> -----Original Message-----
> From: Olin Lathrop [SMTP:olin_piclist@EMBEDINC.COM]
> Sent: Monday, September 01, 2003 2:49 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [PIC:]  Multiplexed-switch design problem
>
> >   .__________.
> >   | 16F628   |
> >   |          |
> >   |      RB0 |---------o-------------| MOSFET 1 gate
> >   |          |         |
> >   |      RB1 |---------|-------o-----| MOSFET 2 gate
> >   |          |         |       |
> >   | RA5      |         |       |
> >   '----------'         |       |
> >      |               |/        o |
> >      |  A>--'\/\/\,--|  NPN      |] Pushbutton N.O.
> >      |         R1    |\>       o |
> >      |                 |       |
> >      |                 |       |
> >      |                (D)      |
> >      |                 |       |
> >      |                 |       |
> >      `-----------------o--|<|--'
> >                        |   1N4148
> >            ,--'\/\/\,--'
> >            |     R2
> >           GNDb
>
> You're making this way too complicated.  Just turn on the internal pullups
> when you want to read the switches and tie the other end of the switches
> to ground.  This even frees up RA5.  (Where did you find such a hairbrain
> design in the first place?)
>
If you check the schematic you will see that RB0 and RB1 are driving MOSFET
gates.  I suspect the original intent is that pressing the switches dosen't
affect this gate drive.

It should be fairly obvious why the circuit dosen't work as you expect: With
5 volts applied to the base of the transistor, a current will be flowing out
of the emitter irrespective of the voltage on the collector.  A further
potential problem is that with RB0 driven low, the BC junction will be
forward biased.  This application either wants a mosfet or a PNP bipolar
configured as a common emitter circuit rather than an emitter follower
(which then inverts the switch polarity).  By putting an extra diode at
point (D), you are reducing the voltage by 0.6 volts which is obviously
enough to fall below the logic threshold of the PIC.  You could also just
lower the value of R2 to acheive the same effect.

Mike




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