>Robert Nansel wrote:
>
>Excellent idea, but check polarity of D3 & D4. With polarity shown in
>you
>schematic SW3 seems to be never scanned.
>
>WBR Dmitry.
>
-------8<--------
>>
>>                        Vcc
>>   SW3                   |
>>      __I__              R1
>>                  +      |                           |
>>  +---o   o---o----D3----o--------o----------o-------| B0
>>  |           |                   |          |       |
>>  R3          |             SW1 | o    SW2 | o       |
>>  |           |                -|         -|         |
>> Gnd          |                 | o        | o       |
>>              |                   |          |       |
>>              |         Vcc       |+         |       |
>>              |          |        D1         D2      |
>>              |          R2       |          |+      |
>>              |   +      |        |          |       |
>>              +----D4----o--------o----------o-------| B1
>>                                                     |
>>
>> To read the status of the switches, you must set the TRIS & data bits so as
>> to cycle B0 and B1 through the following three states:
>>
>>      B1         B0               Meaning
>> ---------------------------------------------------------
>>
>> State 0
>>    out low    in high        Neither SW2 nor SW3 pressed
>>    out low    in low         SW2 OR SW3 pressed
>>
>> State 1
>>    in high    out low        Neither SW1 nor SW3 pressed
>>    in low     out low        SW1 OR SW3 pressed
>>
>> State 2
>>    in low     in low         SW3 pressed
>>    in high    in high        No switch pressed
>>
-------8<--------

The schematic is correct, but I see now that I used a confusing convention
for diode polarity. Thanks for pointing that out.

I meant the "+" sign to indicate the direction current would flow FROM the
diode when forward biased (instead of the more usual direction of current
flow INTO the diode).

Your way is more logical, though, so here's how the original circuit would
look with the "+" representing the ANODE of the diode:


                       Vcc
  SW3                   |
     __I__              R1
                    +   |                           |
 +---o   o---o----D3----o--------o----------o-------| B0
 |           |                   |          |       |
 R3          |             SW1 | o    SW2 | o       |
 |           |                -|         -|         |
Gnd          |                 | o        | o       |
             |                   |          |       |
             |         Vcc       |          |+      |
             |          |        D1         D2      |
             |          R2       |+         |       |
             |      +   |        |          |       |
             +----D4----o--------o----------o-------| B1
                                                    |



Again, the "+" indicates the direction current will flow INTO the diode
when it is forward biased.

With programmable pullups on B0 & B1, R1 & R2 can be eliminated. If you
want to gamble that the code will never set either B0 & B1 as high outputs,
you could even eliminate R3, though I wouldn't.

--BN


----------------------------------------------------------------------------
   Country Robot            "Modular robot components
   69 S. Fremont Ave. # 2     for education and industry"
   Pittsburgh, PA 15202
----------------------------------------------------------------------------