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Using DPI to get the current value of a signal

sdav
I'm using a C++ interface with SystemVerilog DPI to drive my Verilog simulation. From C++ I'd like to call a function which returns the current value of a Verilog signal. Ideally this function would take a string as an input parameter (e.g. "testbench.u1.u2.dout") and then return the reg value of the signal. I'm just not sure how to implement this in Verilog. $display, $monitor, $strobe all display to stdout (I know there are "file" versions of these system tasks, too). The TCL command "value" seems ideal, but how can I access the returned data in Verilog and then return it to C++? Anyway, I'm posting here because I've thought about this for a while and can't come up with an elegant way to do it. But, I'm convinced that there is a way and I'm not seeing it. Thanks, Shirley

  • Hi Shirley.

    I think you need to use some VPI functions to convert the hierarchical path string into a VPI handle to the object (signal). Then there are VPI functions that let you get/set the value of that object.

    It's perfectly OK to call VPI funtions from the DPI, you "just" need to figure out the right functions to call. I was looking for a good worked example but couldn't spot on immediately, sorry. However I suggest you take a look at the examples that ship with IUS, in IUS92/tools.lnx86/examples/vpi/*.

    The NCSim docs also have a whole bunch of info on DPI and VPI, including tutorials and "engineering notebooks" (app notes to use mere mortals) which give a nice cook-book approach to explaining the system.

    Hope this helps.

    Steve.


  • Thank you for the quick response and the pointer to VPI -- I'll take a look at it.  I have used the tutorials and engineering notebooks for DPI, but not for VPI.  I am using vpi_printf already, so it doesn't seem too foreign a concept to use more VPI functions in my environment.

    Shirley

  • I was eventually able to get this to work using VPI.  Here's some of the code from the C++ side:

    # include "vpi_user.h"

    vpiHandle net;              // name of signal to probe
    s_vpi_value cur_val;  // current value of the signal


    net = vpi_handle_by_name("testbench.u1.u2.dout", NULL);  // set the name of the signal you are probing
    vector_size = vpi_get(vpiSize, net);    // returns the size of the signal you are probing
    cur_val.format = vpiVectorVal;             // set the format you want for the returned data
    vpi_get_value(net, &cur_val);              // probe the signal and return the value into cur_val structure

    // The value of the probed net is available in:

    cur_val.value.vector[0].aval   // bits [31:0] of the probed signal
    cur_val.value.vector[0].bval
    cur_val.value.vector[1].aval   // bits [63:32] of the probed signal
    cur_val.value.vector[1].bval
    ... and so on, depending on the vector_size of the net you probed

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