldn't recommend it, but you can limit the likelyhood of damage by having the pin tristated off
most of the time and only enabling the drive a few cycles before reading the pin and disabling the
drive straight afterwards.

Regards,
Simon

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2001\01\18@085942 by Bob Ammerman

picon face
> >Can I set a pin as an
> > output, drive it high and short it to ground with a jumper without
> > damaging the processor?
>
> I wouldn't recommend it, but you can limit the likelyhood of damage by
having the pin tristated off
> most of the time and only enabling the drive a few cycles before reading
the pin and disabling the
> drive straight afterwards.

Yes, this will work, and probably safely. Watch out for the current spike on
Vdd, though. You need to check the decoupling and make sure you won't get a
malfunction because of this.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\01\18@103444 by mike

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On Thu, 18 Jan 2001 10:11:43 +0000, you wrote:

>shawn:
>>Can I set a pin as an
>> output, drive it high and short it to ground with a jumper without
>> damaging the processor?
>
>I wouldn't recommend it, but you can limit the likelyhood of damage by having the pin tristated off
>most of the time and only enabling the drive a few cycles before reading the pin and disabling the
>drive straight afterwards.
This can be a useful method to avoid pullup R's (or re-use existing
output pins) in applications where a very occasional input only is
needed, e.g. to enter a factory calibration/test mode. You can
minimise possible damage by only pulsing the output high for a few
cycles if you expect it to be shorted (e.g. after powerup, to enter a
test mode)

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2001\01\18@121041 by Simon Nield

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me:
>pin tristated off

downside of this is having the pin tristated if it is not pulled low -> possible source of noise
into the chip.

might be a good opportunity to make use of the usually irritating lack of a shadow register for the
port states:

; pin0 of port D is either connected to ground, vcc, or open circuit
; if o/c or vcc pin0 is driven high, if connected to ground pin0 is driven low
bsf   PORTD, 0    ; try and pull the pin high
clrw              ; (and wait a bit to let the voltage rise)
xorwf PORTD, F    ; read state of port D and drive it that way
                 ; if pin0 was pulled low it will now be driven low
                 ; if pin0 was floating or high it will now be driven high

then again, no obvious advantages to that over:

; pin0 of port D is either connected to ground or open circuit
; if o/c C will be set, if connected to ground C will be cleared
bsf   PORTD, 0    ; try and pull the pin high
nop               ; wait a bit to let the voltage rise
rrf   PORTD, W    ; state of pin0 port D into Carry
bcf   PORTD, 0    ; pull the pin low



oh well... probably just pointless rambling, but the use of 'clrw/ xorwf portX, F' does feel like it
_should_ be really useful.

Regards,
Simon

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2001\01\18@123834 by Shawn Yates

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Sounds like something to try.  I will give it a go.  Any other insights or
thoughts are welcome.

Thanks

Shawn

{Original Message removed}

2001\01\18@132406 by Barry Gershenfeld

picon face
>       I remember from the tech data that if you read an output pin it will
> read as the physical state of the pin rather than the output buffer.  SO,
> if I were to set a pin as an output and set it, then pull it low, I would
> read a low.  I am trying to reuse an existing board without any
> 'engineering modifications' (read as CLUDGE).  Can I set a pin as an
> output, drive it high and short it to ground with a jumper without
> damaging the processor?

(suggestions posted...)

>Sounds like something to try.  I will give it a go.  Any other insights or
>thoughts are welcome.
>Thanks
>Shawn

Depending on what the external circuitry is, perhaps you could
arrange for this line to be normally 'high' anyway (use a pullup
resistor.)  The you can set the pin as an input and read it
normally.  After all, if you can't be sure the external circuitry
will leave that line high, then you can't be sure that a low
that you read back has come from this shorting switch you want
to add.

Barry

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2001\01\18@133235 by Shawn Yates

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Barry,

I am trying to avoing the adding of a pull-up.  Till now, the pin had no
purpose.  It is not connected to any circuitry except for a header.  Now I
want to use that pin as an input to allow the user to select a setting (by
shorting or not shorting the header to power or ground.

