Searching \ for '[PIC]: Clamping MCLR' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/microchip/devices.htm?key=pic
Search entire site for: 'Clamping MCLR'.

Exact match. Not showing close matches.
PICList Thread
'[PIC]: Clamping MCLR'
2002\10\22@165738 by llile

flavicon
face
I am using an external signal to shut down a PIC processor via MCLR.  I
have a circuit that can generate a failure condition which will result in
MCLR dropping to zero, shutting off my chip and resulting in a fail-safe
condition.    I am powering MCLR from a circuit that has about 25V half
wave AC, so I need to filter it, voltage divide it, and then clamp it.

I'd like to clamp the voltage going into MCLR so that it doesn't exceed
5V.  Pulling out my trusty 1n5231B zener, I find that you have to crank
the zener current up to 10-20 mA to get this zener to actually "knee" near
5V.  I first tried a zener current of 0.2 mA and got a pitiful soft zener
knee of about 4.8 volts, not enough to reliably hold the PIC out of reset.
 The tiniest spike would change the voltage a lot.  This results in
excessive power dissipation, excessively large capacitors to filter this
input voltage, unexpected resets during normal operation, and lots of
other interesting problems.  I hadn't planned on adding a heat sink to
this area of the circuit (said sarcastically)


Now I've got several approaches I could try:

1.  Are there protection diodes on the MCLR input?  If so, a high
impedance source could possibly drive MCLR without any further adieu.
OTOH, this is not my preferred approach.  Maybe it would even throw the
PIC into programming mode, causing endless trouble and hairpulling?

2.  Mouser and Digikey don't have any low current zeners, the lowest they
go is about 5 mA zener current.  I have heard that there are lower current
zeners out there, 250 uA or so, but am reluctant to specify them if they
are oddball hard-to-find parts.  Also, I need to prototype this with
something ASAP.

3.  What if I clamped this voltage to the 5V rail with a signal diode?
This would put 5.5 volts or so into the MCLR pin, more than 5.0V Vdd.  How
would this affect the chip?  NoNo?

4. What if the input voltage drove an emitter follower?  Emitter of PNP to
the input voltage, base to 5Vdc, and collector to MCLR?  Hmm, this is
getting better.  If I could draw ASCII circuits I would draw this, but I'm
bad at it.


INteresting problem of the day.  Any approaches, PICsters?

--Lawrence

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email spam_OUTlistservTakeThisOuTspammitvma.mit.edu with SET PICList DIGEST in the body


2002\10\22@172105 by Roman Black

flavicon
face
llile@SALTONUSA.COM wrote:
>
> I am using an external signal to shut down a PIC processor via MCLR.  I
> have a circuit that can generate a failure condition which will result in
> MCLR dropping to zero, shutting off my chip and resulting in a fail-safe
> condition.    I am powering MCLR from a circuit that has about 25V half
> wave AC, so I need to filter it, voltage divide it, and then clamp it.
>
> I'd like to clamp the voltage going into MCLR so that it doesn't exceed
> 5V.  Pulling out my trusty 1n5231B zener, I find that you have to crank
> the zener current up to 10-20 mA to get this zener to actually "knee" near
> 5V.  I first tried a zener current of 0.2 mA and got a pitiful soft zener
> knee of about 4.8 volts, not enough to reliably hold the PIC out of reset.
>   The tiniest spike would change the voltage a lot.


Use the resistor and zener you have, and add a
small capacitor across the zener. Unless you need
to change MCLR voltage rapidly that should fix
your spike issues and 0.2mA should be plenty. :o)
-Roman

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email .....listservKILLspamspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body


2002\10\22@174144 by Dale Botkin

flavicon
face
On Tue, 22 Oct 2002 llilespamKILLspamSALTONUSA.COM wrote:

> I am using an external signal to shut down a PIC processor via MCLR.  I
> have a circuit that can generate a failure condition which will result in
> MCLR dropping to zero,

When it's zero, would it be low or high impedance?

If it'll be low impedance, how about a lowly regular old non-Zener diode,
cathode toward the more or less filtered 25V source and anode toward MCLR,
with maybe a 1K pullup resistor on MCLR?  Now it's pulled up to Vdd, but
when the 25V side goes low MCLR follows, at least down to Vf of the diode.
As long as Vdd is something over 3.5V that should be comfortably low to
keep the PIC reset.  I guess you could even add a pull-down resistor on
the cathode side to hold the PIC reset if the 25V side is simply open,
just as an added failsafe.

Would that work?

