Wait statement

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Wait statement

• More appropriate, verstail, and more useful way to manage flow in a process
• It can be used in a process and it causes a process to wait

ways to used wait statement

• 4 ways to use wait statement, some uses are unsynthesizable

  unconditional wait

  • causes a process to freeze until the end of simulation
  • not synthesizable
  • Special case of wait for - wait forever

process
begin
    reset <= '1';
    wait for 200ns;
    reset <= '0';
    wait;
end process;

wait for certain amount of time

• not synthesizable

  process
  begin
      clk <= '1'
      wait for 200ns;
      clk <= '0';
      wait for 200ns;
  end process; -- a clock signal with period = 400ns
  

wait until condition;

• A condition is written to wait on it becoming true

process
begin
    wait until clk'event and clk='1';
    Q <= D;
end process;

wait on list of signals

• A list of signals is written to pause the process till an event happens on them

process
begin
    if sel="00" then
        Q <= A;
    elsif sel="01" then
        Q <= B;
    elsif sel="10" then
        Q <= C;
    else
        Q <= D;
    end if;
    wait on sel, A, B, C, D; -- sensitivity list
end process;

Wait statement and senstivity list

When we use a process we can either use a process with a sensitvity list, or a process that contains wait statements, we cannot use a both at the same time mutually exclusive

Rules to understand about how the process is executed

• At startup, the process executes, so the process always execute at least once when the process starts up
• The process always loops when it ends, it never fails to loop back when it reaches the end process statement
• The process will freeze at a wait statement

  effect of wait statement on events and transactions

• A signal assignment in a process is a transaction not an event
• Transactions become events only when we reach a wait statement
• If there is wait statements in the process and you reach the end of the process without a wait statement, all transactions that have not turned into event are trashed
• A process with a sensivity list is a process with wait on statement at the end of the process
• So the cardinal rule of when transaction turn into an event, is A transaction turns into an event only when it meets wait statement

process
begin
    Din <= conv_std_logic_vector(1,4);
    wait for 200ns;
    Din <= conv_std_logic_vector(2,4);
    wait for 200ns;
    Din <= conv_std_logic_vector(3,4);
    wait for 200ns;
    Din <= conv_std_logic_vector(4,4);
end process;

process
begin
    Din <= conv_std_logic_vector(1,4);
    wait for 200ns;
    Din <= conv_std_logic_vector(2,4);
    wait for 200ns;
    Din <= conv_std_logic_vector(3,4);
    wait for 200ns;
    Din <= conv_std_logic_vector(4,4);
    wait;
end process;

verdict on wait statement

• wait for and unconditional wait statements are generally unsynthesizable and should only be used in the test bench
• wait until statement can be used to create loops or flip flops but there is syntax that is clearer that can do both things
• wait on statement can be used to create processes that behave as if they have a sensitivity list