Erik,

(trying to pull stuff out of 4 levels of comments), after
my experiences, my personal feelings would be:

a) Leave out the cap or put in R1. The cap probably isn't
needed anyway. The chips ALWAYS seem to start up, as long as
the powerup timer is enabled. I don't think simply adding the
diode will save the chip, if R1 is left out.

b) Look at Microchip's datasheet. I think they know there
is a susceptibility on the MCLR pin, and that's why they
show R1 in the circuit, and make the comment they make. R1
definitely limits peak discharge current into the pin to
5v/100ohm = 50mA.

Versus what? 5/10 = .5A ? (whatever the internal MCLR
impedance/resistance is).

c) Personally, I don't think it's the protection diodes that
are being damaged, but rather the programming circuitry internal
to the MCLR pin.

However, w.r.t. the diodes, they are spec'ed at only 25mA max,
and even .1 uf could probably deliver that during discharge.

d) You can determine if any of your chips have been damaged by
measuring the IR drop across the pullup R during normal operation.
I saw the problems with UV-erasable chips, which of course I
re-programmed over and over, and am uncertain about other chip
types - OTP, EErasable. But Mchip didn't say "this refers to
UV chips only" in their datasheet.

e) I have no idea what the internal damage mechanism might be, but
it makes sense that the problem might get worse with time, being
added to every time the system is powered down, and the cap discharges
into the MCLR pin.
----------

All in all, I'd say use R1 if you use the cap (and don't use the
cap if your aren't using R1) - ie, follow the datasheet advice.

- Dan Michaels
Oricom Technologies
http://www.sni.net/~oricom
====================

At 06:03 PM 2/22/00 -0800, you wrote:
>At 05:42 PM 2/22/00 -0700, you wrote:
>>At 01:38 PM 2/22/00 -0800, you wrote:
>>>At 01:19 PM 2/22/00 -0800, you wrote:
>>>>How about adding the diodes as shown in the data book??
>>>>We've been using the pullup (47K), diode, and .01 to .1uF cap for a couple
>>>>of years.
>>>>Any other experiences with this combination?
>>>>Kelly
>>>
>>>I've been doing this (R and C on MCLR) and I haven't seen any damage to the
>>>parts.  I'm planning on adding the diode on the next spin of the board,
>>>because for me to reprogram these suckers with my boot loader I have to
>>>wait like 10 minutes for the cap to drain (unless I grab it with tweezers).
>>> The tweezer approach won't work when the boards are conformal coated!  BTW
>>>I'm using 16F876 parts.
>>>
>>>HTH
>>>
>>
>>Do you also use R1 (the series R between the cap and MCLR)? With R1
>>in there, there will probably never be a problem.
>>
>>Also, if you want to keep the cap, why not add a reset switch to
>>gnd (with 100 ohm in series)?
>>
>
>I'm not using the series resistor to MCLR.
>
>I can't have any external reset switches.  This is going in a completely
>potted case (or at least potted) to get around harsh environment problems.
>
>Do you think I will have units dying in the field if I don't have the
>series resistor or diode there?  I will probably add both to my next spin
>as it can't hurt too much, but I've got some concerns for prototypes going
>out the door next week.  I would think that the protection diodes on MCLR
>wouldn't be hurt by the small amount of charge in a .1 or .01uF cap.
>
>Thanks for the heads up though.
>
>Erik Reikes
>Software Engineer
>Xsilogy, Inc.
>
>ereikes@xsilogy.com
>ph : (858) 535-5113
>fax : (858) 535-5163
>cell : (858) 663-1206
>
>