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

Tuesday 18 December 2012

Sensitivity List

Formal Definition

A list of signals a process is sensitive to.

Simplified Syntax

(signal_name, signal_name, . . .)


The sensitivity list is a compact way of specifying the set of signals, events on which may resume a process. A sensitivity list is specified right after the keyword process (Example 1).

The sensitivity list is equivalent to the wait on statement, which is the last statement of the process statement section.

Only static signal names, for which reading is permitted, may appear in the sensitivity list of a process, i.e. no function calls are allowed in the list.


Example 1

DFF : process (CLK,RST)
  if RST = '1'
    then Q <= '0';
    elsif (CLK'event) and (CLK = '1')
     then Q <= D;
  end if;
end process DFF;
-- DFF : process
-- begin
-- if RST = '1'
-- then Q <= '0';
-- elsif (CLK'event) and (CLK = '1')
-- then Q <= D;
-- end if;
-- wait on RST, CLK;
-- end process DFF;

Here, the process is sensitive to the RST and CLK signals, i.e. an event on any of these signals will cause the process to resume. This process is equivalent to the one described in the comment section.

Important Notes

· A process with a sensitivity list may not contain any explicit wait statements. Also, if such a process statement is a parent of a procedure, then that procedure may not contain a wait statement as well.

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