I'm still quite new to PICs (and have just returned to the list after a
break) but have used 'F84s successfully for a couple of projects. Now that
I can obtain 'F877s I am trying to convert my projects to use the 'F877 so
that I can then expand the projects.

However I'm having problems. I started by simply changing register
addresses to suit the 'F877 but that didn't work. To cut a long story
short I'm now back to trying to get LEDs to flash! I have a simple circuit
on stripboard with an 'F877, voltage reg.,0.1 uF cap. across the supply, 4
MHz crystal with 2 15 pF caps., MCLR tied directly to 5V and 8 LEDs with
resistors on PORTB. The following program works, well kind of - the
LEDs come on one at a time and then go off rather one running up and
down the line, but at least it does something (WDT enabled of course):

;-------------------------------------------------------------
        LIST P=16F877
;
PORTB   EQU     6
TRISB   EQU     86H
OPTREG  EQU     81H
STATUS  EQU     3
CARRY   EQU     0
RP0     EQU     5
MSB     EQU     7              ;BIT POSITION OF LEFTMOST LED
;
        CLRF    PORTB           ;ALL LEDS OFF
        BSF     STATUS,RP0      ;SELECT REGISTER BANK 1
        CLRF    TRISB^80H       ;SET PORTB TO ALL OUTPUTS
        MOVLW   0AH
        MOVWF   OPTREG^80H      ;ASSIGN PRESCALER (1:4) TO WDT
        BCF     STATUS,RP0      ;SELECT REGISTER BANK 0
        INCF    PORTB,F         ;TURN ON RIGHTMOST LED
        BCF     STATUS,CARRY    ;CLEAR CARRY
LEFT    SLEEP                   ;WAIT FOR WDT TIMEOUT
        RLF     PORTB,F         ;TURN ON LED TO LEFT
        BTFSS   PORTB,MSB       ;REACHED LEFTMOST?
        GOTO    LEFT            ;LOOP IF NOT
RIGHT   SLEEP                   ;WAIT FOR WDT TIMEOUT
        RRF     PORTB,F         ;TURN ON LED TO RIGHT
        BTFSS   PORTB,0         ;REACHED RIGHTMOST?
        GOTO    RIGHT           ;LOOP IF NOT
        GOTO    LEFT            ;START NEW CYCLE
        END
;-------------------------------------------------------------

However the following doesn't (WDT off):

;-------------------------------------------------------------
        LIST P=16F877
;
PORTB   EQU     6
TRISB   EQU     86H
OPTREG  EQU     81H
STATUS  EQU     3
CARRY   EQU     0
RP0     EQU     5
MSB     EQU     7              ;BIT POSITION OF LEFTMOST LED
LOOP1   EQU     20H
LOOP2   EQU     21H
;
        CLRF    PORTB           ;ALL LEDS OFF
        BSF     STATUS,RP0      ;SELECT REGISTER BANK 1
        CLRF    TRISB           ;SET PORTB TO ALL OUTPUTS
        MOVLW   0AH
        MOVWF   OPTREG^80H      ;ASSIGN PRESCALER (1:4) TO WDT
        BCF     STATUS,RP0      ;SELECT REGISTER BANK 0
        INCF    PORTB,F         ;TURN ON RIGHTMOST LED
        BCF     STATUS,CARRY    ;CLEAR CARRY

START   MOVLW   0FFH
        MOVWF   LOOP1
L1      MOVLW   0FFH
        MOVWF   LOOP2
L2      DECFSZ  LOOP2
        GOTO    L2
        DECFSZ  LOOP1
        GOTO    L1
        INCF    PORTB,F
        GOTO    START
        END
;-------------------------------------------------------------

I also tried the above with the WDT on and with a CLRWDT in L2 but that
didn't help. I don't think you can get much simpler than this yet I can't
get it to work. Incidentally I've tried two different 'F877s, three
crystals and also with and without a 1k resistor between CLKOUT and the
oscillator in case that was the problem.

So can anyone help? I'm sure I must be overlooking something very simple
but I don't know what.

Thanks,

Mark.