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PICList Thread
'Internal Pull ups (Now realy open drain RA4)'
1998\06\24@064629 by Clewer,Brian

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George wrote:

>On Tue, 23 Jun 1998, Craig Lee wrote:
>
>> If you be meaning the missing pullup on RA4 on the 16F84 for instance,
this
>> is
>> because you may wish to control a device that may have a 12V swing to
it.
>> If a pull-up
>> to the 5V supply was there, you would have an unwanted configuration
>> wouldn't
>> you.
>>
>
>I think the 16F84 specs say that voltage on any pin including RA4 must
not
>exceed supply voltage by more than 0.something volts. Due to the
internal
>clamping diodes, you would have a hard time coming over supply+0.6V
>anyway.
>
>Being able to switch high voltages with RA4 would be a nice feature,
>though. Maybe we could talk Microchip into YAR (yet another revision)...
>;-)
>
>Georg.

The data sheet does say that the voltage on any pin should be -0.6v to
VDD+0.6v (and I guess it's because of the protection diodes, as you quite
rightly say) BUT it also says that the open drain pin (RA4) only has
protection diodes connected to VSS only!  Well, if this is true, I would
think that the voltage on the open drain could go higher.

Has anyone used this pin to control voltages above VDD ?

Brian

1998\06\24@180211 by Marc Heuler

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Hi Clewer,Brian (Clewer,Brian), in <35914910@tarkus> on Jun 24 you wrote:

> The data sheet does say that the voltage on any pin should be -0.6v to
> VDD+0.6v (and I guess it's because of the protection diodes, as you quite
> rightly say) BUT it also says that the open drain pin (RA4) only has
> protection diodes connected to VSS only!  Well, if this is true, I would
> think that the voltage on the open drain could go higher.
>
> Has anyone used this pin to control voltages above VDD ?

Some people on this list have previously pointed out that there maybe
exists a manufacturing test mode that is triggered by RA4= 14V.  You should
not violate the datasheet if you want to be on the safe side.

1998\06\25@163736 by wouter van ooijen

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> > Has anyone used this pin to control voltages above VDD ?

In my WISP programmer I use RA4 to switch the VSS of the 16x84 which is
being programmed.
Works fine.
When in high impedance the voltage on RA4 is ~14V, so their is clearly no
protection diode to
either VDD or VSS (both at ~5V).
Someone on the PICLIST (I don't remember who) said that this was allowed.
For details: http://www.xs4all.nl/~wf/wouter/pic/wisp
regrads,
Wouter.

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