Jose, this isnt going to work.  The spacing has to be the
same for all the bits. You sample them all at the same rate,
and set your recieve register bits according to what you find
at each slice of time.

Take out a comb, like for your hair.  draw on a paper the
pattern for the preamble as black 'o' for on and space for
off.  Lay the comb over the pattern.  Anytime you see white
paper between the teeth, the bit is low.  anytime you see an
'o' the bit is set.  count the number of teeth set for a bit-
it will be one or two or three depending on how big you draw
the dots.  Same for the spaces. If you move the comb so that
the space between 2 teeth is in the middle of each bit, you
have the best chance to detect it.  You could even sample
each piece twice and check that both are the same before
deciding it is on or off.


how about something like this:
wait till 1/2 of a bit has gone by.
check the pin
if the pin is high, set bit #0 high in recieve register
If the pin is low, set bit #0 low in reciever register.
wait a FULL BIT LENGTH, minus the time for the test.
if the pin is now high, set bit #1 high in recieve register.
If the pin is low, set it low.
wait a FULL BIT LENGTH, minus the time for the test.
If you do this 8 times, your recieve register is stuffed with
the incoming byte.
Now you test the byte to see if it was the one you were
looking for.  if it was supposed to be a "U", does the
pattern match or not?  If it doesnt match, start over, maybe
it was just noise.
wait



> If my preamble sir Kevin is 10101010, at 1200 baud,
> is this preamble reception algorithm right?
>
>
> loop1   btfss  PORTC,7
>         goto   loop1
>         call   417us      ;halfbit delay
> loop2   btfsc  PORTC,7
>         goto   loop2
>         call   833us      ;fullbit delay
> loop3   btfss  PORTC,7
>         goto   loop3
>         call   417us      ;halfbit delay
> loop4   btfsc  PORTC,7
>         goto   loop4
>         call   833us      ;fullbit delay
> loop5   btfss  PORTC,7
>         goto   loop5
>         call   417us      ;halfbit delay
> loop6   btfsc  PORTC,7
>         goto   loop6
>         call   833us      ;fullbit delay
> loop7   btfss  PORTC,7
>         goto   loop7
>         call   417us      ;halfbit delay
> loop8   btfsc  PORTC,7
>         goto   loop8
>         call   833us      ;fullbit delay
>
>
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