Bruce Gennette wrote:

> I need to select one-of-eight inputs and display (with an LED) which
> input is currently selected.

> But I need to do this twice, once for the video going out to the class
> room monitors and once for the pre-view monitor.

  OK, let's use efficient LEDs so that 20mA for half the time will
suffice to light each LED.  You won't need transistors.

           LED in row 1              Set Port B pullups ON.  To light
       +---|<|-------+             the LED in row 0, Set PA0 high,
       |  Si  P/B    |             PA1 tri-state (hint, use TRIS
       +-|>|--o/o----+             instruction, not bank select).
____   |   LED in row|0            Clear PORTB and set a mask using
 PBx|--+---|<|-----+ |             TRIS PORTB with the only zero in
    |  |  Si  P/B  | |             the bit corresponding to the LED
    |  +-|>|--o/o--+ |             to be lit.
    |  180 ohm     | |
 PA0|--VVVVV-------+-u--o Other      After 8ms, set up the LED in
    |  180 ohm       |      row 0  row 1 in the same fashion (leave a
 PA1|--VVVVV---------+--o Other    "1" in both bits 0 and 1 of port A.
    |                       row 1   
____|                                To read the row 0 switches, TRIS
                                    PORTB to $FF (inputs), set both
bits 0 and 1 of PORTA to "0", TRIS PORTA bits 1,0 to "10" and read
PORTB which will contain a "0" corresponding to any switch closed.
Read row 1 switches with  "01" in TRIS PORTA bits 1,0.

  The cycle of lighting the row 0 LED for 8ms, lighting the row 1 LED
for 8ms, reading row 0 for about 5µs (i.e., insert a few NOPs between
setting up the ports and reading PORTB) and reading row 1 for 5µs will
both multiplex the LEDs and give you a basis for de-bouncing the push-
buttons.

  You do not need interrupts to perform the timing; a prescale of 32
on TMR0 will allow you to poll its overflow flag for 8192µs intervals
for a 4MHz crystal frequency.  You set up the LED to be lit, go perform
your other code for less than 8ms, then come back and poll the overflow
flag until it shows, *clear the flag*, go set up the next LED and so on.
-- 
  Cheers,
        Paul B.