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'[PICLIST] MCLR?'
2002\03\09@171503 by Pic Dude

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On a 16F872, and 16F84 for that matter, I've gathered that MCLR
should be connected to Vdd during normal operation.  However,
some circuits I run across show this going thru a 1K or other
resistor.  Why is this?  (I expect that there is a good reason for
adding another component).

Cheers,
-Neil.

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2002\03\09@172545 by Rick C.

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IIRC, tying MCLR to Vdd will not allow you to In Circuit Serial Program
and will not allow you to send a reset to the PIC. If you plan to only
program in an external  programmer, and your code will NEVER require a
hardware reset, tie it high, but that isn't what I'd do.
Rick

Pic Dude wrote:

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2002\03\09@172755 by Dwayne Reid

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At 04:21 PM 3/9/02 -0600, Pic Dude wrote:
>On a 16F872, and 16F84 for that matter, I've gathered that MCLR
>should be connected to Vdd during normal operation.  However,
>some circuits I run across show this going thru a 1K or other
>resistor.  Why is this?  (I expect that there is a good reason for
>adding another component).

Having the resistor in circuit allows you to reset the PIC without having
to remove power by simply touching MCLR to GND momentarily.  It also means
that you can add a power-up reset controller if you need to without having
to hack copper traces.

If the design works reliably without needing any reset control, you should
be able to eliminate the resistor.  Be sure you have tested the worst case
PSU rise time and ensure that VDD goes all the way to 0V upon power-down
before committing to a production design.

dwayne


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2002\03\09@173210 by Jinx

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> On a 16F872, and 16F84 for that matter, I've gathered
> that MCLR should be connected to Vdd during normal
> operation.  However, some circuits I run across show this
> going thru a 1K or other resistor.  Why is this?  (I expect
> that there is a good reason for adding another component).

Only if you want a Reset button / control line to MCLR or add
a capacitor from MCLR to ground to stop PSU noise causing
a reset. Otherwise there's nothing wrong with tying MCLR to
Vcc - in fact it generates a POR (power-on reset). There are
some instances, eg PSU with slow rise-time, when a forced
MCLR reset (by using external RC) may be desirable. AN607
covers power-up issues for single and/or networked devices

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2002\03\09@184556 by Diego Sierra

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Hi!

> > On a 16F872, and 16F84 for that matter, I've gathered
> > that MCLR should be connected to Vdd during normal

From Microchip doc DS39582A, page 146 (16F87XA manual):

"The behavior of the ESD protection on the MCLR pin differs from
previous devices on this family. Voltages applied to the pin that exceed
its specification can result in both RESETS and current consumption
outside device specification during RESET event. For this reason,
Microchip recommends that the MCLR pin no longer be tied directly to
Vdd. The use of a RC network ... is suggested"

The figure of the RC network is a resistor to Vdd, >= 1K, and a cap to
Vss of 0.1uF (not critical it says).

Cheers,
Diego.

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2002\03\10@014216 by Pic Dude

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Gotcha, thanks!

----- Original Message -----
From: "Rick C." <.....rixyKILLspamspam@spam@VISUALLINK.COM>
To: <PICLISTspamKILLspamMITVMA.MIT.EDU>
Sent: Saturday, March 09, 2002 4:28 PM
Subject: Re: MCLR?


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2002\03\10@014852 by Pic Dude

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Diego,

I'm going by the ref manual for the 16F872.  I just downloaded/checked
the doc you mentioned.  It covers the 16F87XA (rather than 16F87X)
and also does not cover the 872.

I haven't read this doc cause I'm not using those chips -- how does this
'A' version of the chip differ from its non-'A' counterparts?

Thanks,
-Neil.



{Original Message removed}

2002\03\10@053417 by Diego Sierra

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Hi!

> the doc you mentioned.  It covers the 16F87XA (rather than 16F87X)
> and also does not cover the 872.

Oops, yes, you4re right.

> 'A' version of the chip differ from its non-'A' counterparts?

The addition of two analog comparators and some improve in writing to
the program flash: a word a time instead of 4 words (don4t trust me 100%
on this because I believe I read something about that on microchip site,
but cannot find it now).

