Can we use two wait statements in a single process in VHDL? - vhdl

Question

I have to create a delay of say 20 ms in a process waiting in an input button.
I wrote the following code and it gives an error
wait until clk'EVENT and clk='1';
wait for 20 ms;
Or, can I use a construct like:
wait for 20 ms until clk'EVENT and clk='1';
Any help is highly appreciated.

Answer 1

You can wait for an event, for time, or for both (or neither--but that is a unique case).
Now, I'm not quite sure why your first example gives an error since it is a valid sequential statement. But you have not provided a complete example nor the error code. So, my answer here will be pretty basic.
First, a wait statement can only occur as a sequential statement, essentially meaning only in a process (or called from a process). So, if you are trying to use it concurrently, it is a problem.
Now, if you are using it from a process, it must be in a process without a sensitivity list. That is, the following is illegal:
process(clk)
begin
wait until clk'EVENT and clk='1';
end process;
It must be a bare process, such as:
process
begin
wait until clk'EVENT and clk='1';
end process;
A bit more on your first example (properly placed in a sequential context and does compile):
process
begin
wait until clk'EVENT and clk='1';
wait for 20 ms;
end process;
This code waits for a rising edge on clk, and then waits for 20ms. These are sequentially executed (hence the sequential context in a process).
Your second statement needs to be tweaked to compile. Generally, a wait statement has the form wait until <event> for <time>, where both event and time are optional. For example:
process
begin
wait; -- No event, no time. Wait forever.
wait until clk'event and clk='1'; -- Wait forever for a rising edge on 'clk'
wait for 20 ms; -- Wait for 20 ms
wait until clk'event and clk='1' for 20 ms; --Wait for up to 20 ms for a rising edge on 'clk'
end process;
So, your second example has the order backward for event and time.
Finally, your introductory text indicates you are waiting for 20ms for a push button. This hints that you are trying to create real logic. The wait statement is synthesizable only in very limited use cases. And one of the cases specifically excluded is waiting for a period of time. If you need to wait for 20 ms, you'll need to do it some other way (such as counting clocks). And if you are trying to wait up to 20 ms for the button, it'll have to be a combination of detecting the change on the pushbutton and counting clocks.