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'[PIC]: 100 really obscure factoids about PICs'
2001\08\03@102403 by Lawrence Lile

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Last night insomnia set in, and I was thinking about all the really obscure
factoids you have to keep in mind to use a PIC.  These are the things that
newbies get stumped on, and end by gnashing their teeth, rending their
clothes, and pulling out their hair.  That's why most PIC experts are bald.
(;-)  I wondered if List Members could add enough obscure factoids to reach
100?  Most of us know these by heart, through long arduous trial and error,
and never think about them twice anymore.


1.  On windowed  parts with internal oscillators, you have to read the
OSCCAL register and write it down before you erase the chip.  (12C508JW,
etc.) Once erased, the number is gone for good.

2.  If you write the OSCCAL number on the TOP side of the JW part, the ink
will be erased by the UV EEPROM eraser after 3-4 times, especially if you
forget and leave it in the eraser all night.  Write the number on a sticker
and stick it to the underside of the part.

3.  In-Circuit-Serial-Programming  (ICSP) will not work if the Low Voltage
Programming pin (often RB3) is held at 5 volts.  This happens if you have a
bunch of switch inputs to portB.  Which is natural if you are using the Port
B Interrupt on Change features.  You need to provide a jumper to hold this
pin to zero volts while doing ICSP.

4.  Banked Memory.  Aaargh.  'Nuff said.

5.  The largest array or lookup table  that can be used in a PIC is equal to
the largest free bank of RAM, unless extreme measures are used.

6.  Read-Write-Modify bug - if you read and write to a port in quick
succession, the resulting state of the pins may not be what you expect.
Better to write to a mirror byte, then move that byte to the port as a
whole.

7. Port pins have a backwards diode to the power supply pin. Overdriving a
port pin can feed energy back to the power supply.  This can be useful if
you understand how it works, maddening if you don't.

8. PICs have pretty good static protection, but it is not perfect.  Don't
wear a sweater, work on carpet, and forget your antistatic workstation on a
cold, dry winter's day.  The PIC will work 2 or 3 times once it has been
shocked, making troubleshooting all that much harder.

9.  On some parts (12C508 16C505 etc.) Pin GP3 or RB3 is an input-only pin.
No MPLAB errors result from trying to output to this pin, but your circuit
sure will not work as planned.

10.  On some parts (16C620 etc) Pin RA4 is an open drain output.  Forgetting
the pullup resistor will result in a useless output pin.



There's the first 10.  Anybody else got additions?

--Lawrence Lile

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2001\08\03@104237 by Alan B. Pearce

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>2.  If you write the OSCCAL number on the TOP side of the JW part, the ink
>will be erased by the UV EEPROM eraser after 3-4 times, especially if you
>forget and leave it in the eraser all night.  Write the number on a sticker
>and stick it to the underside of the part.

Or write it on the underneath in pencil. This seems to stay unless you set
to and scrub the chip.

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2001\08\03@120521 by Alan B. Pearce

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Perhaps the following points need to be added

11.    Picstart Plus machines need to be upgraded periodically to keep up
with the new processors

12.    Power supplies for the PicStart Plus periodically need their
electrolytics changing to stop problems as recommended by Roman.

13.    Your PicStart Plus may not communicate with a fast processor. Keep a
slow machine around as your programmer!

14.    Picstart 16 series programmers are not an uncased version of the
Picstart Plus.

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2001\08\03@124539 by uter van ooijen & floortje hanneman

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> There's the first 10.  Anybody else got additions?

ad 3: The fact that leaving RB3 floating does no harm on some chips (when
the moon is right) should not be taken as proof that holding the pin low is
not needed in geneal. And note that 16f62x's uses RB4 as LVP enable.

- At power-up the F87x's default to analog inputs for some port a pins. You
must disable the analog function to use these pins as discrete (digital)
input. Might be the case for other types too?

- beware of SUBLW and SUBWF. Contrary to what you could expect from an
accumulator-architecture these instructions substract W from something, not
something from W. The SUBLW (Substract Literfal from W) menemonic reflects
this, the SUBWF mnemonic was especially choosen to confuse you.

- beware of SUBLW and SUBWF. Take a good look at the instruction description
to see how the carry is set, it might not agree with what you expect!

- The OPTION instruction can - on the chips that support it! - be used
without any problem. Microchip is not suddenly gona drop this instruction
from an existing chip. The comment in the manuals should be understood as
stating that it might wel be dropped from FUTURE 14-bit core chips. So if
you want to have maximum code portability avoid OPTION, otherwise just use
it.

- When you have programmed your first PIC with with a blinking LED and it
blinks at a rate that you did not expect and no matter what you change the
blinking stays at that rate check whether you have disabled the watchdog.

- When you have programmed your first PIC with with a blinking LED no matter
what you change the LED stays on you might well be blinking the LED at a
very high rate. To check: connect it the other way round (to VCC instead of
GND or vice versa).

- The best way to kill a PIC is to connect the VCC/GND backwards. When this
is not your aim you might consider putting the PIC in a (round pin) socket
and solder an anti-parallel diode across the GND/VCC pins. Make sure that
the combo can still be put in a (flat pin) socket. This arrangement also
protects you against broken PIC pins.

