Time in VHDL

VHDL offers support for time-related operations. Here are some examples:

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.NUMERIC_STD.ALL;

entity TimeExample is
end TimeExample;

architecture Behavioral of TimeExample is
    -- Function to simulate printing
    procedure print(s : string) is
    begin
        report s severity note;
    end procedure;

begin
    process
        -- Define variables
        variable current_time : time := now;
        variable specific_time : time := 1 ns;
        variable time_difference : time;
    begin
        -- Get the current simulation time
        print("Current time: " & time'image(current_time));

        -- Create a specific time
        specific_time := 10 ns;
        print("Specific time: " & time'image(specific_time));

        -- Extract components of the specific time
        print("Specific time in ns: " & integer'image(specific_time / 1 ns));
        print("Specific time in ps: " & integer'image(specific_time / 1 ps));

        -- Compare times
        if current_time < specific_time then
            print("Current time is before specific time");
        elsif current_time > specific_time then
            print("Current time is after specific time");
        else
            print("Current time is equal to specific time");
        end if;

        -- Calculate time difference
        time_difference := specific_time - current_time;
        print("Time difference: " & time'image(time_difference));

        -- Advance time
        current_time := current_time + 5 ns;
        print("Advanced time: " & time'image(current_time));

        wait;
    end process;
end Behavioral;

In this VHDL example, we demonstrate various time-related operations:

We start by getting the current simulation time using the now function.
We create a specific time value using the time type.
We can extract components of a time value, such as nanoseconds or picoseconds.
VHDL allows comparison of time values using standard comparison operators.
We can calculate the difference between two time values.
Time values can be advanced by adding a duration.

Note that VHDL’s time handling is simulation-centric, as VHDL is primarily used for hardware description and simulation. The concept of “current time” in VHDL refers to the current simulation time, not the actual wall clock time.

To run this VHDL code, you would typically use a VHDL simulator. The exact commands may vary depending on your simulation environment, but it might look something like this:

$ ghdl -a time_example.vhd
$ ghdl -e TimeExample
$ ghdl -r TimeExample

This will compile the VHDL file, elaborate the design, and run the simulation, displaying the results of our time operations.

VHDL’s time handling is particularly useful for simulating and verifying timing behavior in digital designs. It allows designers to model and analyze the temporal aspects of their hardware systems accurately.

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