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Pullups on individual port B pins

 
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Cuffy



Joined: 27 Sep 2007
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Pullups on individual port B pins
PostPosted: Thu Sep 27, 2007 11:32 am     Reply with quote
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
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PostPosted: Thu Sep 27, 2007 11:40 am     Reply with quote
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
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Pullups on individual port B pins
PostPosted: Thu Sep 27, 2007 11:54 am     Reply with quote
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
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PostPosted: Thu Sep 27, 2007 1:27 pm     Reply with quote
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



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Posts: 21708

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PostPosted: Thu Sep 27, 2007 1:42 pm     Reply with quote
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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