- Section 8.1
wait until condition; wait on signal_list; wait for time; wait;
The wait until statement suspends a process until a change occurs on one or more of the signals in the statement and the condition is evaluated to be true. A rising edge on NET_DATA_VALID and three rising edges on CLK must occur for this process to cycle:
READ_NET: process begin wait until NET_DATA_VALID = '1'; NET_DATA_READ <= '1'; wait until CLK='1'; wait until CLK='1'; NET_BUFFER <= NET_DATA_IN; wait until CLK='1'; NET_DATA_READ <= '0'; end process READ_NET;
The ‘event attribute in the following form of wait statement is in fact redundant, but is required by many synthesis tools:
WAIT_PROC: process begin wait until CLK'event and CLK='1'; Q1 <= D1; end process;
The wait on statement is equivalent to a sensitivity list. These processes will behave identically:
process (A, B) begin -- sequential statements end process; process begin -- identical sequential statements wait on A, B; end process;
Wait for and wait are useful in behavioral models and test benches. Wait on it’s own suspends a process indefinitely:
STIMULUS: process begin EN_1 <= '0'; EN_2 <= '1'; wait for 10 ns; EN_1 <= '1'; EN_2 <= '0'; wait for 10 ns; EN_1 <= '0'; wait for 10 ns; wait; -- end of test end process STIMULUS;
The wait statement cannot be used:
- In a process with a sensitivity list
- In a procedure called from a process with a sensitivity list
- In a function
- In a procedure called from a function
A wait statement may have a label.
Most logic synthesis tools only support a single wait until (clock edge expression) statement in a clocked process.
Some tools support a single wait on statement as an alternative to a sensitive list in a combinational process.
Wait for, unconditional wait, and wait statements in procedures are not supported.