Thanks

Shawn

> read a low.  I am trying to reuse an existing board without any
> 'engineering modifications' (read as CLUDGE).  Can I set a pin as an

Depending on what the external circuitry is, perhaps you could
arrange for this line to be normally 'high' anyway (use a pullup
resistor.)  The you can set the pin as an input and read it
normally.  After all, if you can't be sure the external circuitry
will leave that line high, then you can't be sure that a low
that you read back has come from this shorting switch you want
to add.

Barry

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2001\01\18@134535 by Barry Gershenfeld

picon face
>Barry,
>
>I am trying to avoing the adding of a pull-up.  Till now, the pin had no
>purpose.  It is not connected to any circuitry except for a header.  Now I
>want to use that pin as an input to allow the user to select a setting (by
>shorting or not shorting the header to power or ground.
>Thanks
>Shawn

After I sent the message, I remembered, "without modifying the board". But
I figured if you had to add a switch, you'd have something to attach
a resistor to.  Now I know you're just shorting a connection.

If you're fortunate enough to be using a chip like a 16x74 and haven't
used port B, you can set 'weak pullups' internally and get exactly
what you want.

Well, it was a nice thought, anyway :)

Barry

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2001\01\18@145833 by Bob Ammerman

picon face
> Barry,
>
> I am trying to avoing the adding of a pull-up.  Till now, the pin had no
> purpose.  It is not connected to any circuitry except for a header.  Now I
> want to use that pin as an input to allow the user to select a setting (by
> shorting or not shorting the header to power or ground.

Ah, the light goes on.

Ok, so the pin is connected to either ground, high or nothing.

I am assuming that the pin's connection does _not_ change while the program
is running.

Here is how I would handle it at initialization.

Set pin high
Read pin
if pin is low then
   it is grounded externally
   leave it an input
   goto done.
end if

Set pin low
Read pin
if pin is high then
   it is tied high externally
   leave it an input
   goto done.
endif

if we get here the pin is floating. configure it as an output so it doesn't
float.

done:

Note that this tecnhique lets you detect you three possible states for the
pin (high, low, float).

At worst, you are only driving it for an instruction time or two against the
opposite value.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\01\18@154511 by Dan Michaels

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Shawn Yates wrote:

>I am trying to avoing the adding of a pull-up.  Till now, the pin had no
>purpose.  It is not connected to any circuitry except for a header.  Now I
>want to use that pin as an input to allow the user to select a setting (by
>shorting or not shorting the header to power or ground.
>

Shawn, you have now discovered the reason I always recommend
tying a pulldown [or pullup] on every single unused pin on a
processor, rather than using one of the alternate schemes
[config as output/short to gnd/etc/etc/etc/etc] others have
mentioned recently on other threads.

Additionally, I would strongly recommend using a [small value]
series R inline in pins going to a header. Otherwise, too
durn easy to fry a pin with overvoltage or ESD, or short
an output. Been there, done that, replaced chips [not
superstition].

- danM

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2001\01\18@160942 by Shawn Yates

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Thanks for the heads up.

-----Original Message-----
From: Dan Michaels [spam_OUToricomTakeThisOuTspamUSWEST.NET]
Sent: Thursday, January 18, 2001 3:45 PM
To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
Subject: Re: [PIC]: Reading Outputs as Inputs


Shawn Yates wrote:

>I am trying to avoing the adding of a pull-up.  Till now, the pin had no
>purpose.  It is not connected to any circuitry except for a header.  Now I
>want to use that pin as an input to allow the user to select a setting (by
>shorting or not shorting the header to power or ground.
>

Shawn, you have now discovered the reason I always recommend
tying a pulldown [or pullup] on every single unused pin on a
processor, rather than using one of the alternate schemes
[config as output/short to gnd/etc/etc/etc/etc] others have
mentioned recently on other threads.

Additionally, I would strongly recommend using a [small value]
series R inline in pins going to a header. Otherwise, too
durn easy to fry a pin with overvoltage or ESD, or short
an output. Been there, done that, replaced chips [not
superstition].

- danM

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2001\01\18@162052 by Germain Morbe

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> I am trying to avoing the adding of a pull-up.  Till now, the pin had no
> purpose.  It is not connected to any circuitry except for a header.  Now I
> want to use that pin as an input to allow the user to select a setting (by
> shorting or not shorting the header to power or ground.
>

Shawn,
one thing i frequently use in such situations, even in greater volumes and
without any problems is the following. If the port is totally tristate and
can only be shorted to ground by a jumper, i use the pin and pcb capacity to
examine the jumpers position with this few steps:

1. Have the pin configured as an output and leave it cleared all the time.
2. To read the pin set it to high and configure it as an input within two
consecutive steps.
   Usually you drive current out of the pin for just 2 cycles that way.
3. If open, the pin reads high for AT LEAST 10 to 20 cycles at 4MHz., if
shorted it reads low     just the very next instruction.
You can use a scope to examine the decay time when open. Its pretty slow.

