Can any of you figure this one out?  I have a PIC16F628-based circuit usi=
ng 2=20
multiplexed switches like this...

  .__________.
  | 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


Port B is all outputs, and RA5 is an input.  The point at (D) is just a d=
irect=20
connection for now.  R2 is a 10K pull-down resistor.  Only one output on =
port=20
B will be High at any time, with the rest Low.  When either RB0 or RB1 is=
=20
high, the input value at RA5 is read to determine the state of the=20
corresponding switch.  Well, it works properly.

Oddly, I have another circuit, which is identical with respect to this po=
rtion=20
of the circuit, but it does NOT work properly.  When point A is connected=
 to=20
+5V, the transistor switches on, and is properly detected, but the pushbu=
tton=20
cannot be detected until the transistor is switched off.  However, the=20
circuit will function properly with another diode added at point D (bande=
d=20
end away from the transistor).

In the second circuit, I would think that the emitter serves/acts as a di=
ode=20
when the transistor is on, so that another diode at the emitter is not=20
required.  I do realize that the proper diode behaviour is achieved in th=
e=20
B-E junction of the transistor, but thought that the C-E junction is some=
what=20
unidirectional as well.

I've re-checked the initialization and usage code, but cannot see any=20
difference in this portion of the circuit.  Any idea why this additional=20
diode is required in the second circuit?

Thanks,
-Neil.




---
[This E-mail scanned for viruses]

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.