Dear old friends, help me!

          ; Inicio
          list p=16F84
          radix     dec
          include <P16F84.INC>
          ; Variables
contador   equ d'12'
          org     0x000
          movlw   0x1F
          tris    PORTA
          movlw   0x00
          tris    PORTB

          movlw 0
          movwf contador

loop
          incf contador,f


          movlw contador
          movwf PORTB

          goto loop

          END

Why don't my PORTB pulse? (The level is constant in all BITS of PORTB... ;(  )

Can you see what i'm doing wrong?

Thanks!

Miguel