Hi Dwayne,
        I had similar code to that shown below working using bank
switching. When I used it for another project, but wanted to economize, and
use the OPTION and TRIS commands it stopped working. It took quite a while
to find that changing back to bank switching fixed the problem. The code
below was designed so that the only change is the bank switching for the
OPTION instruction. I tested it and it works/ doesn't work as indicated.

At 1:25 PM 3/1/2000, Dwayne Reid wrote:
>At 02:45 PM 3/1/00 -0500, Heinz Czychun wrote:
>>Hi all,
>>
>>         I've just been playing with the TMR0 on a 16F84, and found that he
>>OPTION command does not work.
>>
>>         It was my understanding that the TRIS, and OPTION instructions,
>>although obsoleted by uChip were still supported on the 'F84. The TRIS
>>instructions still work.
>
>In what way does OPTION fail to work?  Please provide details and code
>snippits.
>
>dwayne
>
        The code below will flash an LED on Port B, Pin 0, and it uses bank
switching to adjust the Option Register.

        When the code segment at the bottom is switched into the program,
the LED no longer flashes, and Port B, Pin 0, stays high. I have assumed
that TMR0 is not incrementing so the subroutine never returns to toggle the
LED.

        What can you make of it?

;______tmr0 Demo_______________________________
;
;Program to demonstrate the use of tmr0
;timer/counter (no interrupts)
;Flashes LED on Port B Pin 0 @ 3.5kHz
;_____________________________________________
;
;Chip & Number system declaration
;
        Processor       16F84
        radix           hex
;_____________________________________________
;       cpu equates (memory map)
;Special function registers on PIC16C6x/7x/8x chip
;
tmr0    equ     0x01            ;Timer/Counter register
status  equ     0x03            ;MCU status register
portA   equ     0x05            ;PIC16F84 port A
portB   equ     0x06            ;PIC16F84 port B
opt_reg equ     0x81            ;Option Reg
;
;Other required Registers
;
;       bit equates
rp0     equ     0x05            ;register file select bit
carry   equ     0               ;MCU carry flag
zflag   equ     2               ;MCU zero flag
led     equ     0               ;led output pin
;___________________________________________
;
;Entry point when MCU is reset
;
    org   0x000                 ;start program at address 0
    goto  start                ;jump to start of main program
;___________________________________________
;
Demoinit
;
;*****************************************************************
;changed code see below
;
        bsf     status, rp0     ;point to register bank at 80h
        clrwt                   ;prep for prescaler assignment
        movlw   b'11011111'     ;clock source internal,
        movwf   opt_reg         ;no prescaler
        bcf     status, rp0     ;point to register bank at 00h
;
;*****************************************************************
;
        movlw   0x00            ;make Port A & B  all outputs
        tris    portA
        tris    portB

;
        bsf     portB,led       ;turn LED off
        return
;___________________________________________
;
;Variable Delay routine
;
TMR_delay
        clrf    tmr0            ;clear TMR0 & start counting
TMR_loop
        btfss   tmr0,7          ;has timer reached terminal
                                ;count
        goto    TMR_loop        ;no, loop again
        return
;___________________________________________
;
start
        call    Demoinit
;
mainloop
        bsf     portB,led
        call    TMR_delay
        nop
        nop
        bcf     portB,led
        call    TMR_delay
        goto    mainloop
;
        End

;changed code in program that doesn't work
;Led pin on Port B stays high.

;*****************************************************************
;
        clrwt                   ;prep for prescaler assignment
        movlw   b'11011111'     ;clock source internal,
        option                  ;no prescaler
;
;*****************************************************************

Heinz