The original problem was:

> >     Petra: I should use any port, and any bit... my idea is to make a
> > subrutine to read a bit using sofware filter...

How is the routine below solving this problem?  It looks useful for DEFINING
a bit to output, not reading a bit in.

>You need to map from a bit number to a bitmask, then use logical
>instructions:
>
>To turn on bit "BitNum" of port "PortNum":
>
>     movf       PortNum,W
>     movwf    FSR
>
>     movf       BitNum,W
>     call         BitToMask
>     iorwf       INDF,F
>
>To turn off bit "BitNum" of port "PortNum":
>
>     movf       PortNum,W
>     movwf    FSR
>
>     movf       BitNum,W
>     call          BitToMask
>     xorlw       0xFF
>     andwf      INDF,F
>
>
>BitToMask:
>     addwf    pcl
>     retlw    1
>     retlw    2
>     retlw    4
>     retlw    8
>     retlw    16
>     retlw    32
>     retlw    64
>     retlw    128

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