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Wednesday, September 8, 2010

Example : D Flip-Flop with Asynchronous Clear,Set and Clock Enable

    As per the request from readers I have decided to post some basic VHDL codes for beginners in VHDL. This is the second one in the series, a basic D Flip-Flop with Asynchronous Clear,Set and Clock Enable(negedge clock).The code is self explanatory and I have added few comments for easy understanding.


--library declaration for the module.
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
--This is a D Flip-Flop with Asynchronous Clear,Set and Clock Enable(negedge clock).
--Note that the clear input has the highest priority,preset being the next highest
--priority and clock enable having the lowest priority
entity example_FDCPE is
   port(
      Q : out std_logic;      -- Data output
      CLK :in std_logic;      -- Clock input
      CE :in std_logic;    -- Clock enable input
      CLR :in std_logic;  -- Asynchronous clear input
      D :in  std_logic;      -- Data input
      PRE : in std_logic   -- Asynchronous set input
   );
end example_FDCPE;

architecture Behavioral of example_FDCPE is  --architecture of the circuit.

begin  --"begin" statement for architecture.

process(CLR,PRE,CLK) --process with sensitivity list.
begin  --"begin" statment for the process.

    if (CLR = '1') then  --Asynchronous clear input
           Q <= '0';
    else
           if(PRE = '1') then  --Asynchronous set input
               Q <= '1';
           else
               if ( CE = '1' and  falling_edge(CLK) ) then
                  Q <= D;      
              end if;
          end if;
   end if;

end process;  --end of process statement.

end Behavioral;

Note :- This is a flip flop which is defined in the Xilinx language template for spartan-3.If you synthesis this design it will use exactly one flip flop and some buffers alone.It will not use any LUT's for the implementation.

6 comments:

  1. Thank you for your wonderful post but as I am newbie in VHDL.

    How about an example of shift register that can increment, shift left , shift right ?

    Thanks again ^_^

    ReplyDelete
  2. may not be exactly as per your requirement but I have written code for a PISO shift register here:
    http://vhdlguru.blogspot.com/2010/09/examples-for-gate-level-and-behavior.html

    Using this you can try to code the module you want.

    ReplyDelete
  3. Can you please post a test bench that could help simulating your code?

    ReplyDelete
  4. Value of q is not changing in simulation

    ReplyDelete
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