I was wondering what is the difference between
process (clk)
if (clk = '1') then
and
process (clk)
if (clk'event and clk = '1') then
I thought the sensitivity list defined on which signals a state change would be monitored so if I am right then the clk'event is redundant.
I have made a simple simulation to verify what I thought and it apparently confirmed my opinion.
1
Yes, you are right, during a simulation both code snippets work identically.
But during synthesis all signals which are read in the process are added implicitly to the sensitivity list.
This means if there is the assignment B<=A; inside the process, A is added to the sensitivity list and at
any time when A is changed B will also change, if clk=’1′.
So without clk’event the code describes a latch and not a flipflop in synthesis.
3
There is no difference in the two posted snippets. Using the ‘event attribute can be used to determine which signal has had an event in a process with multiple signals in the sensitivity list.
For example:
process(a, b)
begin
if a = '1' then
-- do something here
in this case, the if branch is entered when a or b changes, so if you only want to enter the branch when a changes, the check on a'event is also required:
process(a, b)
begin
if a'event and a = '1' then
-- do something here only when a changes to '1'