Wait Statements

Wait Statements
Wait statements are used to make a process wait for a certain condition, signal/variable, or a specific time interval. The following wait statements are used:

● Wait until [condition] and wait on [signal]
wait until [condition] will block the process while checking whether a condition is true or false. The process will remain blocked until the condition being checked becomes true. Meanwhile, wait on [signal] will wait until there is a change in the specified signal. The syntax of wait until and wait on can be synthesized.

process is
begin
    signal1 <= '1';
    signal2 <= '0';

    wait until rst <= '1';

    signal1 <= '0';
    signal2 <= '1';

    wait on clk;
end process;

● Wait for [time period]
wait for [time period] will block the process for the specified time period. The syntax of wait for cannot be synthesized but can be simulated in the waveform using ModelSim. Therefore, wait for is commonly used for a testbench, which will be studied in the next module.

process is
begin
    signal1 <= '1';
    signal2 <= '0';

    wait for 50 ps;

    signal1 <= '0';
    signal2 <= '1';

    wait for 100 ps;
end process;