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Altera_Forum's avatar
Altera_Forum
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15 years ago

Duty cycle VHDL

Hello!

I'm trying to create an A and B pulse and change the duty cycle of that pulse. The program shown below just changes the frequency and creates an A and B pulse, but I'm not able to change the duty cycle.

Has anyone a tip or idea??

library ieee;

use ieee.std_logic_1164.all;

entity freq is

port(

clk :in std_logic;

rst :in std_logic;

S0 :in std_logic;

out_0 :out std_logic;

out_1 :out std_logic

);

end freq;

architecture Behavioral of freq is

signal cnt : integer;

signal temp : std_logic;

signal temp_out : std_logic;

begin

--XXXXXXXXXXXXXXXXXXXXXXXXXXXX

--CLOCK

process(clk)

begin

if(rst='1') then

temp <= '0';

cnt <= 0;

elsif(clk'event and clk='1') then

if(cnt= 1000000) then

temp <= not temp;

cnt <= 0;

else

cnt <= cnt +1 ;

end if;

end if;

--XXXXXXXXXXXXXXXXXXXXXXXXXXXX

--Switch

if (S0='1') then

temp_out <= temp;

else

temp_out <= '0';

end if;

end process;

out_0 <= temp_out;

out_1 <= not temp_out;

end Behavioral;

1 Reply

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    15 years ago

    Hi Frans

    Infact your code simply creates a 50/50 square wave.

    I think this is what you need: 

    library altera;
library IEEE;
use IEEE.std_logic_1164.all; 
use IEEE.numeric_std.all;    -- for the unsigned type
entity pwm_generator is
	port (CLKIN   : in  std_logic;  
		  TPERIOD : in  unsigned(7 downto 0);   
		  TWIDTH  : in  unsigned(7 downto 0);   
			PWM   : out std_logic );
end entity pwm_generator; 
architecture pwm_gen_a of pwm_generator is
	subtype small_int is natural range 0 to 500;
	begin  
		update: process(CLKIN) is  
		variable tcnt : small_int := 0;
		begin
			if rising_edge(CLKIN) then  
				tcnt := tcnt + 1;
				if tcnt = TPERIOD then
					tcnt := 0;
				end if;
				if tcnt >= TWIDTH then
					PWM <= '0';
				else
					PWM <= '1';
				end if;
			end if;  
		end process;   
	end architecture pwm_gen_a;

    You may need to change TPERIOD and TWIDTH bit length to meet your specs.

    You must also add the pwm switching function.