Forum Discussion

Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
9 years ago

wait for [variable] ns - Wait statement for a variable time possible?

Hello Community,

I got a problem with my current coding task: I want to create a small microprocessor in VHDL to work with assembly language via my mif-file. One of my tasks should be a wait statement.

The problem: it should be variable in time. I want to be able to wait as long as the number in my loaded memory is.

This code I tried to implement but it didn't compile: 

when execute_wait =>                
                wait for memory_data_register ns;
                state <= fetch;

In this example my memory_data_register is the content which is read from my memory. It's type is unsigned.

I'd be perfect, if somebody can help me ;)

Best,

orPoG

2 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    9 years ago

    Wait statements are only for simulation and indeed not synthesizable. You'll have to write a timer for this purpose.

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    9 years ago

    Hello PietervanderStar,

    thanks for your hint!

    I've now solved the problem this way: 

    -- wait state
    process
    begin
        wait until clk_50MHZ'event and clk_50MHZ = '1';
            if count_1KHZ /= 24999 then
                count_1KHZ <= count_1KHZ + 1;
            else
                count_1KHZ <= 0;
                clk_1KHZ <= not clk_1KHZ;
            end if;
    end process;
    process (clk_1KHZ)
    begin 
        if (clk_choice = '0') then
            if rising_edge(clk_1KHZ) then
                if (count_individual_timer < timer_time) then
                    count_individual_timer     <= count_individual_timer + 1;
                    individual_timer_EN        <= '0';
                else
                    individual_timer_EN        <= '1';
                    count_individual_timer     <= 0;
                end if;    
            end if;
        else
            individual_timer_EN        <= '1';
        end if;
    end process;