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-----
From: Rodger Richey <Rodger.Richey@microchip.com>
To: jf@detektor.de <jf@detektor.de>; eaejrphd@aol.com
<eaejrphd@aol.com>
Date: Friday, April 30, 1999 11:35 AM
Subject: Application Note AN689


>
>     Jochen, Ed,
>
>     I have been informed of your concerns with the application note
AN689
>     titled Engineer's Assistant Using a PIC16F84A.  The first
concern is
>     that there is the potential of having voltages higher than Vdd
>     connected to an I/O pin on the PICmicro.  First, the voltage is
>     basically shunted to ground and Vdd by the internal protection
diodes.
>     The external voltage will not affect Vdd in any way.  The 1.5K
>     resistor serves as a current limit into the I/O pin.  As long
as the
>     current is limited to the specified limit of 25mA the I/O pin
will not
>     suffer any damage.  In fact, Microchip has another application
note
>     showing the connection of a 120V AC line to an I/O pin on a
PICmicro
>     through a 10M ohm resistor.  The internal protection diodes are
>     capable of withstanding hundreds of milliamps before the device
>     latches up.
>
>     As to your other concerns, I will forward them to the author of
the
>     application note, Voja, who is an engineer from Yugoslavia.
However,
>     I don't think that you can make any assumptions about the
quality of
>     the firmware without actually reviewing the code and seeing the
>     application work.  Often the firmware in applications with this
level
>     of integration on a device with limited resources can appear
complex.
>     The design is actually quite remarkable, considering that a
logic
>     probe, logic analyzer, serial code receiver, frequency counter,
and
>     battery charger are all implemented using a device that has 13
I/O
>     lines , 1K of program memory and 68 bytes of RAM.  Remember
that there
>     are no other devices in the circuit other than passive
components.
>     And it drives a 2x20 LCD display to boot!
>
>     This is an excellent example of how to multiplex I/O pins to
>     facilitate many different tasks.  We are currently using this
>     application as one of our demos at conferences around the world
with
>     flawless operation.
>
>     Please feel free to email me with any comments you have about
>     Microchip documentation in the future.
>
>     Regards,
>
>     Rodger Richey
>     Project Manager
>     AMSG Applications
>     Microchip Technology Incorporated
>
>