Rounding to Even (VHDL) - 2023.1 English

Vivado Design Suite User Guide: Synthesis (UG901)

Document ID
UG901
Release Date
2023-06-09
Version
2023.1 English

Filename: convergentRoundingEven.vhd

-- Convergent rounding(Even) Example which makes use of pattern detect
-- File: convergentRoundingEven.vhd
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

entity convergentRoundingEven is
port (clk : in std_logic;
a : in std_logic_vector (23 downto 0);
b : in std_logic_vector (15 downto 0);
zlast : out std_logic_vector (23 downto 0));
end convergentRoundingEven;

architecture beh of convergentRoundingEven is

signal ar : signed(a'range);
signal br : signed(b'range);
signal z1 : signed(a'length + b'length - 1 downto 0);

signal multaddr : signed(a'length + b'length - 1 downto 0);
signal multadd : signed(a'length + b'length - 1 downto 0);
signal pattern_detect : boolean;

constant pattern : signed(15 downto 0) := (others => '0');
constant c : signed := "0000000000000000000000000111111111111111";

-- Convergent Rounding: LSB Correction Technique
-- ---------------------------------------------
-- For static convergent rounding, the pattern detector can be used
-- to detect the midpoint case. For example, in an 8-bit round, if
-- the decimal place is set at 4, the C input should be set to
-- 0000.0111. Round to even rounding should use CARRYIN = "1" and
-- check for PATTERN "XXXX.0000" and replace the units place with 0
-- if the pattern is matched. See UG193 for more details.

begin

multadd <= z1 + c + 1;

process(clk)
begin
if rising_edge(clk) then
ar <= signed(a);
br <= signed(b);
z1 <= ar * br;
multaddr <= multadd;
if multadd(15 downto 0) = pattern then
pattern_detect <= true;
else
pattern_detect <= false;
end if;
end if;
end process;

-- Unit bit replaced with 0 if pattern is detected
process(clk)
begin
if rising_edge(clk) then
if pattern_detect = true then
zlast <= std_logic_vector(multaddr(39 downto 17)) & "0";
else
zlast <= std_logic_vector(multaddr(39 downto 16));
end if;
end if;
end process;

end beh;