Subversion Repositories pwm_with_dithering

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-- Company:              Aboa Space Research Oy (ASRO)

-- Engineer:             Tero Säntti

-- 

-- Create Date:    14:50:27 01/27/2021 

-- Design Name:          PWM

-- Module Name:    pwm - Behavioral 

-- Target Devices: None / non-specific

-- Tool versions:  None / non-specific

-- Description:          Dithered PWM. Simple implementation. Minimal size, but clock

--                                               frequency is not maximal. Also, may be sensitive to input 

--                                               changes during operation. (Input value jumps to a lower value

--                                               than current counter value.) Fixable with additional register

--                                               which loaded if counter @ max value. The fix is implemented 

--                                               in the pipelined version.

--

-- Revision: 

-- Revision 0.01 - File Created

-- Additional Comments: 

--

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library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.NUMERIC_STD.ALL;

use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity pwm is

    Generic (

                                bits: integer:=16;

                                dithering:integer:=5

                                );

    Port ( clk : in  STD_LOGIC;

           set : in  STD_LOGIC_VECTOR(bits-1 downto 0);

           o   : out  STD_LOGIC);

end pwm;

architecture Behavioral of pwm is

signal o_i:std_logic:='0';

signal cnt:std_logic_vector(bits-1 downto 0):=(others => '0');

signal zeros:std_logic_vector((bits-dithering)-1 downto 0):=(others => '0');

signal target:std_logic_vector((bits-dithering) downto 0);

signal reversed_cnt_top:std_logic_vector((dithering)-1 downto 0);

signal inc:std_logic;

function reverse_and_rebase_bit_order(a: in std_logic_vector)

return std_logic_vector is

  variable result: std_logic_vector(a'high-a'low downto 0);

begin

  for i in a'RANGE loop

    result(a'high-i) := a(i);

  end loop;

  return result;

end;

begin

-- output mapping:

o <= o_i;

zeros <= (others => '0');

normal: if dithering = 0  generate

doit:process(clk)

begin

if rising_edge(clk) then

        cnt <= cnt + 1;

        o_i <= o_i;

        if cnt=zeros then o_i <= '1'; end if;

        if cnt=set then o_i <= '0'; end if;

end if;

end process;

end generate;

dithered: if dithering > 0  generate

reversed_cnt_top <= reverse_and_rebase_bit_order(cnt(bits-1 downto bits-dithering));

inc <= '1' when (reversed_cnt_top < set(dithering-1 downto 0)) else '0';

target <= ('0' & set(bits-1 downto dithering)) + inc;

doit:process(clk)

begin

if rising_edge(clk) then

        cnt <= cnt + 1;

        o_i <= o_i;

        if cnt((bits-dithering)-1 downto 0) = zeros then o_i <= '1'; end if;

        if ('0' & cnt((bits-dithering)-1 downto 0)) = target then o_i <= '0'; end if;

end if;

end process;

end generate;

end Behavioral;