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Pullups on individual port B pins

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Cuffy

Joined: 27 Sep 2007
Posts: 2

Pullups on individual port B pins
Posted: Thu Sep 27, 2007 11:32 am

Is there a way to enable weak pullups on selected port B pins? Specifically I would like to have a weak pull up on RB1 (pin22) only of a 16F877A. Thanks.

PCM programmer

Joined: 06 Sep 2003
Posts: 21708

Posted: Thu Sep 27, 2007 11:40 am

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The data sheet says this:

Quote:

Each of the PORTB pins has a weak internal pull-up.
A single control bit can turn on all the pull-ups.
This is performed by clearing bit RBPU (OPTION_REG<7>).
The weak pull-up is automatically turned off when the
port pin is configured as an output. The pull-ups are
disabled on a Power-on Reset.

Cuffy

Joined: 27 Sep 2007
Posts: 2

Pullups on individual port B pins
Posted: Thu Sep 27, 2007 11:54 am

Thanks for the quick reply. I did see that in the spec. I am hoping that somewhere out there lies a clever solution.

SherpaDoug

Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA

Posted: Thu Sep 27, 2007 1:27 pm

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PCM programmer wrote:

The data sheet says this:

Quote:

Each of the PORTB pins has a weak internal pull-up.
A single control bit can turn on all the pull-ups.
This is performed by clearing bit RBPU (OPTION_REG<7>).
The weak pull-up is automatically turned off when the
port pin is configured as an output. The pull-ups are
disabled on a Power-on Reset.

Does that imply you could turn on pull-ups for the port, and then turn off all but RB1 by making those pins temporarily outputs? It might be worth a try. If it works please let us know, and comment your code thoroughly so you will know what you were doing a few years down the road!
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PCM programmer

Joined: 06 Sep 2003
Posts: 21708

Posted: Thu Sep 27, 2007 1:42 pm

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Code:

Does that imply you could turn on pull-ups for the port, and then turn off all but RB1 by making those pins temporarily outputs

No. If you make the pin into an output and then put it back to an input
the pull-up will be enabled again. There is no trick way to fool the
PIC. It works as stated in the data sheet.

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