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Wait Statement

Used In

  • Process
  • Procedure

Reference Manual

VHDL-93:
  • Section 8.1

Syntax

wait until condition;

wait on signal_list;

wait for time;

wait;

Rules and Examples

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.

Synthesis Issues

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.

See Also