As mentioned earlier, CSA’s allow combinational logic circuits to be implemented. They are not sufficient for implementation of sequential circuits, however. Processes are needed to describe the operation of sequential circuits. Processes have the following form:
-- In the PROCESS statement below (a, b) is a sensitivity list(optional). The
--process executes whenever a signal in this list changes(i.e. has an event).
--======================================================
PROCESS (a, b)
-- this is the declarative part of the process.
-- constants, variables and types are declared here.
BEGIN
-- sequential statements go here
END PROCESS;
Important notes regarding processes:
- Processes execute in zero time.
- Processes execute endlessly unless broken by:
- WAIT statement or
- Sensitivity list (Processes with a sensitivity list have an implied WAIT at the end)
- An Architecture can have multiple processes.
- All processes execute concurrently.
- Variables are local to the process in which they are declared.
- Variables may only be declared within processes
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