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VHDL (VHSIC-HDL, Very High-Speed Integrated Circuit Hardware Description Language) is a hardware description language used in electronic des...

Thursday 20 December 2012



The action of a wait statement when the conditions for which the wait statement is waiting are satisfied.


A suspended process (i.e. a process waiting for a condition specified in a wait statement to be met) is resumed when the condition is met. The execution of resumed process is started immediately in the current simulation cycle (time), unless the process is not postponed. In the latter case, the process execution is postponed to the last simulation cycle at the current simulation time.

A resumed process executes its statements sequentially in a loop until a wait statement is encountered. When this happens, the process becomes suspended again.


Example 1

process (CLK, RST)
  if RST='1'
    then Q <= '0';
    elsif (CLK'event) and (CLK='1')
      then Q <= D;
  end if;
end process;

In this Example 1 of a D flip-flop, the process is sensitive to the two signals: CLK and RST. It will resume when any of the two signals will change its value. Resuming of the process will cause the execution of the 'if' statement (which his the only one statement in this process) and then the process will suspend again, waiting for a change on either RST or CLK.

Important Notes

· A resumed process not necessarily executes all its statements: if there are multiple wait statements the execution suspends on the next 'wait'.

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