long/short sensitivity list

[titi]

In the sample code here after, the sensitivity list is (Reset,Load,F).
Would it be correct to write (Reset,F)?
Why, why not?


machin : process(Reset,Load,F)
begin
if Reset ='1' then
En1 <= '0';
En2 <= '0';
elsif F'event and F ='1' then
En1 <= Load;
En2 <= En1;
end if;
end process;

 

[KJ]

"titi" <titi@nospam.fr> wrote in message
news:esjgpf$117t$1@biggoron.nerim.net...

In the sample code here after, the sensitivity list is (Reset,Load,F).
Would it be correct to write (Reset,F)?
Why, why not?

machin : process(Reset,Load,F)
begin
if Reset ='1' then
En1 <= '0';
En2 <= '0';
elsif F'event and F ='1' then
En1 <= Load;
En2 <= En1;
end if;
end process;

The correct sensitivity list is "Reset, F". The reason is because nothing
happens when the signal "Load" changes (as you have in your list). To see
for yourself, simply take a walk through the code holding all inputs
constant and you'll see that the only way you can get to the lines

En1 <= Load;
En2 <= En1;
are if there has been a rising edge detected on signal 'F'. So change

'Load' all that you want all by itself and it will not affect the outputs of
the process 'En1' and 'En2'. By adding 'Load' to the sensitivity list,
you're telling the simulator to execute this process whenever 'Load'
changes. But as you've seen, none of the outputs will change as a result of
a change in 'Load' so you've caused the simulator to needlessly waste cycles
evaluating something for no reason.

On the plus side, there are much worse things than wasting CPU cycles, the
design as written will simulate and synthesize just fine. There are much
worse things. Ask yourself what would happen if you left 'Reset' out of the
sensitivity list instead (by accident). In that situation, the asynchronous
resetting of 'En1' and 'En2' would not occur. If your clock signal 'F'
happens to be running while 'Reset' is true then the resetting will occur at
the next rising edge of 'F' (i.e. a synchronous reset) but if it's not then
the code after 'if Reset='1' then...' will never execute. Most any
synthesis tool will report this as a warning about an incomplete sensitivity
list and will implement what you intended AS IF you had included 'Reset' in
the sensitivity list. Things like this cause simulation results and actual
implementation results to differ.

I'll also take a moment to encourage the use of
if rising_edge(F) then...
as being clearer than
if F'event and F ='1' then

Kevin Jennings