- When your program worked OK in a windowed chip but fails in an OTP (or the
other way round), or your windowed chips fails in bright light (or in total
darkness, or the other way round)  you might have uninitialized variables.
Cover the windowed chip with something  REALLY black (not just UV-black) and
it should behave like an OTP. The metallic-like things used to make 5.25
inch floppies write-protected are reported to work quite well.

my 0.01 euro
Wouter

PS this list MUST be put on techref!

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2001\08\03@135458 by Jeff DeMaagd

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----- Original Message -----
From: Alan B. Pearce <EraseMEA.B.Pearcespam_OUTspamTakeThisOuTRL.AC.UK>
> Perhaps the following points need to be added
>
> 11.    Picstart Plus machines need to be upgraded periodically to keep up
> with the new processors

Might as well spec the chip needed too.  I know it's in my manual
somewhere...

> 12.    Power supplies for the PicStart Plus periodically need their
> electrolytics changing to stop problems as recommended by Roman.

Huh?

> 13.    Your PicStart Plus may not communicate with a fast processor. Keep
a
> slow machine around as your programmer!

How fast?  What does system CPU speed matter to the serial port?  Is it
because uC is still sticking to Win16 binaries rather than keeping up with a
more accepted Windows API for the newer systems?  Or is it that motherboard
makers are pulling out certain backward compatibilities?

Jeff

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2001\08\04@044301 by Peter L. Peres

picon face
> 2.  If you write the OSCCAL number on the TOP side of the JW part, the
> ink will be erased by the UV EEPROM eraser after 3-4 times, especially
> if you forget and leave it in the eraser all night.  Write the number
> on a sticker and stick it to the underside of the part.

So far the only thing that worked for me (even pencil wears off) was
scribing the value with a steel needle on the brass part on top. Put the
chip in a solid socket with a shim under it (PCB strip) while doing this
to avoid squishing the pins.

Peter

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2001\08\04@044314 by Peter L. Peres

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+ If your 12C508A project uses GPIO.3 (aka Vpp) as a digital A/D input or
otherwise relies on the trigger input levels, and acts erratically,
suspect the pin was subjected to static (ESD) damage. The GPIO.3 pin has
no protection diode to Vdd because it must withstand the programming
voltage. For development purposes add a 15V tranzorb between GPIO.3 and
Vss. Do this in despite of various archived messages complaining about
'unstable' trigger levels on that pin. Once you see erratic behavior
assume it has happened and do not trust the part again.

0.01 Ag.
Peter

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2001\08\04@115719 by Olin Lathrop

face picon face
> There's the first 10.  Anybody else got additions?

11 - Always cover the window on a JW part when using in a circuit.  Flaky
operation can result otherwise.

12 - PICs that have A/Ds wake up with the pins set up for A/D input, not
digital operation as you might expect.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, RemoveMEolinTakeThisOuTspamembedinc.com, http://www.embedinc.com

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2001\08\04@115725 by Olin Lathrop

face picon face
> > 13.    Your PicStart Plus may not communicate with a fast processor.
Keep
> a
> > slow machine around as your programmer!
>
> How fast?  What does system CPU speed matter to the serial port?  Is it
> because uC is still sticking to Win16 binaries rather than keeping up with
a
> more accepted Windows API for the newer systems?  Or is it that
motherboard
> makers are pulling out certain backward compatibilities?

I find this hard to believe too.  I've used my Picstart+ with a 933MHz
Pentium III running Windows 2000 without trouble.  My guess is some people
ran into problems for other reasons and somehow thought the faster machine
was the problem.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, TakeThisOuTolinEraseMEspamspam_OUTembedinc.com, http://www.embedinc.com

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2001\08\04@143655 by Thomas C. Sefranek

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Using mine with a 1GHz processor....
Yea, Olin I finally got a "real" computer in the office.

Olin Lathrop wrote:

{Quote hidden}

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2001\08\04@155554 by Jeff DeMaagd

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I don't think CPU speed is really a problem here, as I don't see 1.0 GHz
breaking anything that works well with 933MHz.  I'd investigate other
things, such as motherboard BIOS or OS settings for the serial port.

Jeff

{Original Message removed}

2001\08\04@193910 by Michael Cook

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I've got one for you: Not all of the parts start the program memory at
the same spot. It took me 3 days to figure out why a program that worked
perfectly on a '84 wouldn't work at all on an '87. I eventually figured
out that my program's first few instructions were overwriting the
interrupt table. I change ONE LITTLE ".ORG" AND IT WORKED PERFECTLY.
AFTER THREE DAYS. AHHHHHHHHHHH.

Hope that helps ;)

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2001\08\04@211102 by Sebastian Garcia

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Hi Lawrence,


----- Original Message -----
From: Lawrence Lile <EraseMEllilespamspamspamBeGonetoastmaster.com>
To: <RemoveMEPICLISTKILLspamspamMITVMA.MIT.EDU>
Sent: Friday, August 03, 2001 11:22 AM
Subject: [PIC]: 100 really obscure factoids about PICs



<SNIP>
...

| 7. Port pins have a backwards diode to the power supply pin. Overdriving a
| port pin can feed energy back to the power supply.  This can be useful if
| you understand how it works, maddening if you don't.
|


The clamping diodes are useful when working with large input voltages
needing only an external current limiting resistor for this purpose.


