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'Re[2]: Open drain'
1997\04\16@191346 by Brian Boles

    Jeff, it is truly open drain, there is no parasitic diode to Vdd.

    Rgds, Brian.


______________________________ Reply Separator _________________________________
Subject: Re: Open drain
Author:  Kathy King <spam_OUTjeffTakeThisOuTMICH.COM> at Internet_Exchange
Date:    4/16/97 3:46 PM


At 10:13 AM 4/16/97 -0700, Brian Boles wrote:
>     Yep, that's why its open drain.  Must limit to +14V and if driving a
>     relay, watch out for back EMF.
>
>     Rgds, Brian.
>
>

Brian:

On RA4 on a 16C62x part, does 'open drain' still apply if the part
is powered down? Or will the part load RA4 down (try to draw power
from) if a voltage appears on RA4 and NOT on the VCC pin?

I can eliminate a part in one of my designs if it truly is a open
drain. It was my impression that the diodes to VCC and GND still
existed since RA4 could also be used as a input. Hence, the part
would only be "open drain" between GND and VCC (+-0.6v).

Regards,

------------------------------------
| Jeff King      Aero Data Systems |
| .....jeffKILLspam@spam@mich.com  P.O. Box 510895   |
| (810)471-1787  Livonia, MI 48151 |
|F(810)471-0279  United States     |
------------------------------------

1997\04\17@115901 by 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.


______________________________ Reply Separator _________________________________
Subject: Re: Open drain
Author:  John Payson <supercatKILLspamMCS.NET> at Internet_Exchange
Date:    4/16/97 8:54 PM


[re port pin RA4]
|     Yep, that's why its open drain.  Must limit to +14V and if driving a
|     relay, watch out for back EMF.

Is the +14 limit documented anyplace?  All of the PIC books I've seen
document the max on "any pin other than /Mclr" as Vdd.  Does Microchip
"officially" allow RA4 higher?

1997\04\17@145254 by Jeff King

I had asked:
>Brian:
>
>On RA4 on a 16C62x part, does 'open drain' still apply if the part
>is powered down? Or will the part load RA4 down (try to draw power
>from) if a voltage appears on RA4 and NOT on the VCC pin?
>

Then you said:


At 04:08 PM 4/16/97 -0700, Brian Boles wrote:
>     Jeff, it is truly open drain, there is no parasitic diode to Vdd.
>
>     Rgds, Brian.
>

OK, so if the PIC is POWERED DOWN, and +5Volts is on RA4, it won't
try and power itself from this pin (a 16C621)? RA4 is also a input
pin using a schmitt trigger. This is what I meant by 'truly open
drain'. I feared that the PIC would try and power itself from
this input (RA4) pin when power was disconnected from VCC.

Sorry to be so anal about this, want to make sure before I change my
circuit design. Right now I have a 2N2222 and I'd love to get rid
of it.

Thanks for you help Brian.
Regards,

------------------------------------
| Jeff King      Aero Data Systems |
| .....jeffKILLspam.....mich.com  P.O. Box 510895   |
| (810)471-1787  Livonia, MI 48151 |
|F(810)471-0279  United States     |
------------------------------------

1997\04\17@154638 by Mike

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