This method is not an idea, it is actually in use on lots of different
series production boards.

Germain Morbe

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2001\01\19@035833 by Roman Black

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Bob Ammerman wrote:
{Quote hidden}

I'm surprised at this! With all the professionals
on this list, I have become used to the "never go
outside the spec" attitude, like never exceeding
clock frequency etc. I really don't like the idea
of driving a pin against a voltage source or sink,
at 5v or 0v, this is as bad as driving a pin into
a short circuit - which I wouldn't do either!

Doing it only for "an instruction or two" doesn't
seem to make it any better, it is still so far
out of spec to be scary. I would much prefer to
overclock a PIC at 1.5x its clock freqency than to
do something like this!
-Roman

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2001\01\19@043553 by Bob Ammerman

picon face
> Bob Ammerman wrote:
> >
> > > Barry,
> > >
> > > I am trying to avoing the adding of a pull-up.  Till now, the pin had
no
> > > purpose.  It is not connected to any circuitry except for a header.
Now I
> > > want to use that pin as an input to allow the user to select a setting
(by
> > > shorting or not shorting the header to power or ground.
> >
> > Ah, the light goes on.
> >
> > Ok, so the pin is connected to either ground, high or nothing.
> >
> > I am assuming that the pin's connection does _not_ change while the
program
{Quote hidden}

doesn't
> > float.
> >
> > done:
> >
> > Note that this tecnhique lets you detect you three possible states for
the
> > pin (high, low, float).
> >
> > At worst, you are only driving it for an instruction time or two against
the
{Quote hidden}

Roman,

I agree that this is _not_ a really good idea, but it isn't as awful as not
being able to perform a required function at all. The original post
indicated that no changes to the board were permitted.

Also, we have to think about the consequences of driving into a short. There
are three that I can think of:

1: Meeting Voh / Vol specs. This isn't  an issue here, or course.

2: Idd spike with attendent Vdd glitch with all kinds of possible trouble -
that's why I mentioned checking for this.

3: Chip damage. This is a thermal issue.

The first question you have to ask: what is the short circuit current.
Looking at a 16F84A as an example (I don't recall what the original poster
was using):

Max output current (low) = 25ma

Max output current (high) = 20ma

Vol @ 8ma = 0.6V (max)

Voh @ 3ma = Vdd - 0.7V (min)

Ok, from the latter two values we can compute a max value for Rdson of the
output FETs of:

0.6V/8ma == 75 ohms (low side)
0.7V/3ma == 233 ohms (high side)

Given that Microchip isn't in the business of giving away silicon, and that
they probably have a pretty good handle on their process, I'm going to go
out on a limb and assume that the minimum Rdson is at least 33% of the max
Rsdon.

Thus:

Rdson (min, low side) = 25 ohms
Rdson (min, high side) = about 80 ohms

Lets take the worst case, the low side.

5V / 25 ohm = 200 ma

Assume we use this code (to minimize the drive time):

   movlw   TRISB,FSR    ; Point at TRIS register
   bcf    PORTB,x             ; Get ready to drive a low
   bcf    INDF,x                  ; Start the short in this instruction
   movf PORTB,W           ; Get the value
   bsf    INDF,x                 ; Clear the short in this instruction
   <now check the value in PORTB at our leisure>

Thus, we are in the shorted condition for only 2 instructions, or 2
microseconds at 4Mhz (the clock rate is another assumption, of course).

So, we are drawing 200ma for 2 microseconds.

Thermally this is the same as 20ma for 20microseconds.

But, we are allowed to draw 20ma indefinitely.

So, unless the thermal time constant of the output driver is less than about
20microseconds, we are not going to thermally damage the output.

Now, I admit that some of the above analysis is not 'worst case', but it is
reasoned, not just a 'try it and see if it works' attitude.

And no, I didn't do this analysis before posting my orignal response, my
answer was based on a 'seat of the pants' engineering judgement (ie: I've
been here before).