...

| 10.  On some parts (16C620 etc) Pin RA4 is an open drain output.
| Forgetting
| the pullup resistor will result in a useless output pin.


I think this is planned for giving the posibility to easy implement a bus
with wired-logic, and to drive odd voltage loads. That's not an obscure
fact.


Best Regards,

S.-

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2001\08\04@221438 by Bob Barr

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Jeff DeMaagd wrote:
>
>I don't think CPU speed is really a problem here, as I don't see 1.0 GHz
>breaking anything that works well with 933MHz.  I'd investigate other
>things, such as motherboard BIOS or OS settings for the serial port.
>

Couldn't CPU speed pose a problem for an earlier release of MPLAB?

If some folks are running an outdated version, any fixes incorporated to
handle fast CPUs may not be present.

As I recall, MPLAB was written with a 16-bit Borland product (Borland C?).
Borland's Turbo Pascal has a known problem in its system unit on faster
Pentiums. (It has to do with an overflow in the timer code.)

Updating to the latest MPLAB version may be one possible solution for people
encountering problems running it on faster machines.

Regards, Bob



_________________________________________________________________
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2001\08\05@043845 by uter van ooijen & floortje hanneman

picon face
> I've got one for you: Not all of the parts start the program memory at
> the same spot.

AFAIK (but correct me if I am wrong!) all (12 and 14 bit core) pics start at
0, except the ones that have an internal osc start at the highest
instruction and wrap around to 0, so for practical purposes (and if you
don't use the highest instrcution!) they can also regarded to start at 0.
Interrupt starts at 4. Now the SX is a bit special, for it starts at the
highest instruction, but interrupt starts at 0.

Wouter

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2001\08\05@095619 by Jerry Merrill

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At 02:43 AM 8/5/01, you wrote:
> > I've got one for you: Not all of the parts start the program memory at
> > the same spot.
>
>AFAIK (but correct me if I am wrong!) all (12 and 14 bit core) pics start at
>0, <SNIP>

ALL 12 bitters start at TOP OF MEMORY and ALL 14 bitters start at 0.  Often
people depend (perhaps without knowing it) on the PC to wrap around from
TOP to 0, making it APPEARS as though it started from 0.

The 12 bitters with internal OSC ARE documented to start at the top (to
load the OSCCAL value into W) and then wrap to 0.


Jerry Merrill

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2001\08\05@120542 by uter van ooijen & floortje hanneman

picon face
> ALL 12 bitters start at TOP OF MEMORY and ALL 14 bitters start at 0.
Often
> people depend (perhaps without knowing it) on the PC to wrap around from
> TOP to 0, making it APPEARS as though it started from 0.
>
> The 12 bitters with internal OSC ARE documented to start at the top (to
> load the OSCCAL value into W) and then wrap to 0.

So you say all 12-bit cores start at the top but for the non-osccal ones
this is not documented? Good candidate for an obscure factoid...

Wouter

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2001\08\05@123528 by Philip Pemberton

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On Fri, 3 Aug 2001 18:01:56 +0200, wouter van ooijen & floortje hanneman
<EraseMEwfspamEraseMEXS4ALL.NL> said:

> > There's the first 10.  Anybody else got additions?
>
> ad 3: The fact that leaving RB3 floating does no harm on some chips (when
> the moon is right) should not be taken as proof that holding the pin low
is
> not needed in geneal. And note that 16f62x's uses RB4 as LVP enable.
>
> - At power-up the F87x's default to analog inputs for some port a pins.
You
> must disable the analog function to use these pins as discrete (digital)
> input. Might be the case for other types too?
>
> - beware of SUBLW and SUBWF. Contrary to what you could expect from an
> accumulator-architecture these instructions substract W from something,
not
> something from W. The SUBLW (Substract Literfal from W) menemonic reflects
> this, the SUBWF mnemonic was especially choosen to confuse you.
>
> - beware of SUBLW and SUBWF. Take a good look at the instruction
description
{Quote hidden}

matter
> what you change the LED stays on you might well be blinking the LED at a
> very high rate. To check: connect it the other way round (to VCC instead
of
> GND or vice versa).
>
> - The best way to kill a PIC is to connect the VCC/GND backwards. When
this
> is not your aim you might consider putting the PIC in a (round pin) socket
> and solder an anti-parallel diode across the GND/VCC pins. Make sure that
> the combo can still be put in a (flat pin) socket. This arrangement also
> protects you against broken PIC pins.
That's unusual. I've never killed a PIC by putting it in backwards. I did
kill a #5 power supply doing that though. The PSU whistled to tell me the
current limit had been exceeded then promptly died. Now I'm using a 4.5V
regulated PSU (that outputs 4.8V) to run my projects. Thanks for the tip,
Wouter.
I have fitted my only PIC16F84-10/P in a high-quality turned-pin socket
though. Tinned beryllium copper contacts (according to my supplier).

> - When your program worked OK in a windowed chip but fails in an OTP (or
the
> other way round), or your windowed chips fails in bright light (or in
total
> darkness, or the other way round)  you might have uninitialized variables.
> Cover the windowed chip with something  REALLY black (not just UV-black)
and
> it should behave like an OTP. The metallic-like things used to make 5.25
> inch floppies write-protected are reported to work quite well.
They work well on EPROMs too. Get the metallized ones - paper labels worketh
not a toss.

