pwm generation using fpga

Question

How to generate a PWM signal using an FPGA? Which is best method to generate a variable duty cycle?

I tried to solve this problem by using the following code but two or three errors occurred.

This is my code:

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

use ieee.numeric_std.all;
--use ieee.float_pkg.all;
---- Uncomment the following library declaration if instantiating
---- any Xilinx primitives in this code.

--library UNISIM;
--use UNISIM.VComponents.all;

entity pwm_sne is
  Generic(
    sys_clk         :integer :=50000000;
    pwm_freq        :integer :=100000;
    bits_resolution :integer :=8
  );
  Port (
    clk : in  STD_LOGIC;
    rst : in  STD_LOGIC;
    k   : in  STD_LOGIC_VECTOR (7 downto 0);
    y   : out STD_LOGIC
  );
end pwm_sne;

architecture Behavioral of pwm_sne is
  signal cnt      :std_logic_vector(7 downto 0);
  signal flag     :std_logic;
  signal reg      :std_logic_vector(7 downto 0);
  --variable duty   :std_logic:=0;
begin
  process(clk,rst)
  begin
    if rst='1' then
      cnt<="00000000";
    elsif(clk'event and clk='1')then
      cnt<=cnt+"00000001";
    elsif cnt="11111111" then
      flag<='0';
      cnt<="00000000";
    end if;
  end process;

  process(clk,flag)
  begin
    if(clk'event and clk='1') then
      reg<=k;
    end if;
  end process;

  process(cnt,reg)
  begin
    if(flag='0')then
    elsif cnt>reg then
      y<=(reg/256)*100;
      --y<=duty;
    elsif cnt=reg then
      y<=(reg/256)*100;
    elsif cnt<=reg then
      y<=period;
      --y<=duty;
    end if;
  end process;
end Behavioral;

The errors occurred in output value y and at the division operation.

Please suggest a good method to solve the problems from above.

Answer 1

Solution : Do maths on numbers, not on untyped bags of bits.

If cnt, reg,k were natural range 0 to 255 and you compared cnt with 0 or 255 and added 1 to it, this would just work. And if k MUST be std_logic_vector, use ONE type conversion function between it and reg.

Also recommended : delete the non-standard std_logic_arith andstd_logic_unsignedlibs and usenumeric_std` instead.