Level Sensitive Wait

Proposal Information

Requirement Summary

Level Sensitive Wait

See ISAC IR2108

The WaitRepeat proposes modifying the language syntax of the wait statement.

A Code Solution

This can easily be accomplished with the code

if A /= Level then
  wait until A = Level ;
end if; 

However, what is being sought is a more concise solution.

Encapsulation with a Procedure

If the following worked, it would be an acceptable solution. However while a signal input to an entity allows expressions with VHDL-2008, a signal input to a parameter of a subprogram does not.

procedure WaitLevel (
  signal condition : in boolean 
) is 
  if not condition then
    wait until condition ;
  end if ; 
end procedure WaitLevel ; 

General support issues: When an expression is passed as input signal parameter, it would have the default value of the type (here false) until a delta cycle passes. Here that works just fine.

Language Based Solution

wait when A /= '1' ; 

Equivalent to and would be defined by the following transformation:

if A /= '1' then
  wait until A = '1' ;
end if; 

An explicit condition_clause (until) would replace the implicit condition clause and hence,

wait when A /= '1' until rising_edge(Clk) and C = '1' ; 

Transforms to

if A /= '1' then
  wait until rising_edge(Clk) and C = '1' ; 
end if; 

It should be noted that the more likely form of the above is shown below. "C" was used to demonstrate the full intent - there were historical misinterpretations of "wait on A until B" as to whether "B" was in the sensitivity list or not (it is not).

wait when A /= '1' until rising_edge(Clk) and A = '1' ; 


wait_statement ::=
    [ label : ] wait [ when_clause ] [ sensitivity_clause ] [ condition_clause ] [ timeout_clause ] ;

when_clause ::= when condition

sensitivity_clause ::= on sensitivity_list

sensitivity_list ::= signal_name { , signal_name }

condition_clause ::= until condition

timeout_clause ::= for time_expression


-- JimLewis - 2020-10-30 As I was coding, this popped into my thoughts, so I captured it. Still forming thoughts about it.

A process with a wait when may never wait, and hence, for simulators that report issues with a "possible infinite loop" will still report that if the only wait is a wait when.

-- JimLewis - 2014-06-22 Personally I am not excited about a syntax based solution as it would potentially make the wait statement even more complex. OTOH, allowing in parameter signals to have an expression and solving this using a procedure in a package would be a good solution.

Arguments Against


Topic revision: r6 - 2020-10-30 - 16:17:58 - JimLewis
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