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Fetching the operating point of a transistor in VerilogA with "$simprobe" always returns 0

CroAndro

I have the following schematic:

"VerilogA_Test" block is written so that it takes the transconductance gm of the transistor MN0 and prints its value.

The VerilogA code is the following:

`include "constants.vams"
`include "disciplines.vams"

module VerilogA_Test;

    string path = "MN0";
    real val5;
    integer count;
    analog begin
            @ (initial_step) begin
            count=0;
        end
        if( count < 5 ) begin
            val5 = $simprobe(path,"gm");
            $display("\n Value of gm in Analog context is: %f\n",val5);
        end
        count = count+1;
     end
endmodule

Whether I run a DC or a transient simulation, the output of the $display function is always:

"Value of gm in Analog context is: 0.000000".

I followed the procedure described in: $simprobe support

What am I doing wrong? I am on Spectre version 21.1.0.389.isr8

Thank you in advance!

  • I should add that the link provided above states "Note: There is a known issue that $simprobe used in expression returns 0.0 ( for example vth = aconst * $simprobe (device, "vth", 1000.0) ), which is fixed in Spectre 21.1ISR5." so this issue should be solved in my version. I have tried doing the $simprobe command to extract nonsense (e.g. $simprobe(path,"ABC")) and it still outputs 0.0 instead of an error.

  • I have solved the problem. In case anyone needs it, the solution is that $simprobe works when using parameter strings and not string literals.

    Here is the working code:

    parameter string inst= "MN0";
    parameter string quant= "gm";

    …

    val5 = $simprobe(inst,quant);

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