Check this link:

http://www.microchip.com/1010/edit/prelease/archive/9_243/index.htm

Cheers,
Diego.

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2002\03\10@054031 by Diego Sierra

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Hi again!

> 'A' version of the chip differ from its non-'A' counterparts?

I found it. There is a doc for migrating to 16F87X to 16F87XA:

http://www.microchip.com/download/lit/migrat/39591a.pdf

It has a table with the differences (I quoted):

- Two analog comparators with input multiplexing have been added. It4s
inputs are mapped with the existing ADC channels on PORTA. Programmable
reference voltage for comparators is provided
- ICSP programming specifications are different
- Write to FLASH program memory in 4-words blocks, instead of 1-word
blocks

Didn4t I say "don4t trust me 100%" on the last :-)

Cheers,
Diego.

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2002\03\10@145213 by uter van ooijen & floortje hanneman

picon face
On a 16F872, and 16F84 for that matter, I've gathered that MCLR
should be connected to Vdd during normal operation.  However,
some circuits I run across show this going thru a 1K or other
resistor.  Why is this?  (I expect that there is a good reason for
adding another component).

Makes ICSP possible?

Wouter van Ooijen
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2002\03\10@145216 by uter van ooijen & floortje hanneman

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> I'm going by the ref manual for the 16F872.  I just downloaded/checked
> the doc you mentioned.  It covers the 16F87XA (rather than 16F87X)
> and also does not cover the 872.
>
> I haven't read this doc cause I'm not using those chips -- how does this
> 'A' version of the chip differ from its non-'A' counterparts?

1. not available yet (DigiKey shows 0 in stock)
2. comparator and voltage reference like 16f62x
3. algorithm for flash-code write is quite different (and the description is
up to the highly acclaimed microchip standard for confusion!)
4. new (additional) way to program (LVP or HVP), looks like 5* faster

Wouter van Ooijen
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2002\03\10@163532 by mike

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On Sun, 10 Mar 2002 14:47:14 +0100, you wrote:

>On a 16F872, and 16F84 for that matter, I've gathered that MCLR
>should be connected to Vdd during normal operation.  However,
>some circuits I run across show this going thru a 1K or other
>resistor.  Why is this?  (I expect that there is a good reason for
>adding another component).
>
>Makes ICSP possible?
Yes.
Also allows reset to be asserted for testing/debugging

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2002\03\10@181618 by Tony Nixon

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wouter van ooijen & floortje hanneman wrote:

> 3. algorithm for flash-code write is quite different (and the description is
> up to the highly acclaimed microchip standard for confusion!)
> 4. new (additional) way to program (LVP or HVP), looks like 5* faster

Hi Wouter,

Where is this info.

I couldn't find it in Microchip's site.

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2002\03\11@010954 by uter van ooijen & floortje hanneman

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> > 3. algorithm for flash-code write is quite different (and the
description is
> > up to the highly acclaimed microchip standard for confusion!)
> > 4. new (additional) way to program (LVP or HVP), looks like 5* faster

3. 16f877a datasheet
4. 16f877a flash memory programming

Wouter van Ooijen
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'[PIC] 47K not small enough on MCLR?'
2010\11\17@050112 by Forrest W Christian
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I have a design which we are having occasional in-field lockup issues with...  I'm still troubleshooting and trying to reproduce (and cure) on the bench.

The specs of relevance:  3.3V supply, PIC18f26K20, 47K external pulling MCLR high.

Tonight I had a locked-up unit.  Curious if a reset by putting MCLR low would resolve the issue, I hooked a jumper to the MCLR end of the 47K resistor - and amazingly (without anything attached to the other end, the unit reset).

The immediate though was "no, really?", so I of course stuck my finger on the end of the jumper cable - immediate reset, and I could hold it in reset until I removed my finger.

So, my thought is that 47K isn't 'strong enough', and may actually be much of my problem, especially in higher rf or EMI environments...   The use of 47K comes from the fact that this seems to often be the recommended value with an ICD, and that design value typically sticks around... although it would be no problem to put a 10K or other lower value in there...

