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'Application Note AN689'
1999\05\01@080216 by Sam Powell

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Where can I find this app note -it is not on the microchip web site. Can you
mail it to me at the below address please?

Thanks

ICQ: 10061444
Web: come.to/thepicarchive
E-Mail: spam_OUTsampowelTakeThisOuTspamglobalnet.co.uk


----- Original Message -----
From: Ed Edmondson <.....EaejrphdKILLspamspam@spam@AOL.COM>
Sent: Friday, April 30, 1999 3:22 AM
Subject: Fwd: Application Note AN689


> A response from Microchip about the AN everyone was discussing last week.
> Any comments?
>
> Dr. Ed
>

1999\05\01@091045 by Russell McMahon

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Rodger

I am writing to comment on your comments on AN689.
I have not yet looked at AN689 myself but it sounds like a very
useful and ingenious application.

However, I have personally experienced problems with PICs (16F84)
when attempting to use the internal protection diodes to limit i/o
pin excursions. The processor operated in an undefined and
intermittent manner. Changing the design so that it guaranteed the
input pin would not sink/source protection diode current cured the
problem totally. The current drawn was much smaller than the PIC
design spec (ie about 1ma max). The application was a simple RS232
input using a 10K ohm series resistor to the PIC pin.

Use of IC protection diodes during normal circuit operation is a
frowned upon design practice and I only did it because it was
actually suggested (by MicroEngineering labs) as a viable RS232 input
method.

In the case of the 16F84 (and I assume all other PICs) the data sheet
specifically gives Vss as the lowest permissible limit for Vil, and
Vdd as the maximum for Vih during operation. (16F84 data sheet, 1998,
DS30430C, table 10.3, DC characteristics). While I have heard MANY
people state that PICs function correctly with protection diode
current flow, I would not recommend this method to anyone and
presumably Microchip would point to the data sheet limits if a
designer complained about problems caused by using this technique.

Where appropriate I have used small external Schottky diodes (eg
BAT85) to clamp I/O pin input transitions slightly outside the supply
rails but not far enough out to allow the internal diodes to conduct.
Even this nominally violates Microchips data sheet specification.

Overall I think that PICs are marvelous (albeit nowadays rather
over-priced) devices in their appropriate areas of application.
However, even PICs have their limits and I suspect that both
Microchip and end users will end up happier if designs aren't pushed
beyond spec-sheet limits.


regards


           Russell McMahon

{Original Message removed}

1999\05\01@095306 by D. F. Welch

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The app note is on the web site under PIC-16/17 applications
towards the bottom of the page.
It is not cross referenced in the index

At 02:43 PM 4/30/99 +0100, you wrote:
>Where can I find this app note -it is not on the microchip web site.

The app note is on the web site under PIC-16/17 applications
towards the bottom of the page.
It is not cross referenced in the index



Daniel F. Welch

Director
Research and Development
American Scientific Associates
Email: amerscispamKILLspamflash.net

1999\05\01@115020 by Bob Blick

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>Where can I find this app note -it is not on the microchip web site.

It's at their ftp site:
ftp.microchip.com/Download/Appnote/Category/PIC16/00689A.pdf

1999\05\03@125145 by John Payson

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|However, I have personally experienced problems with PICs (16F84)
|when attempting to use the internal protection diodes to limit i/o
|pin excursions. The processor operated in an undefined and
|intermittent manner. Changing the design so that it guaranteed the
|input pin would not sink/source protection diode current cured the
|problem totally. The current drawn was much smaller than the PIC
|design spec (ie about 1ma max). The application was a simple RS232
|input using a 10K ohm series resistor to the PIC pin.

I've used the input protection diodes on the PIC for low-current clamping
and have never had any problems, PROVIDED that I ensured that the input
did not pull VDD higher than spec.  Of course, RA4 has different rules,
but for most I/O's the clamps seem to work just fine.

I do wish, though, that the specs were a little more clearly written.  If
the specs are supposed to say what Microchip's ap-note suggests they mean,
it might be clearer if they were written:

DC characteristics:
 Input leakage: (?)uA

Operating conditions:
 Maximum input voltage: VDD + (?)V    (note 1)
 Minimum input voltage: Vss - (?)V    (note 1)
 Maximum input current: +/-(?)mA      (note 1)
 Maximum RA4 voltage:   (?)V          (note 2)
 Maximum RA4 current:   (?)mA         (note 2)


Note 1:
 If the voltage on a pin is within the specified range, the leakage
 current will be as specified.  If the voltage exceeds the specified
 range, excessive current may flow into the input; this is permissible
 iff the current remains below the specified maximum.

Note 2:
 Because RA4 does not have a diode to VDD, overvoltage may cause oxide
 breakdown.  RA4 will start sinking current somewhat below the breakdown
 volage, but the current required for device breakdown is very slight.
 External voltage limitting is thus strongly recommended.

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