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PICList Thread
'Open Drain Pullup'
1999\07\05@022601 by Stewart Pye


I'm interfacing a 16F84 to a 4051 analog mux with a 12V supply.
Is it safe to put a 10k pullup resistor to +12V from the open drain output


1999\07\05@024054 by Jim Robertson

At 23:30 4/07/99 -0700, you wrote:
>RA.4 can be pulled as high as 14V IIRC on the F84 part, should be fine,
>so long as your 12V supply doesn't spike or anything <G>
>  Mark

I am not saying Mark is right or wrong here but I would ask for a second
opinion before doing something as drastic as this. I just had a quick
look at the specs to confirm my suspicion and I did not see anything off hand
that would suggest you can do this. IIRC only the 17Cxx parts have high
pins (and some new 16C5x family member in the future parts list.)


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1999\07\05@024639 by Stewart Pye

All I could see in the data sheet said maximum 6V on all I/O pins too.
What's the point of having an open drain output then?


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1999\07\05@030133 by Peter McAlpine

Hi Stewart,
Could always use a series resistor and a zener
diode to ground or a normal diode to a 5v rail..

Peter Mcalpine - -

{Original Message removed}

1999\07\05@032251 by Stewart Pye

Say I used a diode going to 5v, wouldn't this bring the RA4 output down to
about 5.6v. Where would I take the address line from? I need about 10v to
go to the 4051. Anyway, I think I might use an open collector buffer. (i
have three other outputs going to the 4051. I was going to use transistors
for them.) I'll have to glue the buffer to the board as I've run out of space!


At 05:01 PM 5/07/99 +1000, you wrote:
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>{Original Message removed}

1999\07\05@040038 by Mark Willis

You could be right, Jim.  I think I am  & the datasheets are still not
up to date - the latest datasheets still say "Preliminary" i.e. file
DS30430C.PDF, it says nothing about an upper limit for the open-drain
pins (Parameter D150) - ALL the other parts I've been using (`C66 etc.)
have all been 14V limited on pin RA4, and on /MClr as well.  Not a good
idea to PUSH such limits, of course.  Would someone at Microchip PLEASE
Update Those Data Sheets - and add this specific item, which has been
debated here before?  (The F84 sheets could stand to not say
"Preliminary" any more, at least!  <G>)

Some support for my position here:
>From the PICList Archive, seems it's missing from the F84 data sheets
Message Date: Thu Jan 1 14:29:50 1998
Author: John Payson

> But during programming MCLR is held at the programming voltage,
> so at least for that pin >VDD is allowed.
> So my question is: what voltage can be put on RA4?

According to a rep at Microchip, the max voltage on RA4 for the 16F84
is 14V independent of VDD.
Message Date: Thu, 17 Apr 1997 08:52:47 -0700
Author: Brian Boles

It's in all of the newer ('96-97 or greater) data sheets. It was left
off of the old ones inadvertantly and is still not in the 16F84 or
16C84 data sheet.

However, the picture of the RA4 pin in the data sheet has always
correctly shown only a N-channel tr. on the pin in all data sheets.
Note that this feature is on the 16C55X/6X/6XX/7X/7XX/8X/9XX but not
on the 16C5X.

As Dr. Strangelove said, "But the whole point is lost if you don't
tell the world!"

Rgds, Brian.


Jim Robertson wrote:
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1999\07\06@090324 by Robert K. Johnson

       Probable the results would be rather catastrophic, There is a 200µa li
in current injected into any pin. If for ANY reason the protection diodes
are forward biased the chip will go into melt down. This comdition has been
known to burn labels off of 'JW' parts... One of the sure ways to destruct
a chip.

               Robert K. Johnson

At 04:30 PM 7/5/99 +1000, you wrote:
>I'm interfacing a 16F84 to a 4051 analog mux with a 12V supply.
>Is it safe to put a 10k pullup resistor to +12V from the open drain output

1999\07\06@114625 by Sean Breheny

face picon face
Hi Robert,

I don't think there are any protection diodes (or at least, none to Vdd) on
RA4, just because of this. In addition, there is no way the chip could go
into melt down through a 10K resistor.


At 10:02 PM 7/5/99 -0700, you wrote:
>        Probable the results would be rather catastrophic, There is a
200µa limit
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1999\07\06@152322 by Mark Willis

Read the .PDF file; no protection diodes to Vdd on /MClr or RA4,

I'll ask my favorite FAE, though I'm pretty certain.


Sean Breheny wrote:
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