Convert 8bit binary number to BCD in VHDL

Question

The algorithm is well known, you do 8 left shifts and check the units, tens or hundreds bits (4 each) after each shift. If they are above 4 you add 3 to the group and so on.

Answer 1

At least two issues appear:

• Adding is done after shift, and not before as described in the Double dabble algorithm

• The bcd shift goes bcd(11 downto 1), but should be bcd(10 downto 0)

So try with the code:

process ( hex_in )
    variable hex_src : std_logic_vector (7 downto 0) ;
    variable bcd     : std_logic_vector (11 downto 0) ;
begin
    hex_src := hex_in ;
    bcd     := (others => '0') ;

    for i in 0 to 7 loop
        if bcd(3 downto 0) > "0100" then
            bcd(3 downto 0) := bcd(3 downto 0) + "0011" ;
        end if ;
        if bcd(7 downto 4) > "0100" then
            bcd(7 downto 4) := bcd(7 downto 4) + "0011" ;
        end if ;
        if bcd(11 downto 8) > "0100" then
            bcd(11 downto 8) := bcd(11 downto 8) + "0011" ;
        end if ;

        bcd := bcd(10 downto 0) & hex_src(7) ; -- shift bcd + 1 new entry
        hex_src := hex_src(6 downto 0) & '0' ; -- shift src + pad with 0
    end loop ;

    bcd_hun <= bcd(11 downto 8) ;
    bcd_ten <= bcd(7  downto 4) ;
    bcd_uni <= bcd(3  downto 0) ;

end process ;

However, the implementation may require a slow clock...

Based on Davids observations in the comments, the code be optimized to:

process ( hex_in )
    variable hex_src : std_logic_vector (4 downto 0) ;
    variable bcd     : std_logic_vector (11 downto 0) ;
begin
    bcd             := (others => '0') ;
    bcd(2 downto 0) := hex_in(7 downto 5) ;
    hex_src         := hex_in(4 downto 0) ;

    for i in hex_src'range loop
        if bcd(3 downto 0) > "0100" then
            bcd(3 downto 0) := bcd(3 downto 0) + "0011" ;
        end if ;
        if bcd(7 downto 4) > "0100" then
            bcd(7 downto 4) := bcd(7 downto 4) + "0011" ;
        end if ;
        -- No roll over for hundred digit, since in 0 .. 2

        bcd := bcd(10 downto 0) & hex_src(hex_src'left) ; -- shift bcd + 1 new entry
        hex_src := hex_src(hex_src'left - 1 downto hex_src'right) & '0' ; -- shift src + pad with 0
    end loop ;

    bcd_hun <= bcd(11 downto 8) ;
    bcd_ten <= bcd(7  downto 4) ;
    bcd_uni <= bcd(3  downto 0) ;
end process ;