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PICList Thread
'[PIC]: No resistor to base of NPN - revisit'
2001\01\05@090253 by Drew Vassallo

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Hey all,

The subject came up a few days ago regarding the lack of a resistor to the
base of an NPN - the original question was with RA4.

I'm trying to drive an LCD by re-using RB6 and RB7.  They are connected to
input switches via external pullups, and those pullups are switched via two
NPN transistors to RA2.  During the time that the switch inputs are not
being polled, I switch off the pullups.  However, in my haste to breadboard
up a setup, I failed to connect a resistor to the base.
My question is: what happens if there is no resistor to the base of RA2 in
this case?  I seem to be having problems writing to the display - sometimes
it works fine, then stops working after only a few messages, other times it
just simply doesn't initialize/output the first message at all.

I wonder if there isn't something happening to the clock/data lines when the
pullups are turned on, at which point these lines are shared with the
pullups.

Thanks,

--Andrew
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2001\01\05@185731 by Matt Bennett

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Drew Vassallo wrote:
>
> Hey all,
>
> The subject came up a few days ago regarding the lack of a resistor to the
> base of an NPN - the original question was with RA4.
>
> I'm trying to drive an LCD by re-using RB6 and RB7.  They are connected to
> input switches via external pullups, and those pullups are switched via two
> NPN transistors to RA2.  During the time that the switch inputs are not
> being polled, I switch off the pullups.  However, in my haste to breadboard
> up a setup, I failed to connect a resistor to the base.
> My question is: what happens if there is no resistor to the base of RA2 in
> this case?  I seem to be having problems writing to the display - sometimes
> it works fine, then stops working after only a few messages, other times it
> just simply doesn't initialize/output the first message at all.
>
> I wonder if there isn't something happening to the clock/data lines when the
> pullups are turned on, at which point these lines are shared with the
> pullups.

I think I can see what you're trying to do- correct me if I am mistaken,
but I think you have the transistors hooked up as emitter followers
(base to the output pin, collector to +V, emitter to the pullups you
mention).  If this is a source of your problems or not, depends on what
you mean by "turning off" RA2.  If you are clearing the bit- you've just
forced that output pin into current limit.  You can turn it off by
tristating that pin (RA2).

Matt

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