I've just started to learn about PICs and have started with some basic
"hello world" programs.  I'm using a 16F628 and started by doing a slightly
modified version of the bincnt.asm program on the Projects page. Got this to
work fine with LEDs on the PORTB output pins arranged to light up when the
pin is outputting a 1.

Then tried to add a pushbutton as an input and have the program branch to a
different output to show that the pushbutton was acknowledged. The problem
is that the input pin does not seem to be an input, but still an output. The
pushbutton is from PORTB<4> (although I've also tried pins on PORTA) to
Ground.  When the button is not pressed, I expected the pin to be high, but
it is low, and actively low - if I tie it through a resistor to Vcc, it
stays low.

With the code below, the LEDs count properly, until the count overflows to
PORTB<4>, at which time the pin goes high (as it would if it's an output),
and, after the one second timer delay, all LEDs go on, showing that it has
branched properly.

I've tried setting the pin direction using the TRIS instruction as well as
bank switching and loading the TRIS register as in the code below.  Neither
seems to set the pin as an input. So far I'm baffled.

I haven't done any assembler programming for 25 years, and not much even
then, so I'm a newbie - be gentle.

Dennis

       LIST P=16F628
        INCLUDE "p16f628.inc"
        ERRORLEVEL -224
        __CONFIG _PWRTE_ON & _INTRC_OSC_NOCLKOUT & _WDT_OFF & _CP_OFF
&_MCLRE_OFF &_LVP_OFF

        ORG     0               ; start at program memory location zero
pin     equ     H'0004'
        clrf    CCP1CON         ; turn off compare module
        movlw   0x07
        movwf   CMCON           ; turn off comparators
        clrf    PORTA           ; reset latch
        clrf    PORTB
        bsf     STATUS, RP0     ; set bank 1
        bcf     STATUS, RP1
        bcf     PCON, OSCF      ; set internal oscillator to 37kHz
        movlw   B'00000000'
        movwf   TRISA           ; set PORTA as outputs
        movlw   B'00010000'     ; set PORTB as outputs except bit 4
        movwf   TRISB
        movlw   B'00000100'     ; configure with pull ups active and
        movwf   OPTION_REG      ; prescaler to TMR0 at 1:32
        bsf     STATUS, RP0     ; set bank 0
;-----------------------------------------------------------------------;
;       Previous section turns off compare module, comparators and      ;
;       sets PORTA<0:7> and B<0:3,5:7> as outputs.                      ;
;       Then OPTION_REG is set to make pull-ups active and
;
;       sets prescaler to TMR0 at 1:32 so it will                       ;
;       overflow and interrupt approximately every second.  PORTB<4> is ;
;       the pushbutton input to change patterns.  Button must be held   ;
;       for about a second because it's only checked on timer overfolw  ;
;-----------------------------------------------------------------------;

;-----------------------------------------------------------------------;
;                      Main program                                     ;
;-----------------------------------------------------------------------;

        clrf    PORTB
loop:   incf    PORTB, f
        btfss   INTCON, T0IF    ; wait on T0IF to be set
        goto    $ -1
        bcf     INTCON, T0IF
        btfss   PORTB, pin      ; if button not pressed (high) break out and
set all LEDS on
        goto    loop
        movlw   B'11111111'
        movwf   PORTB
        nop
        nop
        btfsc   PORTB, pin      ; if button pressed,  go back to counting
        goto    $ -1
        goto    loop

         end

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