I'm probably going to change the flags to disable MCLR on future devices, but I'm wondering, does anyone know the 'real' recommended value?

-forres

2010\11\17@060941 by Jan-Erik Soderholm

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I have a hard time beleaving that what you describes
(which seems as the usual EMF-pickup by high-impedence
inputs) would be rellevant with a 47 Kohm pullup.
You need probably several 10's of Mohms to get that.

There is no other I/O pin that (by misstake) happens
to be an input *and* unconnected ?

Or maybe it isn't a 47 Kohm restor.
Or maybe it isn't connected to +3.3V at the other end.



On 2010-11-17 11:01, Forrest W Christian wrote:
{Quote hidden}

> -forres

2010\11\17@062450 by Forrest Christian

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 To answer your questions.

1) I had a hard time believing it as well...
2) I can double check.. but, I have pretty much isolated it to that input - a 2-3 ft jumper wire to the mclr side of the resistor permits the 'touch effect' on the non-resistor end.   I've also done the 'finger rub down' on both sides of the pic, and the effect is only on the MCLR pin
3) VDD is defiitely connected.  The net runs across that pad to get to everything else on the board, and I did a top-side continuity test (that trace is on the bottom).
4) And the 47K resistor has been verified - both color bands (1/8W 5% 47K), and with actual measurement with an ohmmeter.

I haven't replaced the resistor yet - wanting to verify regular lockups with this unit before doing so.

-forrest

On 11/17/10 4:08 AM, Jan-Erik Soderholm wrote:
{Quote hidden}

>> -forrest

2010\11\17@070350 by Jan-Erik Soderholm

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OK... :-)

Then I'm lost.
It is a bit weird that a 47 Kohm pull-up can't
tolerate touching with a finger.

We'll see what others might add... :-)

Jan-Erik.


On 2010-11-17 12:24, Forrest Christian wrote:
{Quote hidden}

2010\11\17@074720 by Michael Rigby-Jones

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> -----Original Message-----
> From: piclist-bouncesspamspam_OUTmit.edu [@spam@piclist-bouncesKILLspamspammit.edu] On
Behalf
> Of Forrest W Christian
> Sent: 17 November 2010 10:01
> To: Microcontroller discussion list - Public.
> Subject: [PIC] 47K not small enough on MCLR?
>
> I have a design which we are having occasional in-field lockup issues
> with...  I'm still troubleshooting and trying to reproduce (and cure)
on
> the bench.
>
> The specs of relevance:  3.3V supply, PIC18f26K20, 47K external
pulling
> MCLR high.
>
Have you actually confirmed the voltage on the MCLR pin is 3.3v under
normal operation? e.g. no excessive leakage currents pulling the voltage
down close to logic 0 threshold?

Regards

Mike

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2010\11\17@084018 by Forrest Christian

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 On 11/17/10 5:47 AM, Michael Rigby-Jones wrote:
> Have you actually confirmed the voltage on the MCLR pin is 3.3v under
> normal operation? e.g. no excessive leakage currents pulling the voltage
> down close to logic 0 threshold?
I'm about 99% sure the answer to this is yes...  but will have to double check tomorrow..

The good news as far as testing goes is that this board seems to be locking up every 2-3 hours.  So after 3-4 lockups, I've jumpered across the resistor (I.E. inserted a 0K resistor in it's place ;) and we'll see how long it runs in that configuration.

The even better news is that if this turns out to be the cause of the instability that I likely can release a firmware update (these are field upgradeable) that reprograms the configuration registers (self-reprogrammable on the 'K20) to use that pin as an input not a reset...

-forres

2010\11\17@085859 by Picbits Sales

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I've had a similar thing happen to me twice.

Once was my own fault, the other was a freak occurance.

Both involved decoupling capacitors.

The first time I left off the decoupling capacitor - that was back in my early days of designing PIC related stuff. While the unit worked for 99% of the time there was the occasional glitch/reset/lockup.

