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'[PIC]: Need advice on using the Open-collector out'
2001\10\24@163709 by Allen Mahurin

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I'm using a 16F84, so you guessed it ... I'm talking
about RA4.  It's the only unused pin I have for a
project, and I'm wanting to clock a decade counter
4017 with it.  I know it can't source much, so I want
it to sink current (trigger the counter on a negative
going pulse).  Anyway, the question is:

The PIC runs on 5V, while the 4017 runs on 9V.  If I
tie the clock input of the 4017 to 9V through a 4.7k
or 10k resistor, and use the PIN input to pull it low
(tied between the resistor and 4017 input), will this
hurt the PIC?  I know it would be sinking current, and
it would be low, which is OK.  But will putting 9V
into the open-collector output cause problems?  I'd
like to see if anyone knows from experience before I
kill a good chip.  :)   If this won't work, does
anyone have any other low-parts count alternatives?

Thanks,

ATM

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2001\10\24@171703 by Bob Blick

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Yes, it will work fine. I have not tested it with a 16F84A but on the
16F84 and 16C71 I have projects running continuously that have ~10 volts
on RA4 no problem for two years now.

On one of them I thought it had hurt the chip but it turned out to be
something else stupid I'd done, not related to RA4.

Cheers,

Bob Blick

On Wed, 24 Oct 2001, Allen Mahurin wrote:

{Quote hidden}

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2001\10\24@173154 by Olin Lathrop

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> But will putting 9V
> into the open-collector output cause problems?

Read the data sheet.  The maximum voltage for the open drain output is well
specified.


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2001\10\24@183315 by Byron A Jeff

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On Wed, Oct 24, 2001 at 05:21:51PM -0400, Olin Lathrop wrote:
> > But will putting 9V
> > into the open-collector output cause problems?
>
> Read the data sheet.  The maximum voltage for the open drain output is well
> specified.

Just took a quick peek at the 16F87X page. 8.5V max. Looks like you'll need
a transistor. Also 7407 buffers and 7406 inverters (both OC) are rated up to
30V.

Hope that helps,

BAJ

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2001\10\24@202306 by Bob Barr

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Bob Blick wrote:
>
>Yes, it will work fine. I have not tested it with a 16F84A but on the
>16F84 and 16C71 I have projects running continuously that have ~10 volts
>on RA4 no problem for two years now.
>

Glad to hear it works but the 16F84A prelim datasheet (I've gotta get the
new one some day) does list +8.5v as the absolute max rating for RA4. I
don't know what the spec is for the F84 or C71.

Regards, Bob


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2001\10\24@203119 by Gennette, Bruce

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About RA4 - Open-collector Output, Schmitt trigger Input

1 - It can't source *ANYTHING*  itself
2 - It can sink quite a bit, and it has in-built protection diodes to drain
off excess voltage (at the level you would use).

As you have suggested the solution for your problem is to make the pin an
output and 'pull up' its output with a resistor from +9V.
The +9V either goes to the 4017 when the PIC +5V output is Hi, or is pulled
into RA4 when it is Lo (thus providing a level shifted output, exactly what
RA4 was designed to do).

If it doesn't reliably clock then add a 1K between the 4017 and the pull up
so that the PIC will always win a struggle to sink the voltage.

You should design for a minimum 1mA @ 9V to trigger the 4017, so from R=V/I
= 9/0.001 = 9K, use 4K7 for a 'more than strong enough' signal.

Bye.

{Original Message removed}

2001\10\25@020751 by Bob Blick

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Olin Lathrop writes:

>Read the data sheet.  The maximum voltage for the open drain output is well
>specified.

Thanks for taking the time to post that helpful answer.

I have the 16C71 datasheet here. Maximum voltage on RA4 is 14 volts, same
as MCLR. The original question was regarding the 16F84. The data sheet for
the 16F84 still specifies 14 volts for MCLR but ignores RA4 completely, and
says the max voltage on any pin is VDD plus 0.6 volts. That is pure
baloney, and what is also pure baloney is the data sheet for the 16F84 is
still marked "preliminary" like any Microchip product still in production.
They don't stop being preliminary until they are discontinued.

I have also tested and used the 16F84 with 10 volts on RA4 with no problems.

So basically I disagree that the voltage on the open drain output being
clearly specified for the 16F84. It is ignored completely. Electrical
specifications have always been a weak spot for Microchip.