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2001\08\05@125741 by Bob Ammerman

picon face
> > > 13.    Your PicStart Plus may not communicate with a fast processor.
> Keep
> > a
> > > slow machine around as your programmer!
> >
> > How fast?  What does system CPU speed matter to the serial port?  Is it
> > because uC is still sticking to Win16 binaries rather than keeping up
with
> a
> > more accepted Windows API for the newer systems?  Or is it that
> motherboard
> > makers are pulling out certain backward compatibilities?
>
> I find this hard to believe too.  I've used my Picstart+ with a 933MHz
> Pentium III running Windows 2000 without trouble.  My guess is some people
> ran into problems for other reasons and somehow thought the faster machine
> was the problem.

I think this may be related to a combination of a fast PC and an old version
of the Picstart Plus firmware.

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

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2001\08\05@130806 by Jerry Merrill

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No. I'm saying that all 12 bitters are documented to start at the top; just
sometimes people don't realize it and 'get away with' starting at the
bottom because the PC typically wraps around to 0 and the blank instruction
at the top of memory has no ill side effects.  I don't believe this
behavior (wrapping from top to 0) is necessarily guaranteed.

However, the 12 bit devices with internal OSC ARE DOCUMENTED to execute the
instruction at the top of memory and THEN WRAP to 0.

When a 12 bit device is reset, its PC is set to TOP OF MEMORY but its page
select bits are set to page 0.  Therefore, a JMP at the reset vector (top
of memory) will land in PAGE 0.


At 10:52 AM 8/5/01, you wrote:
{Quote hidden}

Jerry Merrill

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2001\08\05@132051 by Bob Ammerman

picon face
> | 7. Port pins have a backwards diode to the power supply pin. Overdriving
a
> | port pin can feed energy back to the power supply.  This can be useful
if
> | you understand how it works, maddening if you don't.
> |
>
>
> The clamping diodes are useful when working with large input voltages
> needing only an external current limiting resistor for this purpose.
>

This comment is likely to reignite a religious war here on PICLIST. There
are those that state that you must have more than a simple series resistor.
Others claim that this technique is valid.

The datasheets give two specifications that seem to apply:

1: in the absolute maximum ratings section: pin voltage is limited to
Vss-0.3 to Vdd+0.3

2: in the dc parameters the maximum current is limited to (for some typical
chips) 20 ma.

The 'series-resistor-is-enough' camp believe that the series resistor will
handle the current limiting, and the on-chip clamping diodes will limit the
pin voltage.

The 'something-more-is-needed' group insist that you cannot depend on the
clamping diodes to do the voltage limiting.

There are several app notes from mChip that seem to endorse the
'series-resistor-is-enough' line of thinking.

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

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2001\08\05@135448 by David VanHorn

flavicon
face
>
> > The clamping diodes are useful when working with large input voltages
> > needing only an external current limiting resistor for this purpose.
> >
>
>This comment is likely to reignite a religious war here on PICLIST. There
>are those that state that you must have more than a simple series resistor.
>Others claim that this technique is valid.

Most devices have similar clamping diodes.
The problem is that most manufacturers won't specify how much current you
can put into a pin in this manner. So, if you end up depending on this,
you're into a grey area as far as a dependable production design.

If you get it wrong, you risk SCR latchup on the die, and the only way out
is to power down, and let the device cool. (assuming it hasn't been toasted)

>2: in the dc parameters the maximum current is limited to (for some typical
>chips) 20 ma.

Output current rating, not related to how much you can safely dump through
the protection diodes.


Why is it though, that the PIC line seems to have so many exceptions,
gotchas, and "features"?


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2001\08\05@141542 by Bob Ammerman

picon face
> >
> > > The clamping diodes are useful when working with large input voltages
> > > needing only an external current limiting resistor for this purpose.
> > >
> >
> >This comment is likely to reignite a religious war here on PICLIST. There
> >are those that state that you must have more than a simple series
resistor.
> >Others claim that this technique is valid.
>
> Most devices have similar clamping diodes.
> The problem is that most manufacturers won't specify how much current you
> can put into a pin in this manner. So, if you end up depending on this,
> you're into a grey area as far as a dependable production design.
>
> If you get it wrong, you risk SCR latchup on the die, and the only way out
> is to power down, and let the device cool. (assuming it hasn't been
toasted)
>
> >2: in the dc parameters the maximum current is limited to (for some
typical
> >chips) 20 ma.
>
> Output current rating, not related to how much you can safely dump through
> the protection diodes.
>

For the '876 line at least, there are specs in the 'absolute maximum
ratings' section that state:

Input clamp current, IIK (VI < 0 or VI > Vdd)  == +/- 20mA
Output clamp current, IOK (VO < 0 or VO > Vdd) == +/- 20mA

Don't get me wrong: you gotta build to be in specs. But you should know what
the specs really are, and not just design  based on old-wives-tales and the
like.

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

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2001\08\05@141956 by uter van ooijen & floortje hanneman

picon face
> Why is it though, that the PIC line seems to have so many exceptions,
> gotchas, and "features"?

I don't think the PICs have more of those than other chips, but there are
more (communicating!) people using (and often abusing) pics.