The second time I had a 100nF decoupling capacitor across the power pins. When my heater kicked in in the workshop or if I waved my hand near the PIC it would sometimes but not always do strange things. This happened more when I was using the PLL. I debugged and debugged, doublechecked MCLR was disabled then put a 1uF capacitor across the power pins near the PIC. This solved the problem. I then took the 100nF capacitor off and tested it - it was a duff one !!!

Worth double checking on yours - might be a bit of a long shot but make sure you have adequate decoupling as close to the power pins as you can manage.

Dom

{Original Message removed}

2010\11\17@090205 by Olin Lathrop

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Forrest W Christian wrote:
> I'm probably going to change the flags to disable MCLR on future
> devices, but I'm wondering, does anyone know the 'real' recommended
> value?

I use 20K Ohms pullup to Vdd.  That's a good compromise between low enough
impedence to overcome stray noise, but high enough to not get in the way of
most programmers and in-circuit debuggers.

Of course the other part of this is to not leave the MCRL line dangling
around where it can pick up noise in the first place.


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(978) 742-9014.  Gold level PIC consultants since 2000

2010\11\17@090350 by Jan-Erik Soderholm

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On 2010-11-17 14:40, Forrest Christian wrote:
>    On 11/17/10 5:47 AM, Michael Rigby-Jones wrote:
>> Have you actually confirmed the voltage on the MCLR pin is 3.3v under
>> normal operation? e.g. no excessive leakage currents pulling the voltage
>> down close to logic 0 threshold?
> I'm about 99% sure the answer to this is yes...  but will have to double
> check tomorrow..
>
> The good news as far as testing goes is that this board seems to be
> locking up every 2-3 hours.  So after 3-4 lockups, I've jumpered across
> the resistor (I.E. inserted a 0K resistor in it's place ;) and we'll see
> how long it runs in that configuration.
>
> The even better news is that if this turns out to be the cause of the
> instability that I likely can release a firmware update (these are field
> upgradeable) that reprograms the configuration registers
> (self-reprogrammable on the 'K20) to use that pin as an input not a
> reset...
>

And you do have a programmer that supports "internal-MCLR" ?
Or maybe you are using a bootloader. And maybe that isn't
an issue with these newer PICs... :-)




> -forres

2010\11\17@092701 by Bob Axtell

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My experience is that 22K is the maximum reliable value.
I usually use 15K if VDD=5V, or 12K if VDD=3.3V.

--Bob

On 11/17/2010 4:08 AM, Jan-Erik Soderholm wrote:
{Quote hidden}

>> -forrest

2010\11\17@095323 by Wouter van Ooijen

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On 17/11/2010 3:26 PM, Bob Axtell wrote:
> My experience is that 22K is the maximum reliable value.
> I usually use 15K if VDD=5V, or 12K if VDD=3.3V.

I always use 33k, never seen an MCLR-related problem.

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2010\11\23@080616 by James Hillman

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>I have a design which we are having occasional in-field lockup issues
> with...  I'm still troubleshooting and trying to reproduce (and cure) on
> the bench.
>
> The specs of relevance:  3.3V supply, PIC18f26K20, 47K external pulling
> MCLR high.
>
> Tonight I had a locked-up unit.  Curious if a reset by putting MCLR low
> would resolve the issue, I hooked a jumper to the MCLR end of the 47K
> resistor - and amazingly (without anything attached to the other end,
> the unit reset).

We had problems with the 18f45k20 locking up after being switched on.
It needed some series resistance into Vdd to limit the rise time.

I looked in the 18f26k20 errata sheet for revisions A4 and A5 (80379A) and found the same fault (section 18):

"The part may hang in the Reset state when VDD
falls to the POR rearm threshold of approximately
1.2 volts then rises at a rate faster than 7500 volts
per second to the operating range. Recovery from
the hung state is possible only by first lowering
VDD to below the POR rearm threshold followed by
raising VDD to the operating range."

If you happen to be using that silicon revision this might be related.

James
The part may hang in the Reset state when VDD falls to the POR rearm threshold of approximately 1.2 volts then rises at a rate faster than 7500 volts per second to the operating range. Recovery from the hung state is possible only by first lowering VDD to below the POR rearm threshold followed by raising VDD to the operating range.

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