Cheerful regards,

Bob Blick

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2001\10\25@034028 by uter van ooijen & floortje hanneman

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> But will putting 9V
> into the open-collector output cause problems?  I'd
> like to see if anyone knows from experience before I
> kill a good chip.  :)   If this won't work, does
> anyone have any other low-parts count alternatives?

In my experience it will not hurt the chip. My WISP programmer uses RA4 of a
16x84 to switch the (13V) VPP to the target chip. But the relevant figure is
not in the datasheet. Note that for a 16f62x the 'maximum open drain
voltage' IS in the datasheet: 8.5 V.

Wouter van Ooijen

Van Ooijen Technische Informatica: http://www.voti.nl
Jal compiler for PIC uC's:  http://www.xs4all.nl/~wf/wouter/pic/jal

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2001\10\25@035200 by Vasile Surducan

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On Wed, 24 Oct 2001, Bob Blick wrote:

>
> So basically I disagree that the voltage on the open drain output being
> clearly specified for the 16F84. It is ignored completely. Electrical
> specifications have always been a weak spot for Microchip.
>
 Agree with your conclusions ( there also many other lacks on Microchip
data sheets and too many revisions, what is writing in rev b there isn't
in rev c so you must list all datasheet revisions and understand what you
want. )
Cheers and respectfully,
Vasile

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2001\10\25@035343 by uter van ooijen & floortje hanneman

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> Read the data sheet.
> The maximum voltage for the open drain output is well specified.
Olin, can you name the exact datasheet and page you are referring to? Maybe
16F84A?

I once got an answer to this question
from a uChip guy who said that on a (nowadays extinct) 16c84 14V was fine.

Wouter van Ooijen

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2001\10\25@080120 by Allen Mahurin

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Thanks guys!  This and Bob's feedback are what I was
looking for.  It's not hard to "read the datasheet",
but unfortunately, the datasheets for any chip list
what it's rated to do.  While you *should* stay within
these limits, I was wondering what experience you guys
have had actually using it.

On a side note, the 4017 is only rated to source about
10mA per output (if I recall correctly), but I've
built a circuit which drives 20mA per output ... built
the same circuit at least a couple of dozen times, and
never had any problems.  Again, this may not be
"optimal" design practices - but I'm not
mass-marketing anything, or putting it into a
life-and-death type of circuit.  Thanks for the
feedback and experience.   :)

ATM

--- "Gennette, Bruce" <.....bruce.gennetteKILLspamspam@spam@TAFE.NSW.EDU.AU>
wrote:
{Quote hidden}

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2001\10\25@090904 by Olin Lathrop

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> About RA4 - Open-collector Output, Schmitt trigger Input
>
> ... and it has in-built protection diodes to drain
> off excess voltage (at the level you would use).

No it doesn't.  It can't because the voltage allowed on the open drain is
higher than the supply voltage by much more than a diode drop.


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2001\10\25@090924 by Olin Lathrop

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> So basically I disagree that the voltage on the open drain output being
> clearly specified for the 16F84. It is ignored completely. Electrical
> specifications have always been a weak spot for Microchip.

I said it was in the manual because it is in the manual for other PICs, I've
never used an 16F84, and don't have a manual around.  Your comment caused me
to get out the CD and check.  Yes, it's annoying that the recent 16F84 data
sheet is "preliminary", even though this chip has been out quite a while.

However, the spec sheet does clearly state that the maximum voltage of all
input pins except MCLR is Vdd + 600mV.  There is no special mention of RA4
in places where other PIC data sheets do mention it.  I take this as
unambiguously meaning that RA4 shouldn't be pulled higher than 600mV above
Vdd.  This is only unclear if you have a preconcieved notion that RA4 can
tolerate a higher voltage.  Now that I've read the spec sheet, I would not
put a 16F84 into production with an RA4 voltage more than 600mV above Vdd.
If you need RA4 to tolerate a higher voltage, don't use the 16F84.


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(978) 742-9014, .....olinKILLspamspam.....embedinc.com, http://www.embedinc.com

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2001\10\25@092344 by Douglas Butler

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I bet +8.5V is more that enough for a logic 1 for a 9V 4017.  Use a 1K
pull up and a 15k pull down.  That will give you 8.4V for a logic 1 and
the PIC cal pull a logic 0.

