Output for only one clock cycle

[Altera_Forum]

Hi all, 

 I have a counter that counts up every time an input of '1' is seen. At the max count value a flag should output '1' and the count resets. I can achieve this, however the output lasts for as long as the input stays at '1' on the final count.  How do you code for an output to last for just one clock cycle?  Thanks

[Altera_Forum]

--- Quote Start ---  Hi all,  I have a counter that counts up every time an input of '1' is seen. At the max count value a flag should output '1' and the count resets. I can achieve this, however the output lasts for as long as the input stays at '1' on the final count.  How do you code for an output to last for just one clock cycle?  Thanks  --- Quote End ---    my guess. you haven't put else statement or default value for that output. where is your code.

[Altera_Forum]

I'm in work and its on my laptop, let me type it over.

[Altera_Forum]

here: 

 process begin if reset = '1' then count <= 0; elsif rising_edge (clk) then   if count < 255 then  if input = '1' then count <= count + 1; end if; end if; end;

[Altera_Forum]

you are asking your count to stay at 255 once it reaches there and where is the output?

[Altera_Forum]

sorry, second process is: 

 process begin if count = 255 then flag <= '1'; else flag = '0'; end if; end;

[Altera_Forum]

well comment out that if count < 255 and its fellow end if. then if your counter is 8 bits it will move back to zero by itself on enable (or on reset)

[Altera_Forum]

Thanks, I removed that, it made no difference to the simulation at all. When input stays '1' the output changes every +ve clk edge. I need the output to only change once for each time the input goes '1', then change again when the input next goes '1' again (after it has been '0').

[Altera_Forum]

now your requirement changed. You want input 0=>1 to enable count and not just 1 so you need to redesign your counter on that basis. 

 inside process: input_d <= input; if input = '1' and input_d = '0' then count <= count + 1; ...

[Altera_Forum]

Cheers, 

 got to get back to work but i'll give it a shot later and get back to you.

[Altera_Forum]

Couldn't get this to work, heres my code in full. Sorry if I confused the question. What happens is that whenever on_input = '1' then the count increases every time the clk goes +ve. I require it to count up only once no mater how long the on_input is held at '1'. I can see why the design I have operates as it does, but I can't work out how to change it to what I need. I need the overflow to act in a similar way. Thanks. 

  entity counter is port (clk : in std_logic;  on_input : in std_logic;  reset : in std_logic;  overflow : out std_logic;  values : out integer range 0 to 255); end counter;  architecture Behavioral of counter is  signal count : integer range 0 to 255;    begin   counter_process : process (clk, reset, on_input)  begin if reset = '1' then count <= 0;  elsif rising_edge (clk) and on_input = '1' then   if count < 255 then  count <= count + 1;   end if;  else count <= count;  end if;     end process counter_process;  overflow_process : process (count) begin if count > 255 then overflow <= '1';  else overflow <= '0'; end if; end process overflow_process;   values <= count;  end Behavioral;

[Altera_Forum]

you wanted count to go from 0 to 255 and back, yet you are asking if count > 255 which will never occur in your new code. 

 Here is code for my understanding of your plans earlier in your post   entity counter is port (clk : in std_logic; on_input : in std_logic; reset : in std_logic; overflow : out std_logic; values : out integer range 0 to 255); end counter; architecture Behavioral of counter is signal count : integer range 0 to 255; signal on_input_d: std_logic := '0'; begin process (clk, reset) begin if reset = '1' then count <= 0; on_input_d <= '0'; elsif rising_edge (clk) then on_input_d <= on_input; if on_input = '1' and on_input_d = '0' then count <= count + 1; end if; end if; end process; process (count) begin if count = 255 then overflow <= '1'; else overflow <= '0'; end if; end process; values <= count; end Behavioral;

[Altera_Forum]

Hey! Thanks for that it worked a treat! And I understand how it works. A real help, thank you!