- Section 9.2
optional_label: process ( -- optional sensitivity list ) -- declarations begin -- sequential statements end process optional_label;
The sensitivity list is a list of signals. A change in value on one or more of these signals, causes the process to be activated:
process (ALARM_TIME, CURRENT_TIME) begin if (ALARM_TIME = CURRENT_TIME) then SOUND_ALARM <= '1'; else SOUND_ALARM <= '0'; end if; end process;
Alternatively, process activation and suspension may be controlled via the wait statement:
process begin if (ALARM_TIME = CURRENT_TIME) then SOUND_ALARM <= '1'; else SOUND_ALARM <= '0'; end if; wait on ALARM_TIME, CURRENT_TIME; end process;
A process cannot have both a sensitivity list and wait statements. A process may contain any sequential statement.
The keyword process (or the sensitivity list, if there is one) may be followed by the keyword is for clarity and consistency.
A postponed process may be defined. Such a process runs when all normal processes have completed at a particular point in simulated time. Postponed processes cannot schedule any further zero-delay events. Their main use is to perform timing or functional checks, based on the “steady-state” values of signals.
Processes are synthesizable, provided they match certain typical forms, some of which are shown below.
A “clocked process” with either a wait statement or sensitivity list. For such a process, registers are inferred on all signals which have assignments to them:
WAIT_PROC: process begin wait until CLK'event and CLK='1'; Q1 <= D1; end process; SENSE_PROC: process (CLK) begin if CLK'event and CLK='1' then Q2 <= D2; end if; end process;
A “combinational process” must have a sensitivity list containing all the signals which it reads (inputs), and must always update the signals which it assigns (outputs):
process (A, B, SEL) begin Z <= B; if (SEL = '1') then Z <= A; end if; end process;