Something just occurred to me as I was calculating the current-limiting=20
resistor and pull-up resistor values for this.... why do I need a (the us=
ual)=20
current-limiting resistor for an open-collector output, when the current =
in=20
the ON state (base is high, open collector o/p is off/floating) will come=
=20
from the pull-up resistor.

So this is the usual cmos o/p ckt...

                                  | /
PIC O/P >-----'\/\/\,-------------|<
                 Rb               | \


I added this resistor f/the open collector o/p...

                  Rp
            .--'\/\/\,---> V+
            |                       =20
            |                     | /
PIC O/P >---'--'\/\/\,------------|<
                  Rb              | \


But this seems like a workable option to me...

                  Rp
            .--'\/\/\,---> V+
            |                       =20
            |                     | /
PIC O/P >---'---------------------|<
                                  | \


In each of these scenarios, the transistor collector is connected to the =
7-seg=20
common (display is CC) and the emitter is connected to ground.  When the =
pic=20
pin is low, the transistor base will be driven low, forcing it to stay of=
f. =20
When the pic pin is high, it's output floats, so it gets pulled up by Rp.=
  Am=20
I missing anything here?

Cheers,
-Neil.




On Tuesday 06 May 2003 21:19, Picdude scribbled:
> Using a 16F872 to it's max i/o capacity, and down to using RA2, RA4, an=
d
> RA5 to drive the commons of three 7-seg led displays (via npn transisto=
rs
> w/base resistors, of course).  The display is common cathode.  But RA4 =
is
> open collector o/p and the other 2 are cmos.  I'm thinking I can pull R=
A4
> high with a resistor, then let the pic drive it low in code, but what i=
f I
> switched to pnp transistors, and used a common-anode display?  Can I sa=
ve
> from using the base pull-up resistor?  (I'd still leave the base
> current-limiting resistor though).
>
> Cheers,
> -Neil.

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu