waiting on vector change

A funny thing is happening in my test bench.

Given an entity with a vector in the list of ports:

entity foo is
port (
in bar : std_logic_vector(7 downto 0);
... );
end entity foo;

and a greatly-simplified architecture

architecture bletch of foo is
begin
proc : process is
begin
wait on bar; -- wait for change
doSomething;
end process proc;
end architecture bletch;

The problem is that the wait is triggered when the vector bar changes
from all undriven ('UUUUUUUU') to some reasonable value. Then
doSomething is called, and then it goes back to waiting for bar to
change. The higher-level module that uses foo definitely changes bar
again, and I can see it change in foo, but the process proc is hung and
apparently doesn't see bar changing.

My assumption is that this should work but it doesn't. I'm using
ModelSim XE Starter 5.8c

Ideas?

thanks,
-a

Andy Peters

Andy Peters wrote:
> A funny thing is happening in my test bench.
>
> Given an entity with a vector in the list of ports:
>
> entity foo is
> port (
> in bar : std_logic_vector(7 downto 0);
> ... );
> end entity foo;
>
> and a greatly-simplified architecture
>
> architecture bletch of foo is
> begin
> proc : process is
> begin
> wait on bar; -- wait for change
> doSomething;
> end process proc;
> end architecture bletch;
>
> The problem is that the wait is triggered when the vector bar changes
> from all undriven ('UUUUUUUU') to some reasonable value. Then
> doSomething is called, and then it goes back to waiting for bar to
> change. The higher-level module that uses foo definitely changes bar
> again, and I can see it change in foo, but the process proc is hung and
> apparently doesn't see bar changing.

You don't appear to have included enough code. Either you need a
sensitivity list, or you need an inifinite wait in the process.
Otherwise, Modelsim should give you an error when you attempt to compile
that. Do you have a sensitivity list that includes bar?

Duane Clark

Duane Clark wrote:
>
> You don't appear to have included enough code. Either you need a
> sensitivity list, or you need an inifinite wait in the process.
> Otherwise, Modelsim should give you an error when you attempt to compile
> that. Do you have a sensitivity list that includes bar?

Hmm.. my bad. Any wait is okay, so ignore that, sorry.

Duane Clark

Andy Peters wrote:

> The problem is that the wait is triggered when the vector bar changes
> from all undriven ('UUUUUUUU') to some reasonable value. Then
> doSomething is called, and then it goes back to waiting for bar to
> change.

Maybe bar changes during doSomething;
instead of during the wait. You only
have half of a handshake here.

-- Mike Treseler

Mike Treseler

> Maybe bar changes during doSomething;
> instead of during the wait. You only
> have half of a handshake here.

Mike,
That's exactly it. I added a "Busy" flag that the logic must monitor
and know not to change bar until Busy goes away.
Thanks,
-a

Andy Peters