Binary to Decimal Converter for Microchip PIC18f

This is a very simple program for the Microchip PIC18f that makes a good programming project. Once I figured out what many of the code samples were doing, it became very clear and easy to understand.  It’s too bad that all the code samples I ran across were not (or at least very poorly) ‘documented’.

[stextbox id=”info”]In my opinion, “Documented” means a LOT more than throwing a few vague words after a semicolon, scattered sparsely throughout your code.  If you can’t read the steps somewhere in plain-English, then others can’t follow, and it ain’t documented.[/stextbox]

 DAW – Decimal Adjust W Register

Since we will be adding two BCD numbers, the DAW command will take care of ‘adjusting’ the resulting the result back into a BCD number.  I believe this DAW command is only on the PIC18 family (may be on higher families, I don’t know).  Note this only works when adding two BCD numbers – it won’t work for squat with a regular binary add.

From the Microchip PIC18 instruction set. Under “Operation” you can see the steps the DAW command executes.  If your processor does not have this feature, you could easily create some code to do this same thing.

This command assumes that the register you are using was the result of the addition of two BCD numbers.  It will only make a mess of a ‘normal’ binary number, with one exception.

We can ‘get away with’ using this command because, for numbers less than or equal to 0x09, binary and BCD are the same.  And our first operation will always result in a number less than 0x09.  So we start converting to BCD then, and doubling the BCD number each time from then onward.

 

 

 

 

Double and add carry

The code uses the double and add carry approach.  Below is a fragment that explains the process: this is repeated for each BCD output ‘pair’.  (each byte holds two BCD numbers)

Start by rotating the binary number to the left, putting the MSB into the carry bit

    RCLF    Bin0, f     ; for first byte to convert

Move the BCD byte from memory into the W register, then add it to itself (double) with carry, from the above command.

    MOVF    BCD0, w     ; move first BCD pair into Wreg
    ADDWFC  BCD0, w     ; add it to itself (double) and add carry from rotate of binary

Decimal Adjust Wreg (re-align & correct BCD numbers

DAW                 ; Decimal adjust BCD word

Store the result back where you got it from

 movwf   BCD0        ; Save result back to BCDx register

Repeat this for each “BCD pair”.  If there is a carry to the next BCD byte, it will be handled via the Add-and-carry for the next byte.

Binary to BCD code for PIC18

;*******************************************************************************
;*    Filename: Bin2BCD.asm
;*    Date: 5/15/15
;*    File Version:  0.0
;*    Author:    Brian Volken
;*    Company:     Brady Volken Enterprises
;*    Description: Convert binary number into BCD string
;*******************************************************************************
;*    Revision History:
;*    0.0 Initial Version
;*******************************************************************************

;*******************************************************************************
; Processor Inclusion
;*******************************************************************************
   #include p18f4580.inc

    UDATA_ACS
BCD0        		RES	1       ; holder for lowest BCD pair
BCD1        		RES	1       ; holder for next BCD pair
BCD2		        RES	1       ; continue as necessary to hold answer
;BCD3        		RES	1
Bin0        RES 1       ; holder for binary number to convert, lowest byte
Bin1        RES 1       ; holder for binary number higher byte
;Bin2        RES 1       ; repeat as necessary
BitCtr		      RES	1       ; counter of number of bits to convert

;*******************************************************************************
;* Defines
;*******************************************************************************
#define     bcount   .16     ; number of binary bits to be converted to BCD


;*******************************************************************************
; "Exposed" sub routine elements (variables & code)
; GLOBAL is used to make a lable visible to other files.
; EXTERN must be used in the file that uses (calls) the lable to make it visible
;*******************************************************************************
Bin2BCD CODE

; Init variables, load BitCtr with number of bits in binary source
BinToBCD:
    CLRF    BCD0
    CLRF    BCD1
    CLRF    BCD2
    MOVLW   bcount
    MOVWF   BitCtr

;*******************************************************************************
;* ConvertBit
;*******************************************************************************

ConvertBit:
; rotate binary word(s) left one bit
    RCLF    Bin0, f     ; for first byte to convert
    RCLF    Bin1, f     ; for 2nd byte to convert
;   RCLF    Bin2, f     ; for 3rd byte to convert
    movf    BCD0, w     ; move first BCD pair into Wreg
    addwfc  BCD0, w     ; add it to itself (double) and add carry from 
                        ; rotate of binary 
    daw                 ; Decimal adjust BCD word
    movwf   BCD0        ; Save result back to BCDx register
    movf    BCD1,W      ; repeat for BCDx+1 as many times as necessary
    addwfc  BCD1,W
    daw
    movwf   BCD1
; add more BCD 'holders' for longer binary strings
;    movf    BCD2,W
;    addwfc  BCD2,W
;    daw
;    movwf   BCD2
    decfsz  BitCtr      ; decrement Bit Counter
    bra     ConvertBit  ; keep going until last bit rotated out of binary source
    return

	END