A parameterized PWM module written in VHDL

PWM.vhd

-- Written By Jon Carrier

library IEEE;
use IEEE.STD_LOGIC_1164.all;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
use ieee.numeric_std.all;

entity PWM is
  generic (
    prescale: integer := 0; --frequency(PWM_clk)=frequency(i_CLK/(prescale+1))
    quantas:  integer := 8  --PWM stages, determines granularity    
  );
  port (
    i_CLK:  in  std_logic;  --Main input clock signal 
    i_PWM:  in  std_logic_vector(31 downto 0); --Controls the Duty Cycle of o_PWM
    o_PWM:  out std_logic := '1' --The Pulse Width Modulated output signal
  );
end PWM;

architecture PWM_arch of PWM is
  signal  PWM_clk : std_logic :='0';  
  signal  PWM_acc : std_logic_vector(31 downto 0) := x"00000000"; 
begin

--PRESCALING PROCESS
--Generates the clock divided signal, frequency(PWM_clk)=frequency(i_CLK/(prescale+1))
--PWM_clk drives the rest of the PWM logic
  prescale_proc: process(i_CLK)
    variable  prescale_cnt : std_logic_vector(31 downto 0) := x"00000000";
  begin
    if rising_edge(i_CLK) then
      if prescale_cnt >= prescale then
        prescale_cnt := x"00000000";
        PWM_clk <= not PWM_clk;
      else
        prescale_cnt := prescale_cnt + 1;
      end if;
    end if;
  end process;
  
--ACCUMULATOR PROCESS
--Generates the accumlation variable PWM_acc
--This variable is used in the PWM as a means to determine
--when the PWM signal should switch.
  accumulate: process(PWM_clk)
  begin
    if falling_edge(PWM_clk) then
      if PWM_acc >= std_logic_vector(to_unsigned(quantas - 1, PWM_acc'length)) then
        PWM_acc <= x"00000000";
      else
        PWM_acc  <=  PWM_acc + 1;
      end if;
    end if;
  end process;
  
--PWM PROCESS
--Generates the PWM output signal, o_PWM
--i_PWM controls the output and specifies how many Time Quantas
--out of a Total Quanta the output must be set HIGH
  modulate: process(PWM_clk, i_PWM)
  begin
    if rising_edge(PWM_clk) then
      if PWM_acc >= i_PWM then
        o_PWM <= '0';
      elsif PWM_acc = x"00000000" then
        o_PWM <= '1';
      end if;     
    end if;
  end process;
  
end PWM_arch;

@Eguy358, I will skip on writing the VHDL/Verilog code as I do not have a dev environment setup for this right now, but I can help with the math.

A 50Hz PWM will have a period of 20ms. You can break this period into fractions of time using time quantas. To get a nice even 2ms pulse width you could use a value that is a multiple of 10. A higher value will give you better granularity if you need to provide a PWM output for a different pulse width or duty cycle. So we can go with quantas = 200.

With this, we can now calculate the prescaler to make this work which follows the form of:

PWM frequency = Input Clock Frequency / (prescaler + 1) / quantas
50Hz = 12MHz / (prescaler + 1) / 200

This results in:
prescaler = 1199

Then with this configuration, each unit of i_PWM is equal to:
i_PWM unit of time = (1/50)/200

or

i_PWM unit of time = 100us

So a 2ms pulse would be created using:
i_PWM = 2ms / 100us = 20

To summarize, to get a 2ms pulse width on a 50Hz PWM using a 12MHz clock, the following parameters could be used (there many possible solutions to this problem):

• prescaler = 1199 (creates a PWM_clk of 10KHz)
• quantas = 200 (200 quantas for a 10KHz PWM_clk = 20ms or 50Hz)
• i_PWM = 20

Anyways hope that helps and I did not mix any math up.