Wouter

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2001\08\05@142215 by uter van ooijen & floortje hanneman

picon face
> For the '876 line at least, there are specs in the 'absolute maximum
> ratings' section that state:

IMHO the 'absolute maxima ratings' are not the specs under which the device
will function according to all other specs, rather the specs under which the
device will not die (in other words: function according to all its specs
once you apply the 'rated conditions').

Wouter

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2001\08\05@160728 by Bob Ammerman

picon face
----- Original Message -----
From: "wouter van ooijen & floortje hanneman" <.....wfspam_OUTspamXS4ALL.NL>
To: <TakeThisOuTPICLIST.....spamTakeThisOuTMITVMA.MIT.EDU>
Sent: Sunday, August 05, 2001 2:21 PM
Subject: Re: [PIC]: 100 really obscure factoids about PICs


> > For the '876 line at least, there are specs in the 'absolute maximum
> > ratings' section that state:
>
> IMHO the 'absolute maxima ratings' are not the specs under which the
device
> will function according to all other specs, rather the specs under which
the
> device will not die (in other words: function according to all its specs
> once you apply the 'rated conditions').
>
> Wouter
>

Wouter,

You are, of course, right.

I would interpret a 20ma absolute maximum rating on clamp current to
indicate that such a current for a brief interval would not permanently
damage the chip (ie: no SCR lock-up), but that there would be no guarantee
of proper operation during the 20ma pulse.

However, the reality of it is that there is nothing magic about the internal
clamping diodes. mChip has given us information that 20ma will not
permanently damage them, so I'd feel comfortable depending on them to
continuously clamp perhaps 1/100 or even 1/10 as much current, or 0.2 ma or
2 ma.

It would be a trade off. For a high quantity, highly cost sensitive
application, I'd be more inclined to go with the minimum possible parts
count and to depend on the diodes.

Also, note that many of the circuits offered for more extensive (expensive)
clamping are little better than the single resistor at modest voltages.

A typical circuit uses two resistors in series and adds silicon diodes to
clamp the junction of the resistors to Gnd and Vdd, thus limiting that
junction to the range Vss-0.6 to Vdd+0.6 or so. This is _still_ out of
specification for the PIC pin! Saying that 0.6 volts above the rail is only
a little bit out of spec is like a woman saying she is just a little bit
pregnant. You either live strictly within the spec or you accept the fact
that you are not a virgin.

Using a 5.1V 5% zener to clamp the high side is another fallacy. The actual
zener voltage could be as high as 5.36 and Vdd (if also 5%) could be as low
as 4.75. The differece is 0.6V, again well above the specified 0.3V.

As yet another example: imagine a typical keypad interface circuit
consisting of a pullup resistor (or the built in weak pullup) and a series R
to the keypad button. In many cases the voltage waveform induced at the pin
would well exceed the clamping limits if it were not for the clamping
diodes. This particular application is almost always considered acceptable.

Finally, I am sure there is some safe clamping current that will work in
normal operation. I'd love to see mChip document it (even if just as a
'design guidance' type of number).

When I started this discussion I mentioned that talking about depending on
the internal clamping diode would restart a religious war. It looks like I
was at least partly right. :-)

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

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2001\08\05@164327 by Sebastian Garcia

flavicon
face
Hi Bob,


----- Original Message -----
From: Bob Ammerman <TakeThisOuTRAMMERMANKILLspamspamspamPRODIGY.NET>
To: <.....PICLISTspamRemoveMEMITVMA.MIT.EDU>
Sent: Sunday, August 05, 2001 2:16 PM
Subject: Re: [PIC]: 100 really obscure factoids about PICs


| > | 7. Port pins have a backwards diode to the power supply pin.
Overdriving
| a
| > | port pin can feed energy back to the power supply.  This can be useful
| if
| > | you understand how it works, maddening if you don't.
| > |
| >
| >
| > The clamping diodes are useful when working with large input voltages
| > needing only an external current limiting resistor for this purpose.
| >
|
| This comment is likely to reignite a religious war here on PICLIST. There
| are those that state that you must have more than a simple series
| resistor.
| Others claim that this technique is valid.


:)


| The datasheets give two specifications that seem to apply:
|
| 1: in the absolute maximum ratings section: pin voltage is limited to
| Vss-0.3 to Vdd+0.3


That's for the 'F877. For the 'F84 and other devices, the 'delta' is 0.6 V
and not 0.3 V, so when the diodes come in action, and assuming there is
an external current limitation under the maximum rated [your point (2)],
the voltage in the pin gets clipped to (Vdd + 0.6 V) or (Vss - 0.6 V),
according to the spec.

This point it's not totally clear for me. In some document the Input Clamp
Current is defined as: "The current through the diode to Vss/Vdd IF PIN
VOLTAGE EXCEEDS SPECIFICATION"

Then, under this definition, there's no problem about the input voltage
limitation, and the only issue is the current limitation that can be tackled
with only a series resistor.


| 2: in the dc parameters the maximum current is limited to (for some
| typical chips) 20 ma.


OK.


| The 'series-resistor-is-enough' camp believe that the series resistor will
| handle the current limiting, and the on-chip clamping diodes will limit
| the pin voltage.