Dale

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email .....listservKILLspamspam.....mitvma.mit.edu with SET PICList DIGEST in the body


2002\10\22@174552 by Dwayne Reid

flavicon
face
At 03:49 PM 10/22/02 -0500, EraseMEllilespam_OUTspamTakeThisOuTSALTONUSA.COM wrote:

>4. What if the input voltage drove an emitter follower?  Emitter of PNP to
>the input voltage, base to 5Vdc, and collector to MCLR?  Hmm, this is
>getting better.  If I could draw ASCII circuits I would draw this, but I'm
>bad at it.

The approach I have used in past is very similar to what you mentioned
above: cheap PNP transistor (2n4403) with C to GND, B to Vdd, E to
MCLR.  MCLR goes 1 diode drop above Vdd which has not been a problem (so
far).  Note that this will NOT raise Vdd like a diode to Vdd might.

But I've used zener diodes before and they have worked OK as well.  I don't
even remember who makes the ones that we use - they come from Future or
Arrow and have nice a sharp knee at less than 1 mA.

You can also use a 2 diode 2 transistor clamp similar to the one that Peter
Pere has mentioned previously - this will constrain MCLR between Vdd & gnd.

dwayne

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

Celebrating 18 years of Engineering Innovation (1984 - 2002)
 .-.   .-.   .-.   .-.   .-.   .-.   .-.   .-.   .-.   .-
    `-'   `-'   `-'   `-'   `-'   `-'   `-'   `-'   `-'
Do NOT send unsolicited commercial email to this email address.
This message neither grants consent to receive unsolicited
commercial email nor is intended to solicit commercial email.

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email @spam@listservKILLspamspammitvma.mit.edu with SET PICList DIGEST in the body


2002\10\22@174936 by Olin Lathrop

face picon face
> I am using an external signal to shut down a PIC processor via MCLR.  I
> have a circuit that can generate a failure condition which will result in
> MCLR dropping to zero, shutting off my chip and resulting in a fail-safe
> condition.    I am powering MCLR from a circuit that has about 25V half
> wave AC, so I need to filter it, voltage divide it, and then clamp it.

I'm assuming you don't need to toggle MCLR very fast.  In that case you can
add a large resistor like 100K in series with the signal and clamp it to the
PIC power with a diode.

> 1.  Are there protection diodes on the MCLR input?

I think there is to GND, but definitely not to Vdd because high voltage
programming would otherwise not be possible.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email KILLspamlistservKILLspamspammitvma.mit.edu with SET PICList DIGEST in the body


2002\10\22@181617 by mike

flavicon
face
On Tue, 22 Oct 2002 15:49:35 -0500, you wrote:

{Quote hidden}

to accept high voltages for programming and test respectively.
The chip will misbehave if either pin rises much above 6V
--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email RemoveMElistservTakeThisOuTspammitvma.mit.edu with SET PICList DIGEST in the body


2002\10\23@120053 by llile

flavicon
face
I constructed a circuit using an NPN transistor yesterday, C to VDD, E to
MCLR, B to my filtered input voltage. (many ways are possible).  I am not
happy with it because of excessive parts count, and also the whole shebang
rebooted a time or two when a noise pulse came along on the Vdd line.

I'll try your simpler diode clamp to VDD today and see how far I get with
it.  Apparently people are saying if MCLR does not rise more than a diode
drop above Vdd it should be safe, although I cannot determine that from
the spec sheets.


-- Lawrence Lile





Olin Lathrop <spamBeGoneolin_piclistspamBeGonespamEMBEDINC.COM>
Sent by: pic microcontroller discussion list <TakeThisOuTPICLISTEraseMEspamspam_OUTMITVMA.MIT.EDU>
10/22/02 04:47 PM
Please respond to pic microcontroller discussion list


       To:     RemoveMEPICLISTspamTakeThisOuTMITVMA.MIT.EDU
       cc:
       Subject:        Re: [PIC]: Clamping MCLR


> I am using an external signal to shut down a PIC processor via MCLR.  I
> have a circuit that can generate a failure condition which will result
in
> MCLR dropping to zero, shutting off my chip and resulting in a fail-safe
> condition.    I am powering MCLR from a circuit that has about 25V half
> wave AC, so I need to filter it, voltage divide it, and then clamp it.

I'm assuming you don't need to toggle MCLR very fast.  In that case you
can
add a large resistor like 100K in series with the signal and clamp it to
the
PIC power with a diode.

> 1.  Are there protection diodes on the MCLR input?

