Barry Gershenfeld <barry_g <at> zmicro.com> writes:

> As your morse code programs are getting smaller and smaller, they are in 
> danger of disappearing!  This code trades efficiency for novelty.  I saw 
> this idea at a ham club talk (in the '70's) and have been doing it this way 
> ever since.  Since you guys use assembler more than anything else, I'll 
> post it that way.  I do the same thing in other languages.
> 
> ; Morse code, tree structured.
> ; Example fragment assumes the basic elements have been previously defined:
> 
> E:
>      call    dit
>      goto    interchar
> 
> N:
>      call    dah
>      goto    E
> 
> G:
>      call    dah
>      goto    N
> 
> P:
>      call    dit
>      goto    G
> 
> T:
>      call    dah
>      goto    interchar
> 
> A:
>      call    dit
>      goto    T

Or, rewritten a little bit:

P: call dit
G: call dah
N: call dah
E: call dit
   goto interchar

O: call dah
M: call dah
T: call dah
   goto interchar

A: call dit
   goto T

; etc

But it is more interesting to analyze this for morse decoding. A binary tree
approach can be used, pointing into a retlw table, with halving distances for
each succesive symbol, e.g.:

(Note: decoding of 'V' shown as example in comments)

;; dit half of table, to symbol count of 4
      retlw 'H'
      retlw 'S'   ;                      step(3); dit: A -= Y; Y=Y/2
      retlw 'V'   ;                      step(4); daw: (end)

      retlw 'I'   ;                      step(2); dit: A -= Y; Y=Y/2 

      retlw 'F'
      retlw 'U'
      retlw m0011 ; dit dit dah dah

Ddit  retlw 'E'   ; ex: Y=4;A=Ddit; 'V': step(1); dit: A -= Y; Y=Y/2

      retlw 'L'
      retlw 'R'
      retlw m0101

      retlw 'A'

      retlw m0110
      retlw 'W'
      retlw 'J'

;; daw half

      retlw 'B'
      retlw 'D'
      retlw 'X'

      retlw 'N'

      retlw 'Y'
      retlw 'K'
      retlw 'C'

Ddaw  retlw 'T'

      retlw 'Z'
      retlw 'G'
      retlw 'Q'
    
      retlw 'M'

      retlw m1110
      retlw 'O'
      retlw m1111
       
; to decode morse, call a subroutine that times symbols and outputs a bit vector
; like 0001bbbb where bbbb are morse symbols, 1 for dah and 0 for dit. The 1
; is a start symbol. If the start symbol is in bit 5 and 6,7 are 0 then it's a
; number, so treat it differently. Else use this table (bit vector is in Morse):
; (else use a different table with prosigns etc)
Sdecode_morse4:
  movlw 4
  movwf Y ; step size

  movlw Ddit      ; 1st symbol
  btfsc Morse,3   ; zero: start at Ddit
  movlw Ddaw      ; one:  start at Ddaw

decode_morse4:
  rlf   Morse,f   ; next bit in bit 3

  btfss Morse,3   ; dit: go up
  subwf Y,w       ; do it
  btfsc Morse,3   ; daw: go down
  addwf Y,w       ; do it

  rrf   Y,f       ; Y /= 2
  incf  Y,f       ; +1
  decfsz Fcount,f ; -1, check Z
  goto   decode_morse4

  ;; check page/bank bits here
  movwf  PC       ; return with character code in W

A suitable table can be found on wikipedia (scroll down to the dichotomic search
table):

  http://en.wikipedia.org/wiki/Morse_code

Peter P.


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