PIC Microcontroller Radix Math Method

4 Bit Binary to 1 ASCII HEX digit

Converting a Nybble to ASCII

This is a question that comes up all the time when the contents of a byte are to be displayed/output. Does your code need to be fast, or does your code need to be small?

4 Bit Binary to 1 ASCII HEX digit FAST

The most obvious and fastest way of doing this is:

NybbletoASCII:
; Call with binary nybble in W. Returns ASCII value in W.
; From: http://www.myke.com/basic.htm
  andlw H'0F' ; (optional )
  addwf  PCL, f            ;  Add the Contents of the Nybble to PCL/ 
  dt	 "0123456789ABCDEF"  ;  return the ASCII as a Table Offset

4 Bit Binary to 1 ASCII HEX digit SMALL

Several people have independently reduced the core of nybble-to-ASCII conversion to 4 instructions in size, zero RAM. Can you spot that core inside these 6 instruction subroutines?

NybbletoASCII:
; from Sean H. Breheny [shb7 at CORNELL.EDU]
; Sean H. Breheny KA3YXM [at rocket-roar.com]
    andlw H'0F' ; (optional )
    addlw 0-D'10'               ; test: Is W < 10 ?
    skpnc ; btfsc STATUS,C  ; If 0 <= W <= 9, skip ahead
        addlw 'A'-'0'-D'10' ; If A <= W <= F, add ASCII char 'A', and
                             ; subtract enough to make the next
                             ; line have no effect
    addlw '0'+D'10'            ; Add ASCII character '0' as well as
                             ; replace the original 10 that was subtracted
    return

Another:

NybbletoASCII:
; From: Scott Dattalo
    andlw H'0F' ; (optional )
    addlw    6
    skpndc
      addlw   'A'-('9'+1)
    addlw    '0'-6
    return

example

All of the above routines are interchangeable. Some test code that calls those routines with all possible values of W:

test_nybble_to_ascii:
    ; for( i = 0; i < 256; i++){ nybble_to_ascii(i) };
    movlw h'00' ;
    movwf index
nybble_test_for_loop:
    ; nybble_to_ascii(i);
    movf index,W
    call nybble_to_ascii
    nop ; place to put breakpoint to see if it converted correctly
nybble_test_for_test:
    decfsz index,f
    goto nybble_test_for_loop
    ; ... continue
    return    

Comments:

• Nevermind, the ASCII table is in a slightly different order than I was thinking. I see.+
• Dear Piclist Keepers,

  I just wanted to say a big "THANKS!" for all the great help, tips, and first hand knowledge you provide. I've been able to get my PIC projects running much better with your help.

  Take care, and Thanks,
  baumgarc AT gps.caltech.edu+

Questions:

• hvymtlsteve at charter.net asks: " Can't you convert a 4-bit nybble to ASCII by simply adding 0x30 to it?" James Newton replies: No, because the ASCII values above "9" are ":", ";", "<", "=", ">", and "?" rather than "A" thru "F". To convert a nibble to hex, you need to add "A"-10 (55 or 0x37) if it is 10 (0x0A or 1010b) or more and "0" (48 or 0x30) if it is 9 (0x09 or 1001b) or less. Or you can add "0" as you suggest, then test to see if it is ":" or greater and then add 7 more ("A" - ":") if it is.+

  +

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