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Rev 5 → Rev 6

/trunk/rtl/BCD_adder.vhd
0,0 → 1,79
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity BCD_adder is
generic(
--DEC_SIZE is the amount of decimal digits. e.g. BCD_ADD_DEC_SIZE = 4 means the highest representable integer is 9999
DEC_SIZE : positive := 4
);
port(
clk_i : in STD_LOGIC;
a_i : in STD_LOGIC_VECTOR(4*DEC_SIZE-1 downto 0);
b_i : in STD_LOGIC_VECTOR(4*DEC_SIZE-1 downto 0);
sum_o : out STD_LOGIC_VECTOR(4*DEC_SIZE-1 downto 0);
cin_i : in STD_LOGIC;
cout_o : out STD_LOGIC
);
end entity;
architecture behavioral of BCD_adder is
 
--input and output registers
signal a_r : unsigned(4*DEC_SIZE downto 0);
signal b_r : unsigned(4*DEC_SIZE downto 0);
signal sum_r : STD_LOGIC_VECTOR(4*DEC_SIZE-1 downto 0);
signal cin_r : STD_LOGIC;
signal cout_r : STD_LOGIC;
--increment6: a function to increment a 4 bit unsigned number by 6.
function increment6 (number : unsigned(3 downto 0)) return unsigned is
begin
return ((number(3) or number(2) or number(1)) & ((number(2) or number(1)) nand (number(2) nand number(1))) & not(number(1)) & number(0));
end function;
--decrement6: a function to decrement a 4 bit unsigned number by 6.
function decrement6 (number : unsigned(3 downto 0)) return unsigned is
begin
return ((number(3) and number(2) and number(1)) & (number(2) xor number(1)) & not(number(1)) & number(0));
end function;
begin
process(clk_i)
--BCD_SIZE is the amount of binary digits that are needed for the BCD number. Each decimal digit is 4 bits, so 4*BCD_ADD_DEC_SIZE.
constant BCD_SIZE : integer := 4*DEC_SIZE;
variable sum_v : unsigned(BCD_SIZE downto 0);
variable a_v : unsigned(BCD_SIZE downto 0);
variable b_v : unsigned(BCD_SIZE downto 0);
variable cin_v : STD_LOGIC;
begin
if rising_edge(clk_i) then
--Put the inputs in the variables, add a leading '0' to store a later carry.
a_r <= unsigned('0' & a_i);
b_r <= unsigned('0' & b_i);
cin_r <= cin_i;
a_v := a_r;
b_v := b_r;
cin_v := cin_r;
--increment every decimal digit of operand b by 6
for i in 0 to DEC_SIZE-1 loop
b_v(4*i+3 downto 4*i) := increment6(b_v(4*i+3 downto 4*i));
end loop;
--add a, b and the carry_in to form a temporary sum that needs to be corrected
sum_v := a_v + b_v + ("" & cin_v);
--correction: if the sum of two decimal digits exceeded 9, subtract 6 from the temporary sum
for j in 0 to DEC_SIZE-1 loop
if (sum_v(4*j+4) = (a_v(4*j+4) xor b_v(4*j+4))) then
sum_v(4*j+3 downto 4*j) := decrement6(sum_v(4*j+3 downto 4*j));
end if;
end loop;
sum_r <= STD_LOGIC_VECTOR(sum_v(BCD_SIZE-1 downto 0));
cout_r <= sum_v(BCD_SIZE);
end if;
end process;
sum_o <= sum_r;
cout_o <= cout_r;
end behavioral;

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