I have a module for an oscillator that works as follows:  When the ENB (enable_bar, so active low) is set low, the output goes to zero.  Great.  When ENB is set high, the output resumes.  Also great.   However, the timebase of the oscillator is as it was before the reset.  That is to say, it behaves like a continuouslly running oscillator with an outout enable.

I want to modify the module so that when ENB is set low, the oscillator will always start from the same state, regardless of the abstime.  I've tried messing with the n and next variables, making them hold position while ENB is high, but am getting nowhere.

If anyone has a suggestion, I would like to hear it.  I'm a novice, so, as always, if you see something silly or wrong in the module, I'm always open to learning a better way!

Here is the module:
module osc (enb, out);

input enb;
output out;
voltage enb,out;

parameter real      freq=1 from (0:inf),            // output frequency
                                               vl=-1,                        // high output voltage
                                               vh=1,                        // low output voltage
                                               vth=(vh-vl)/2,
                                               tt=0.01/freq from (0:inf);      // transition time of output

integer n, enable;
real next;

analog begin

           @(cross(V(enb) - vth, -1 )) enable = 1;
           @(cross(V(enb) - vth, +1 )) enable = 0;
   
               @(initial_step)
                             next = (0.5/freq + $abstime);
                     
               @(timer(next)) begin
                             n = !n;
                             next = next + 0.5/freq;
                    end
                       
                       V(out) <+ transition(n ? enable*vh : vl, 0, tt);

     end
endmodule

Thanks!
John

Replacing the cross() event function calls in your code by above() event function calls will achieve the same as the conditional outside the event.

Code:

@(above(V(enb) - vth)) enable = 0;
@(above(vth - V(enb))) begin enable = 1; next = $abstime; end 

I'm puzzled that you do not use the period argument of the timer() event function and just update the reference on every falling edge of enb.

Code:

@(timer(next, 0.5/freq)) n = !n; 

If you want the oscillator to have 0 output voltage when disabled and a voltage that toggles between vh and vl you will need still a different output statement:

Code:

V(out) <+ transition(enable ? (n ? vh : vl) : 0, 0, tt); 

Cheers,
Marq

Yes, 1000 was arbitrary. It just has to be larger than any possible stop time.

-Ken