My only doubt here is in reliability since, as Wouter has said, these are
"Absolute Maximum Ratings", and the device under these is not running at
normal operating conditions. But assuming the latter definition of Input
Clamp Current is correct, there's no problem if we limit the current
under (and not equal to) the absolute maximum rated.


| The 'something-more-is-needed' group insist that you cannot depend on the
| clamping diodes to do the voltage limiting.
|
| There are several app notes from mChip that seem to endorse the
| 'series-resistor-is-enough' line of thinking.


Yep. So their engineers promote these practices.


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


Best Regards,

S.-

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2001\08\05@164330 by Sebastian Garcia

flavicon
face
Hi Wouter,

----- Original Message -----
From: wouter van ooijen & floortje hanneman <RemoveMEwfspamspamBeGoneXS4ALL.NL>
To: <spamBeGonePICLIST@spam@spamspam_OUTMITVMA.MIT.EDU>
Sent: Sunday, August 05, 2001 3:21 PM
Subject: Re: [PIC]: 100 really obscure factoids about PICs


| > For the '876 line at least, there are specs in the 'absolute maximum
| > ratings' section that state:
|
| IMHO the 'absolute maxima ratings' are not the specs under which the
device
| will function according to all other specs, rather the specs under which
the
| device will not die (in other words: function according to all its specs
| once you apply the 'rated conditions').
|
| Wouter


Yes, the "Absolute Maximum Ratings" are stress ratings. In practice, maybe
it's not that severe, but the manufacturer always specificate it that way
to avoid legal troubles.

And a so called "good engineering practice" should be mantainig below these
specs.



Regards,

S.-

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2001\08\05@170814 by Andy Jancura

picon face
Hi,

I forgot the subject, again



Hello Bob,

>A typical circuit uses two resistors in series and adds silicon diodes
>to clamp the junction of the resistors to Gnd and Vdd, thus limiting that
>junction to the range Vss-0.6 to Vdd+0.6 or so. This is _still_ out of
>specification for the PIC pin!

in this case you have cascaded R-2D, R-2D (the second 2D inside the chip),
so the second R has limit only 0.3V from 0.6V at the first stage. If this
isn't o.k. you can still choose schottky. But the questions should be :
which voltage are you protecting the pins from? Depending on the answer, you
should go different ways.

Andrej


_________________________________________________________________

_________________________________________________________________
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2001\08\05@173345 by Spehro Pefhany

picon face
At 05:36 PM 8/5/01 -0300, you wrote:
>
>And a so called "good engineering practice" should be mantainig below these
>specs.

Not just below, but WELL below. The definition of "well" depending on the
nature
of the stress and failure modes. For example, latchup tendency increases
greatly with junction temperature.

Using the protection diodes can be a useful technique, but take care, there
have
been cases of oddball problems such as current leaking into analog inputs
on the same port (but I'm not sure that was on PICs).

Best regards,

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2001\08\06@052426 by Alan B. Pearce

face picon face
> > 13.    Your PicStart Plus may not communicate with a fast processor.
Keep
> a
> > slow machine around as your programmer!
>
> How fast?  What does system CPU speed matter to the serial port?  Is it
> because uC is still sticking to Win16 binaries rather than keeping up with
a
> more accepted Windows API for the newer systems?  Or is it that
motherboard
> makers are pulling out certain backward compatibilities?

>I find this hard to believe too.  I've used my Picstart+ with a 933MHz
>Pentium III running Windows 2000 without trouble.  My guess is some people
>ran into problems for other reasons and somehow thought the faster machine
>was the problem.


Well I tried programming a 17C44 to update the programmer firmware, and had
problems on my Dual 1GHz Pentium machine, but the programmer worked alright
on my single Pentium 350MHz machine. After update I tried it again on the
Dual Pentium, and it still has communication problems.

Dual Pentium is running Windows 2000 Professional, and MPLAB 5.31, single
Pentium is NT4 and MPLAB 4.something. Picstart firmware was updated using
the PSF23000.hex file.

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2001\08\06@053502 by Alan B. Pearce
face picon face
>I have fitted my only PIC16F84-10/P in a high-quality turned-pin socket
>though. Tinned beryllium copper contacts (according to my supplier).

These are the only sort of sockets that I ever consider using. Had too much
trouble with cheaper ones over the years.

They also make great "pin savers" for putting a chip into that is going to
be fitted and removed many times, such as UV erasable chips that are used
for development. The socket pins have the nice touch that they will fit into
the same sort of socket and make reliable contact. Then if you bend and
break a pin on your chip saver socket, you put the chip into another socket.

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2001\08\06@054124 by Alan B. Pearce

face picon face
> Why is it though, that the PIC line seems to have so many exceptions,
> gotchas, and "features"?

Is it not just that the PIC community is adept at pushing the boundaries of
the devices they use? I have not seen any specs in the pin protection area
of these devices that is really different to any other CMOS chips. It is
just that "our" community is so into doing things with minimum parts :)

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2001\08\06@054954 by Roman Black

flavicon
face
Alan B. Pearce wrote:
>
> >I have fitted my only PIC16F84-10/P in a high-quality turned-pin socket
> >though. Tinned beryllium copper contacts (according to my supplier).
>
> These are the only sort of sockets that I ever consider using. Had too much
> trouble with cheaper ones over the years.
>
> They also make great "pin savers" for putting a chip into that is going to
> be fitted and removed many times, such as UV erasable chips that are used
> for development. The socket pins have the nice touch that they will fit into
> the same sort of socket and make reliable contact. Then if you bend and
> break a pin on your chip saver socket, you put the chip into another socket.