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\01\19@045746 by Alan B. Pearce

face picon face
> I'm surprised at this! With all the professionals
> on this list, I have become used to the "never go
> outside the spec" attitude, like never exceeding
> clock frequency etc. I really don't like the idea

Is it really that different to driving directly into the base of a transistor?
The impression I have is most people on this list would do that to drive a relay
or other high current load without a resistor between the output from the PIC
and the base of the transistor - we get enough messages on the list about
reading back the port state when doing this.

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2001\01\19@054842 by Simon Nield

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following on from bob's suggestion of detecting the open circuit state too, how about:

; if pin n port x is pulled high  => result = 0xfe, pin is driven high
; if pin n port x is pulled low   => result = 0x00, pin is driven low
; if pin n port x is open circuit => result = 0xff, pin is driven high
; i.e. bit0 set => open circuit, bits7..1 = pin state
bsf    PORTx, n
clrf   result     ; h:00 o:00 l:00
btfsc  PORTx, n
decf   result, f  ; h:ff o:ff l:00

bcf    PORTx, n
decf   result, f  ; h:fe o:fe l:ff
btfss  PORTx, n
incfsz result, f  ; h:fe o:ff l:00
bsf    PORTx, n   ; set pin high to match state


regards,
Simon

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2001\01\19@083507 by mike

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On Fri, 19 Jan 2001 19:54:39 +1100, you wrote:

>Bob Ammerman wrote:
>>
>> > Barry,
>> >
>> > I am trying to avoing the adding of a pull-up.  Till now, the pin had no
>> > purpose.  It is not connected to any circuitry except for a header.  Now I
>> > want to use that pin as an input to allow the user to select a setting (by
>> > shorting or not shorting the header to power or ground.
<snip>
{Quote hidden}

When you've seen a JW pic with bond wires GLOWING and it still works
afterwards you have a lot of faith in the PICs robustness.
>Doing it only for "an instruction or two" doesn't
>seem to make it any better, it is still so far
>out of spec to be scary. It does make a big difference - the only damage mechanism is thermal
heating. If you limit it to a few microseconds there isn't much chance
of heating even something as small as an on-chip transistor enough to
burn it. Also, a short pulse will be supplied by the decoupling cap, so you
don;t risk causing a brownout.

>I would much prefer to
>overclock a PIC at 1.5x its clock freqency than to
>do something like this!
I would disagree - overclockability will be more dependent on
part-to-part differences than the heating effect of a few
microseconds' output short. I would still only use it for 'very occasional' use - things like
production test though.

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2001\01\19@094642 by Roman Black

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Bob Ammerman wrote:

{Quote hidden}

Hi Bob, I hear everything you're saying, and I didn't
question your (or the other person's) professionality
for a second. :o)

It just seems somewhat un-"piclist like" to discuss the
correct way to drive a pic pin into a dead short as
a "normal" way of measuring the pin state. It surprised
me!

Your argument is sound re the input and output impedances,
although I do question the calc based on max sourcing and
sinking as these are recommended max I, not actual max I.
I'm sure the PIC pin will drive more than 20-25mA if
driven into a short...

But it still raises the point, why are people so harsh
re the overclocking argument but quite happy to discuss
the "correct" method of driving a PIC pin into a dead
short. You could probably come up with some good arguments
why overclocking by 20% will probably perform great too,
but I'm not going to rush to try either grey area.
Out of spec is out of spec. Next we'll be discussing the
best way to drive a 10amp load with a 3amp transistor,
using time constants and such as our argument??? ;o)
-Roman

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2001\01\19@121104 by Bob Ammerman

picon face
> Your argument is sound re the input and output impedances,
> although I do question the calc based on max sourcing and
> sinking as these are recommended max I, not actual max I.
> I'm sure the PIC pin will drive more than 20-25mA if
> driven into a short...

My thinking, as I mentioned, is that these values are based on the Rdson
with determines max specced output current. Mchip isn't about to give you a
lot more silicon than they need to to get the Vol and Voh values, therefore
Rdson can't be much smaller than the numbers I gave. If you really wanted to
be sure, you could measure Rdson across a range of temps and Vcc values.

