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How can I take a real value and assign it to a node in this case the output port of my module

Wasif Absar

In the module below there is an error that says I cannot assign a real value to a node. How can I solve the issue? 


module VA_measureDelay(INPUT_CLOCK,OUTPUT_CLOCK,TIME_DELAY);
input INPUT_CLOCK;
input OUTPUT_CLOCK;
output TIME_DELAY;

electrical INPUT_CLOCK, OUTPUT_CLOCK, TIME_DELAY;

parameter real vdd = 830m;

real time_INPUT_CLOCK, time_OUTPUT_CLOCK;


analog begin

@(cross( V(INPUT_CLOCK)-vdd/2, 1)) begin
time_INPUT_CLOCK = $abstime();
end

@(cross( V(OUTPUT_CLOCK)-vdd/2, 1)) begin
time_OUTPUT_CLOCK = $abstime();
end

TIME_DELAY = time_OUTPUT_CLOCK - time_INPUT_CLOCK;


end

endmodule

  • To update a node (an electrical node in this case) you need to use a contribution statement such as:

    V(TIME_DELAY) <+ time_OUTPUT_CLOCK - time_INPUT_CLOCK;

    However, you shouldn't do that directly because it would then be discontinuous (and discontinuities are bad for a circuit simulator). So instead something like:

    V(TIME_DELAY) <+ transition(time_OUTPUT_CLOCK - time_INPUT_CLOCK,10p,10p); set an appropriate delay and rise time for the changing signal

    Another issue that you would need to deal with is that if you are using voltages to represent time, the resulting voltages may be very small (I don't know what timescales you are dealing with here) and lower than the normal tolerance settings for voltages. Two approaches to this are either to use a different discipline than electrical for outputting time (you could create your own discipline with an appropriate abstol) or just scale the times to be within normal voltage ranges for the simulator.

    Alternatively, if this is for monitoring purposes, you could keep it as a real number and then use saveahdlvars or use the Op parameter saving capability in ADE to save the specific variables you want which would allow you to plot them anyway - they then end up just being numbers rather than electrical signals.

    Andrew

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