Turned-pin sockets (even the best gold plated ones)
are not good for multiple insertions. Or should I say
the socket is good for it but the chip won't be.
I keep two types of sockets, turned-pin gold plated
for finished devices, where the chip is inserted once
and stays in there, and the cheaper double flat sprung
type for anything where the chip will be pulled in/out
a number of times, like development and test boards.

The turned pin sockets work by CRUSHING the chips pins
and getting good contact. But if that chip is pulled
in/out 3 or more times you will feel it get noticably
looser as the soft chip pins wear on their edges where
the crush happens. This is bad. They are not meant
for multiple insertions.

The double flat sprung sockets have two spring devices
and press flat against the flat chip pins, and are ok
for hundreds of insertions.
:o)
-Roman

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2001\08\06@063140 by Russell McMahon

picon face
> Also, note that many of the circuits offered for more extensive
(expensive)
> clamping are little better than the single resistor at modest voltages.
>
> A typical circuit uses two resistors in series and adds silicon diodes to
> clamp the junction of the resistors to Gnd and Vdd, thus limiting that
> junction to the range Vss-0.6 to Vdd+0.6 or so. This is _still_ out of
> specification for the PIC pin! Saying that 0.6 volts above the rail is
only
> a little bit out of spec is like a woman saying she is just a little bit
> pregnant. You either live strictly within the spec or you accept the fact
> that you are not a virgin.
>
> Using a 5.1V 5% zener to clamp the high side is another fallacy. The
actual
> zener voltage could be as high as 5.36 and Vdd (if also 5%) could be as
low
> as 4.75. The differece is 0.6V, again well above the specified 0.3V.
>
> As yet another example: imagine a typical keypad interface circuit
> consisting of a pullup resistor (or the built in weak pullup) and a series
R
> to the keypad button. In many cases the voltage waveform induced at the
pin
> would well exceed the clamping limits if it were not for the clamping
> diodes. This particular application is almost always considered
acceptable.
>
> Finally, I am sure there is some safe clamping current that will work in
> normal operation. I'd love to see mChip document it (even if just as a
> 'design guidance' type of number).
>
> When I started this discussion I mentioned that talking about depending on
> the internal clamping diode would restart a religious war. It looks like I
> was at least partly right. :-)


It's not a matter of religious war - it's a matter of whether one is stupid
enough to exceed the operating specification for ANY reason and expect it
not to cost you at some unknown future time. (There, that should help get a
Jihad going ;-) )

As noted, the 20 mA current limit is a stress rating under absolute maximum
ratings and doesn't say anything about whether the IC will function as
intended.

I can assure you that, in SOME cases, using a simple series resistor and a
higher input voltage into a PC pin will definitely cause indeterminate
occasional program malfunction in a 16F84-04P.  Guess how I know. (It was a
simple "RS232" receive application). I can be fairly certain that it will
also happen on most other PICs as well.

Injecting clamping current into the PIC substrate using the protection
diodes produces voltages and currents in the PIC where they may not be
expected during normal operation and any operating failures should not be
unexpected.

I can also report that using a series string of 2 x 10k resistors from an
input voltage to a PIC pin and clamping the middle point of the two
resistors to ground and Vcc with 2 reverse biased small signal (1N9148)
diodes results in good operation for the applications I have tried it in
:-). I suspect that JUST raising the PIC pin voltage to 0.6 volts this way
is liable to be safe in almost every case but, of course, it also risks
exceeding spec by a small margin and one could not complain if this
sometimes caused problems.

As noted, use of a 5.1v zener is not a guaranteed method - but not just
because the zener is 5.1v. A zener has a very "soft" knee and its voltage
will rise substantially with increasing current A zener is typically rated
at 10 mA and will have a higher voltage at higher current.

The "ideal" safe clamp is a pair of normally reverse biased schottky diodes
from PIC pin to Vcc and ground. These are relatively expensive compared to
other solutions but cheap compared to malfunction in most cases. Be aware
that an external voltage clamped to supply with diodes (whether external
Schottky, silicon or PIC internal clamp diodes can "pump up" the PIC Vcc
rail if the PIC and associated circuitry draws less than the clamped current
and the power supply has no provision for clamping over-voltage. This would
be an unusual situation but in some cases could lead to circuit destruction.



.   Russell McMahon
    apptechEraseMEspamclear.net.nz

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2001\08\06@095129 by Lawrence Lile

flavicon
face
> This comment is likely to reignite a religious war here on PICLIST. There
> are those that state that you must have more than a simple series
resistor.
> Others claim that this technique is valid.
>

Death to the Infidels!

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2001\08\06@121912 by Harold M Hallikainen

picon face
       Same goes for RAM! Watch what you put in that first CBLOCK!