> But it still raises the point, why are people so harsh
> re the overclocking argument but quite happy to discuss
> the "correct" method of driving a PIC pin into a dead
> short. You could probably come up with some good arguments
> why overclocking by 20% will probably perform great too,
> but I'm not going to rush to try either grey area.

No, overclocking is a very different thing. At a given clock speed,
temperature, Vcc, aging of chip, etc. a signal inside the chip is either on
time or not on time. If they test to a given limit I would _not_ want to go
beyond it (unless I was willing to test to my new limits myself). This
disregards the thermal issues in overclocking, which are probably pretty
small for modest overclocking rates.

> Out of spec is out of spec. Next we'll be discussing the
> best way to drive a 10amp load with a 3amp transistor,
> using time constants and such as our argument??? ;o)

This is often legitimately done. Some transistors are specced at other than
100% duty cycle just for this reason.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\01\19@122006 by Alan B. Pearce

face picon face
>> Out of spec is out of spec. Next we'll be discussing the
>> best way to drive a 10amp load with a 3amp transistor,
>> using time constants and such as our argument??? ;o)

>This is often legitimately done. Some transistors are specced at other than
>100% duty cycle just for this reason.

Certainly power diodes and LED's are. I cannot recall seeing transistors spec'd
this way, but then have not really wanted to try this sort of trick with them.

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2001\01\19@133804 by Dan Michaels

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Alan B. Pearce wrote:
>> I'm surprised at this! With all the professionals
>> on this list, I have become used to the "never go
>> outside the spec" attitude, like never exceeding
>> clock frequency etc. I really don't like the idea
>
>Is it really that different to driving directly into the base of a transistor?
>The impression I have is

most

people on this list would do that to drive a relay
>or other high current load without a resistor between the output from the PIC
>and the base of the transistor - we get enough messages on the list about
>reading back the port state when doing this.
>

I think you should change "most" here to "some".

It always gives me spinal-willies to hear about people doing
this sort of thing. I always assume [perhaps wrongly] that
these people are simply not trained as EE's.

- danM

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2001\01\19@160214 by Dwayne Reid

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At 09:57 AM 1/19/01 +0000, Alan B. Pearce wrote:
> > I'm surprised at this! With all the professionals
> > on this list, I have become used to the "never go
> > outside the spec" attitude, like never exceeding
> > clock frequency etc. I really don't like the idea
>
>Is it really that different to driving directly into the base of a transistor?
>The impression I have is most people on this list would do that to drive a
>relay
>or other high current load without a resistor between the output from the PIC
>and the base of the transistor - we get enough messages on the list about
>reading back the port state when doing this.

Nope - you missed the point in that discussion, which involved only the
open drain pin RA4.  It is quite legal and safe to use a pullup resistor on
pin RA4 and connect the pin directly to the base of a transistor (emitter
to ground).  The drawback in doing so is that R-M-W instructions read back
a logic 0 on RA4 - that is what the discussion was about.

dwayne



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

Celebrating 17 years of Engineering Innovation (1984 - 2001)

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2001\01\20@023508 by Roman Black

flavicon
face
Dan Michaels wrote:
{Quote hidden}

I agree Dan, the bulk of my practical experience is from
repairing things, although I have done a lot of design
over the years. The things I have to repair are always
due to engineers pushing the spec. Like using a 5amp
transistor to switch 5amps.

So if your little car has 150hp you are going to drive
everywhere with the motor screaming at peak hp at
redline towing a big caravan?? How long would it last?
About as long as your new Taiwanese VCR power supply does.

After my initial trade as an industrial electrician
I did a couple of years of industrial electronics,
a very practical training mainly about microprocessors
and how to interface them with everything from special
sensors to huge motors. We were taught that good
design over-rates parts at 10x. So you use a 10w
resistor if it is known to dissipate 1w. This gives
a long term reliability guaranteed to work 24 hours
7 days, even in a hot environment.

So anything less is simply trading reliability for
cost, if you don't care that a large percentage
of your product will fail within a couple of years
that's fine. Some poor sucker like me will end up
being paid to fix it, or should I say improve it,
because I will be putting a decent sized transistor
in it. ;o)

Back on topic, I feel uneasy driving a PIC pin
more than 10mA, I prefer about 5mA if I can.
I don't want it to fail in two years. Driving it
into a short gives me the heebie jeebies. How
much does a resistor cost? 1 cent?
-Roman

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