I think there is to GND, but definitely not to Vdd because high voltage
programming would otherwise not be possible.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listservEraseMEspam.....mitvma.mit.edu with SET PICList DIGEST in the body

--
http://www.piclist.com hint: To leave the PICList
EraseMEpiclist-unsubscribe-requestspammitvma.mit.edu


2002\10\23@130328 by Bob Barr

flavicon
face
On Wed, 23 Oct 2002 10:58:41 -0500, Lawrence Lile wrote:

<snip>
>I'll try your simpler diode clamp to VDD today and see how far I get with
>it.  Apparently people are saying if MCLR does not rise more than a diode
>drop above Vdd it should be safe, although I cannot determine that from
>the spec sheets.

It's in the Absolute Maximum Ratings section.


For the 16C745/765, (from DS41124C, page 127):

Any pin except Vdd, MCLR/, and RA4: -0.3V to (Vdd+0.3V)

Vdd -0.3V to +7.5V

MCLR/  -0.3V to +13.25V

RA4 -0.3V to +10.5V


Regards, Bob

--
http://www.piclist.com hint: To leave the PICList
RemoveMEpiclist-unsubscribe-requestEraseMEspamEraseMEmitvma.mit.edu


2002\10\23@131158 by Peter L. Peres

picon face
On Tue, 22 Oct 2002, Dwayne Reid wrote:

*>At 03:49 PM 10/22/02 -0500, RemoveMEllilespam_OUTspamKILLspamSALTONUSA.COM wrote:
*>
*>>4. What if the input voltage drove an emitter follower?  Emitter of PNP to
*>>the input voltage, base to 5Vdc, and collector to MCLR?  Hmm, this is
*>>getting better.  If I could draw ASCII circuits I would draw this, but I'm
*>>bad at it.
*>
*>The approach I have used in past is very similar to what you mentioned
*>above: cheap PNP transistor (2n4403) with C to GND, B to Vdd, E to
*>MCLR.  MCLR goes 1 diode drop above Vdd which has not been a problem (so
*>far).  Note that this will NOT raise Vdd like a diode to Vdd might.
*>
*>But I've used zener diodes before and they have worked OK as well.  I don't
*>even remember who makes the ones that we use - they come from Future or
*>Arrow and have nice a sharp knee at less than 1 mA.
*>
*>You can also use a 2 diode 2 transistor clamp similar to the one that Peter
*>Pere has mentioned previously - this will constrain MCLR between Vdd & gnd.

Actually only one half of it is needed (the upper half), but as you said,
for MCLR it's ok to go slightly beyond Vdd so your approach is better,
using only one transistor.

My full clamp circuit is useful when there are AD inputs in use on the
chip. The half clamp is used for video sync separation ...

Peter

--
http://www.piclist.com hint: To leave the PICList
RemoveMEpiclist-unsubscribe-requestTakeThisOuTspamspammitvma.mit.edu


2002\10\23@131159 by Peter L. Peres

picon face
On Tue, 22 Oct 2002, EraseMEllilespamspamspamBeGoneSALTONUSA.COM wrote:

*>4. What if the input voltage drove an emitter follower?  Emitter of PNP to
*>the input voltage, base to 5Vdc, and collector to MCLR?  Hmm, this is
*>getting better.  If I could draw ASCII circuits I would draw this, but I'm
*>bad at it.

You are approaching my clamp circuit ... just one more diode or
transistor. But the MCLR should be ok to be clamped with a diode towards
Vdd as the programming mode entering requires a lot more voltage.

Peter

--
http://www.piclist.com hint: To leave the PICList
RemoveMEpiclist-unsubscribe-requestKILLspamspammitvma.mit.edu


2002\10\23@131206 by Peter L. Peres

picon face
On Wed, 23 Oct 2002, llileSTOPspamspamspam_OUTSALTONUSA.COM wrote:

*>I constructed a circuit using an NPN transistor yesterday, C to VDD, E to
*>MCLR, B to my filtered input voltage. (many ways are possible).  I am not
*>happy with it because of excessive parts count, and also the whole shebang
*>rebooted a time or two when a noise pulse came along on the Vdd line.

Excessive parts count ? I thought you say part, not parts, for a single
transistor, no ? You should try  Dwayne Ried's circuit. PNP with C to GND,
B to Vdd, E to MCLR, and input your signal to MCLR through a resistor.

*>I'll try your simpler diode clamp to VDD today and see how far I get with
*>it.  Apparently people are saying if MCLR does not rise more than a diode
*>drop above Vdd it should be safe, although I cannot determine that from
*>the spec sheets.

Look at the absolute maximum ratings. It should be there somehwere.

Peter

--
http://www.piclist.com hint: To leave the PICList
spamBeGonepiclist-unsubscribe-requestSTOPspamspamEraseMEmitvma.mit.edu


More... (looser matching)
- Last day of these posts
- In 2002 , 2003 only
- Today
- New search...