Harold


On Sat, 4 Aug 2001 18:37:14 -0500 Michael Cook
<RemoveMEfoobarsoftEraseMEspamspam_OUTFOOBARSOFT.COM> writes:
{Quote hidden}

FCC Rules Online at http://hallikainen.com/FccRules
Lighting control for theatre and television at http://www.dovesystems.com

________________________________________________________________
GET INTERNET ACCESS FROM JUNO!
Juno offers FREE or PREMIUM Internet access for less!
Join Juno today!  For your FREE software, visit:
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2001\08\06@135124 by uter van ooijen & floortje hanneman

picon face
> >I have fitted my only PIC16F84-10/P in a high-quality turned-pin socket
> >though. Tinned beryllium copper contacts (according to my supplier).
>
> These are the only sort of sockets that I ever consider using. Had too
much
> trouble with cheaper ones over the years.
>
> They also make great "pin savers" for putting a chip into that is going to
> be fitted and removed many times, such as UV erasable chips that are used
> for development. The socket pins have the nice touch that they will fit
into
> the same sort of socket and make reliable contact. Then if you bend and
> break a pin on your chip saver socket, you put the chip into another
socket.

In my experience the round socket pins fit well into the cheaper flat pin
sockets. The round pins are much wider than the chip's pins, so contact is
much better.

Wouter

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2001\08\06@135136 by uter van ooijen & floortje hanneman

picon face
> The "ideal" safe clamp is a pair of normally reverse biased schottky
diodes
> from PIC pin to Vcc and ground. These are relatively expensive compared to
> other solutions but cheap compared to malfunction in most cases. Be aware
> that an external voltage clamped to supply with diodes (whether external
> Schottky, silicon or PIC internal clamp diodes can "pump up" the PIC Vcc
> rail if the PIC and associated circuitry draws less than the clamped
current
> and the power supply has no provision for clamping over-voltage. This
would
> be an unusual situation but in some cases could lead to circuit
destruction.

In WLoader I use a clamp (for RS232) with a resistor, a 4V7 zener (limits
voltage at that point to -0.6 .. +5V) and a R divider from that point to Vcc
that raises the -0.6 V to just above 0 V. The first resistor is large enough
to avoid big currents through the zener. IMHO this should be safe, but from
what I heard it works 'almost always', which is maybe acceptable for a
hobby-design but of course not enough for serious work. Which goes to tell
that going to the extremes might save some components but adds a lot of
work...

Wouter

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2001\08\06@170313 by Russell McMahon

picon face
> Alan B. Pearce wrote:
> >
> > >I have fitted my only PIC16F84-10/P in a high-quality turned-pin socket
> > >though. Tinned beryllium copper contacts (according to my supplier).
> >
> > These are the only sort of sockets that I ever consider using. Had too
much
> > trouble with cheaper ones over the years.
> >
> > They also make great "pin savers" for putting a chip into that is going
to
> > be fitted and removed many times, such as UV erasable chips that are
used
> > for development. The socket pins have the nice touch that they will fit
into
> > the same sort of socket and make reliable contact. Then if you bend and
> > break a pin on your chip saver socket, you put the chip into another
socket.
{Quote hidden}

Agree.
The AMP Champ double wipe sockets are relatively cheap and superb in
reliability.
They work well for multiple insertion use.


RM
.

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2001\08\06@172616 by George Tyler

flavicon
face
I had a lot of problems using a pic 12c508 to sense the zero crossing of the
mains. If you use a high value resistor (10M)  from a pin to rectified mains
this pin is fed from a high impedamce, and is very sensitive to electrical
interference as well as strange things happening when current flow into that
pin. I personally do not allow any current to flow in or out of an input. I
also got caught by putting a zener (4V7) on the input, it allows current as
the circuit fires up. I now put a transistor inverter before any input if
there is a possibility of that input signal going outside of the supply
range.

(see, you where right about starting a Jihad!)

{Original Message removed}

2001\08\06@174735 by Dale Botkin

flavicon
face
On Mon, 6 Aug 2001, Alan B. Pearce wrote:

> > Why is it though, that the PIC line seems to have so many exceptions,
> > gotchas, and "features"?
>
> Is it not just that the PIC community is adept at pushing the boundaries of
> the devices they use? I have not seen any specs in the pin protection area
> of these devices that is really different to any other CMOS chips. It is
> just that "our" community is so into doing things with minimum parts :)

I think Microchip tends to document some things that other maufacturers
tend not to, also.  I could be wrong, but I never recall seeing the level
of detail MCHP goes into in the data sheets for, say, an 8051.  Things
like the details of how I/O pins are driven, that sort of thing.   think
we just have more information at our disposal.

Dale
--
A train stops at a train station.  A bus stops at a bus station.
On my desk I have a workstation...

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2001\08\07@144422 by Lawrence Lile

flavicon
face
Well we have two religious wars going now, one about input protection
diodes, and the other about the definition of a "really obscure factoid".
This is great!

Not having wieghed in on the input diode fight, except to say "death to the
Infidels!"  I would like to weigh in on religious war #2 - the definition.

Having started this thread, I claim the right to define a "really obscure
factoid" .

DEFINITION:  Obscure Factoid: (N) a fact about PICS  which, undiscovered,
causes one to gnash one's teeth, emit gutteral growls, and bash 12C508's
against one's workbench with a hammer for hours or even days while
fruitlessly building useless circuits that won't work right.  Once
discovered,knowledge of  these facts will often cause the discoverer to emit
the word "D'Oh!" and whack his or her palm against his or her forehead.


There.  Does that clear it up any?

We're up to around 20 or 30 factoids, so far. I'll post them all when we
have run out.

-- Lawrence Lile

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.


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