Hi..


>(Alternatively, make B0-B3
>outputs, A2 input, strobe each in turn, and look for corresponding
>strobe on A2).

Yeah I'd do it this way:

Connect one side of each switch to A2 with a 10k pullup resistor to Vdd.
Connect the other side of the switches to B0-B3.

bring B0 low, with B1-3 high.
if RA2 is low key 1 is pressed.

bring B1 low, the others high
if RA2 is low key 2 is pressed.
etc

BTW if you had a pin on portb spare you could use an internal pullup so
wouldn't need the external one.


Regards,

Stewart Pye

At 12:15 AM 13/06/99 -0700, you wrote:
>Hi,
>
>I'm building a project which requires 4 pushbuttons to be attached to
>a bus interface. I *could* obviously use a tristate buffer, but I'm
>trying to keep the chip count down. I do have one spare pin on the
>PIC (A2), so my initial solution was to have this pin as the common
>connector to 4 switches, and have the other side of the switches
>wired to B0-B3, (which is in use as part of a bus interface).
>When the bus in in use, I float A2, so pressing a button doesn't
>interfere with the bus operation. When I'm not using the bus, I flip
>B0-B3 to inputs, set A2 high, read B0-B3, set A2 low, read B0-B3 again
>and figure out which switches are closed.






>
>Of course, a moment's thought demonstrates that if two switches are
>closed at the same time, unspeakable confusion will occur, because
>two of the bus lines will be shorted together, regardless of whether
>or not A2 is floating, and when the bus *is* in use, disaster will
>occur.
>
>So, I'm thinking - presumably if I got a transistor for each line,
>arranged in an open-collector configuration, such that B0-B3 remain
>outputs, each driving a transistor, I could take the second
>approach described above (use A2 as input), and it wouldn't matter
>if two buttons were pressed simultaneously. (I don't need to detect
>simultaneous presses - it's just that they can't interfere with
>anything!).
>
>My question is, (and once again, I'm forced to demonstrate my
>complete lack of elec eng knowledge...), how exactly do I connect
>the transistors? I need a series resistor on the base, yes? If so,
>what sort of value should I use?
>
>Alternatively, if anyone can suggest a simpler way to get around
>this problem, I'm all ears!
>
>Cheers,
>Ben
>