Hey PICers,

I wanted to try and use the RTCC
register to detect an input and
then grab a data nibble and store
it. I know you can't read the RTCC
pin directly but you can detect
L->H transits by fiddling with the
OPTION register and looking for
increments in the RTCC register.

This would be great because you
could save an extra pin in your
data registers. I've got a 4x4
keypad hooked up to a 74C922 whose
DA goes to RTCC on a 16C57C and
Data outputs 1-4 got to the PIC's
PORTA.
(It's set up on PROTOBOARD, not a
PCB)

The programme supposedly just grabs
the data, stores it in "KeyData"
and dumps it to LEDs on PORTB (not
shown). Trouble is, nothing much
seems to be happening. Works OK on
MPLAB but you have to increment
RTCC yourself, can't really test
its response to the rising edge.

OK, from reading the data sheet, it
seems to me this trick orta work.
But nothing doing. Anyone got any
ideas - am I missing something
really dumb here?

Thnax for advice - PJH

****************************************************************

Programme snips :-

        org      0x00
        goto     main

;*********************************************************************

;FETCH - monitors the DA pin on the
74C922, grabs the data nibble and
;stores it in KeyData. It then sets
b3 in flags before returning.
;This routine uses the transits on
TOCK1 to determine if an input is
;present. RTCC=TOCK1 in the newer
PIC 16C5*C series?
;OPTION.3=1 .. prescaler assigned
to the WDT
;OPTION.4=0 .. increment on L->H
transit on TOCK1
;OPTION.5=1 .. transition on TOCK1
pin
;OPTION.0,1,2=0 ..
;Restore the default OPTION setting
before we leave?
;Clear DA in the MAIN PROGRAMME,
NOT in this subroutine.
;*******************************************************************

fetch   clrf     TMR0        ;Clear
TMR0, 0x001
        nop
        movlw    B'11101000'
        OPTION
;Configure the PRESCALER
        nop
        btfss    TMR0,0      ;Has
TMR0 been incremented?
         goto    $+4         ;No  -
exit the subroutine.
        movf     PORTA,0     ;Yes -
there are data present.
        movwf    KeyData     ;Store
the data
        bsf      flags,DA    ;Tell
the main programme
        nop                  ;Jump
to here if no input
        retlw    0x00

;****************************************************************

;Beginning of main programme from
the the upper half of PAGE #0
;****************************************************************

        org     0x0100

main    nop
        call    init
;Initialise the ports ..
PORTA=INPUT

look    goto    pwr_on      ;Power
up the device
        nop
;*****************************************************************

;Now grab the data from the 74C922.

;*****************************************************************

        bcf     flags,VT
        call    fetch
        btfss   flags,VT    ;Are
data present?
         goto   look+1      ;No  ..
keep looking
        goto    table       ;Yes -
jump to the selected routine

        ..SNIP..