Sherpa Doug

> {Original Message removed}

2001\10\25@113731 by Bob Blick

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On Thu, 25 Oct 2001, Olin Lathrop wrote:
>
> However, the spec sheet does clearly state that the maximum voltage of all
> input pins except MCLR is Vdd + 600mV.  There is no special mention of RA4
> in places where other PIC data sheets do mention it.  I take this as
> unambiguously meaning that RA4 shouldn't be pulled higher than 600mV above
> Vdd.  This is only unclear if you have a preconcieved notion that RA4 can
> tolerate a higher voltage.  Now that I've read the spec sheet, I would not
> put a 16F84 into production with an RA4 voltage more than 600mV above Vdd.
> If you need RA4 to tolerate a higher voltage, don't use the 16F84.

However, since the spec sheet is clearly labeled "preliminary", who is to
say? If you really wanted "to be safe", you would not use ANY part that
had only a preliminary data sheet, under any conditions, in a production
device.

Look at what is actually connected to RA4 and decide for yourself. There's
an N-mosfet and an input buffer. The mosfet is not going to be bothered by
14 volts, so the input buffer is the only question mark. Have Microchip
changed the design of the input buffer from the 16C71 and 16C84 to the
16F84? Sure they've shrunk the die, but have there been changes that would
lower the maximum voltage?

The data sheet may be marked "preliminary", but the maximum voltage has
been tested by people here, and 14 volts is fine. Until Microchip gets
their trip together with regards to specs, we have to look at the old data
sheets and test by ourselves. That is the only way to make a
high-performance and efficient design. If we operated strictly by the data
sheets, we would never have used the 16F84 because it is still
preliminary, or else we would have added extra parts that clearly aren't
needed.

Ever looked inside the connector on a parallel-port Quickcam? There's a
PIC in it, running at 24 MHz. Didn't they sell a few MILLION of these
overclocked cameras with no problems related to overclocking? Actually
testing something yourself is a GOOD THING instead of taking the(incorrect
or omissive) data sheets as word of law. Use the data sheets as part of a
healthy breakfast, not your whole diet. And use the experience of the
members of the piclist as a resource :-)

Cheerful regards,

Bob Blick

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2001\10\25@121539 by Ned Konz

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On Thursday 25 October 2001 08:35 am, Bob Blick wrote:

> Look at what is actually connected to RA4 and decide for yourself. There's
> an N-mosfet and an input buffer. The mosfet is not going to be bothered by
> 14 volts, so the input buffer is the only question mark. Have Microchip
> changed the design of the input buffer from the 16C71 and 16C84 to the
> 16F84? Sure they've shrunk the die, but have there been changes that would
> lower the maximum voltage?

Referring to the 16F84 datasheet,
www.microchip.com/download/lit/pline/picmicro/families/16f8x/30430c.pdf
, the indication is that the RA4 pin shares the same input buffer as the
/MCLR pin (its leakage and Vih voltage specs are the same). So I'd guess that
the protection diodes on the standard ports are missing on this pin. They are
apparently missing on the /MCLR pin, because of the warning about latchup due
to voltages below VSS. I wonder if the same warning applies to RA4 (that is,
should you use a series resistor to limit current from negative spikes when
using it as an output?).

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2001\10\29@020730 by Bala Chandar

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Bob Blick wrote:

> Actually testing something yourself is a GOOD THING
> instead of taking the(incorrect or omissive) data sheets
> as word of law. Use the data sheets as part of a
> healthy breakfast, not your whole diet. And use the
> experience of the members of the piclist as a resource :-)

Very well said, Bob.

In many cases, people go through the data sheets and search for a clue to
solve their problems or clarify their doubts. When they don't find what they
are looking for, they send their questions to this list. I for one have
benefited quite a bit by the collective wisdom and experience of Piclist
experts.

It will be really nice if Microchip has a system of updating the data sheets
as often as needed, on the basis of the questions, clarifications &
discussions in Piclist.

Regards,
Bala

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2001\10\29@045625 by Alan B. Pearce

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>It will be really nice if Microchip has a system of updating the data
sheets
>as often as needed, on the basis of the questions, clarifications &
>discussions in Piclist.

Especially as these things are essentially electronically distributed these
days, so they are not investing in a lot of paper to replace a pile already
in a warehouse, which is what would have happened in the past.

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2001\10\29@053653 by Jinx

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> Especially as these things are essentially electronically distributed
> these days, so they are not investing in a lot of paper to replace a
> pile already in a warehouse, which is what would have happened in
> the past.

It's been my experience that Microchip are one of, if not the, least
helpful micro suppliers, and not only in the datasheet department

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