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-- Author : Timo Alho
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-- e-mail : timo.a.alho@tut.fi
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-- Date : 28.06.2004 18:19:03
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-- File : Serial_multiplier4idct.vhd
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-- Design : VHDL Entity DCT_RC_DA.Serial_multiplier4idct.rtl
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------------------------------------------------------------------------------
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-- Description : Serial multiplier (with signed numbers) using shift-and-add
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-- topology. This version is specially tailored for idct usage.
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--
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-- Usage: Multiplicand is fed into input accoring to the bits of the
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-- multiplier (starting with multipliers LSB).
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--
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-- Example:
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-- 01011 = 11
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-- * 01101 = 13
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-- _________
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-- 01011
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-- 00000
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-- 01011
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-- 01011
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-- 00000
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-- _________
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-- 010001111 = 143
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--
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-- The input sequence in this case should be 11, 0, 11, 11, 0.
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-- Before the first input value, signal start should be held high during one
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-- clock cycle. During the last input value, signal last_value should be held
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-- high.
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--
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-- Two's complement multiplication:
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-- A sequence of two's complement additions of shifted multiplicands expect for
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-- last step where the shifted multiplicand corresponfing to MSB must be
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-- negated.
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-- Before adding a shifted multiplicand to the partial product, an additional
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-- bit is added to the left of the partial product using sign extension.
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------------------------------------------------------------------------------
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LIBRARY ieee;
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USE ieee.std_logic_1164.ALL;
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USE ieee.std_logic_arith.ALL;
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ENTITY Serial_multiplier4idct IS
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GENERIC(
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coeffw_g : integer := 14;
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i_dataw_g : integer := 18;
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round_val_g : integer := 64
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);
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PORT(
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clk : IN std_logic;
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last_value : IN std_logic;
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mul_in : IN std_logic_vector (coeffw_g-1 DOWNTO 0);
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rst_n : IN std_logic;
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start : IN std_logic;
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mul_out : OUT std_logic_vector (i_dataw_g-1 DOWNTO 0)
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);
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-- Declarations
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END Serial_multiplier4idct ;
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--
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ARCHITECTURE rtl OF Serial_multiplier4idct IS
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SIGNAL partial_res_r : std_logic_vector(i_dataw_g+2 DOWNTO 0);
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-- paritial result is stored here
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SIGNAL extended_input : signed(coeffw_g DOWNTO 0);
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SIGNAL shifted_partres : signed(coeffw_g DOWNTO 0);
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SIGNAL sum_out : signed(coeffw_g DOWNTO 0);
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BEGIN
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-- purpose: adds one bit to the left of input using sign extension
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-- type : combinational
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-- inputs : mul_in
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-- outputs: extended_input
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ext_in : PROCESS (mul_in)
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BEGIN -- PROCESS ext_in
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extended_input(coeffw_g) <= mul_in(coeffw_g-1); --MSB
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extended_input(coeffw_g-1 DOWNTO 0) <= signed(mul_in);
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END PROCESS ext_in;
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-- purpose: shifts partial result (arithmeticly) by one
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-- type : combinational
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-- inputs : partial_res_r
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-- outputs: shifted_partres
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sh_partres : PROCESS (partial_res_r)
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VARIABLE temp : signed(coeffw_g DOWNTO 0);
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BEGIN -- PROCESS sh_res
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temp := shr(signed(partial_res_r(i_dataw_g+2 DOWNTO i_dataw_g-coeffw_g+2)),
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conv_unsigned(1, 1));
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shifted_partres <= temp; --(coeffw_g DOWNTO 0);
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END PROCESS sh_partres;
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-- purpose: adds or subtracts partial result and input
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-- type : combinational
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-- inputs : shifted_input, shifted_sum, last_value
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-- outputs: sum_out
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sum : PROCESS (extended_input, shifted_partres, last_value)
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BEGIN -- PROCESS sum
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IF (last_value = '1') THEN
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sum_out <= shifted_partres - extended_input;
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ELSE
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sum_out <= shifted_partres + extended_input;
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END IF;
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END PROCESS sum;
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clocked : PROCESS (clk, rst_n)
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VARIABLE i : integer;
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BEGIN -- PROCESS clocked
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IF rst_n = '0' THEN -- asynchronous reset (active low)
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partial_res_r <= (OTHERS => '0');
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ELSIF clk'event AND clk = '1' THEN -- rising clock edge
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IF (start = '1') THEN
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--put rounding value into partial result register
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partial_res_r <= conv_std_logic_vector(round_val_g, i_dataw_g+1+2);
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ELSE
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--save sum into partial result register
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partial_res_r(i_dataw_g+2 DOWNTO i_dataw_g-coeffw_g+2) <=
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conv_std_logic_vector(sum_out, coeffw_g+1);
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FOR i IN i_dataw_g-coeffw_g+2 DOWNTO 1 LOOP
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partial_res_r(i-1) <= partial_res_r(i); --shift result
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END LOOP; -- i
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END IF;
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END IF;
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END PROCESS clocked;
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--FOR IDCT INPUT: (12bit input -> 9bit output)
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--3 MSB bits of partial result are insignificant (for final result)
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mul_out <= partial_res_r(i_dataw_g-1 DOWNTO 0);
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END rtl;
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