CODEWITHSUNDEEP
tclmodelsim
nsrdn
nsrdn

Reputation: 17

How to write the equivalent tcl script for "wait until (SomeSignal)" in ModelSim

I am writing tcl scripts as a wrapper for my VHDL simulation and I am trying to implement the equivalent of the following VHDL pseudo code snippet:

Data <= SomeData;
wait until (strobe = '1');
Data <= NewData;

I tried to do that by creating a loop in tcl that checks the value of the strobe signal for some amount of time but it is extremely inefficient. As far as I see "when" clause is not going to help as well. Are there any suggestions?

proc wait_until {sig val} {
  for {set i 0} {$i < 50} {incr i} {
    set dummy [examine -radix unsigned $sig]
    if {$dummy eq $val} {
      break
    }
    run 10 ns
  }
  }

you run it:

wait_until strobe 1

Upvotes: 0

Views: 419

Answers (1)

Donal Fellows
Donal Fellows

Reputation: 137587

Short answer: vwait does "wait until variable is written to (by some event callback)", and that's when you reevaluate the expression to see if it has changed, going back to waiting if it hasn't. The variable name given to vwait must be a global variable (or the qualified name of a namespace variable); it cannot be a local variable.

Longer answer: You can use a variable trace to determine whether to reevaluate the expression, and then write (an arbitrary value) to some sort of trigger variable that vwait notices. The trigger variable has to be global, but the trace can be on a local variable if needed. (You formally take a performance hit for that, but it doesn't matter if that's what you need.)

However, in all cases you'll need some sort of event (or rather a callback in response to an event) somewhere that causes the variable in the expression to change.

Upvotes: 1

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