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---------------------------------------------------------------------------
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-- (c) 2020 mark watson
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-- I am happy for anyone to use this for non-commercial use.
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-- If my vhdl files are used commercially or otherwise sold,
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-- please contact me for explicit permission at scrameta (gmail).
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-- This applies for source and binary form and derived works.
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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.numeric_std.all;
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ENTITY PSG_noise IS
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PORT
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(
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CLK : IN STD_LOGIC;
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RESET_N : IN STD_LOGIC;
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ENABLE : IN STD_LOGIC;
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TICK : IN STD_LOGIC;
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BIT_OUT : OUT STD_LOGIC
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);
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END PSG_noise;
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ARCHITECTURE vhdl OF PSG_noise IS
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signal shift_reg: std_logic_vector(16 downto 0);
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signal shift_next: std_logic_vector(16 downto 0);
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signal noise_reg : std_logic;
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signal noise_next : std_logic;
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BEGIN
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-- register
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process(clk, reset_n)
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begin
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if (reset_n = '0') then
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shift_reg <= (others=>'0');
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noise_reg <= '0';
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elsif (clk'event and clk='1') then
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shift_reg <= shift_next;
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noise_reg <= noise_next;
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end if;
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end process;
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-- next state lfsr
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process(shift_reg,enable)
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begin
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shift_next <= shift_reg;
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if (enable = '1') then
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shift_next <= (shift_reg(3) xnor shift_reg(0))&shift_reg(16 downto 1);
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end if;
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end process;
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-- next state output
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process(shift_reg,noise_reg,tick)
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begin
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noise_next <= noise_reg;
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if (tick = '1') then
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noise_next <= shift_reg(0);
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end if;
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end process;
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-- output
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bit_out <= noise_reg;
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END vhdl;
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