Verilog Procedural Interface (VPI) The Verilog Procedural Interface (VPI) is a C-programming interface for the Verilog Hardware Description Language (HDL). VPI is the third-generation procedural interface to Verilog HDL.

 VPI provides consistent, object-oriented access to the complete Verilog HDL.

VPI consists of a set of access and utility routines that you call from standard C programming language functions. These routines interact with the instantiated simulation objects contained in your Verilog HDL design.
VPI is the same as Verilog PLI 1.0 and PLI 2.0, so if something is working in PLI 2.0, it can be easily ported to VPI. VPI is cleaner than earlier PLI 2.0. All the functions and tasks start with vpi_*. Example vpi_printf().

Steps : Writing Application Using VPI
Following steps need to be performed to write a C application and interface with Verilog simulator.

• Writing a C function
• Associating C Functions with a New System Task
• Registering New System Tasks
• Invoking System Tasks

Below is an example code of a simple C function:

1 #include "vpi_user.h"

 2

 3 void hello() {

 4   vpi_printf("\n\nHello Deepak\n\n");

 5 }

Associating C Functions with a New System Task

To associate your C function with a system task, create a data structure of type s_vpi_systf_data and a pointer to that structure. The vpi_systf_data data type is defined in the vpi_user.h include file. Below is the data structure of s_vpi_systf_data.

1 typedef struct t_vpi_systf_data {

 2   PLI_INT32 type;     // vpiSysTask, vpiSysFunc 

 3   PLI_INT32 sysfunctype; // vpiSysTask, vpi[Int,Real,Time,Sized, SizedSigned]Func 

 4   PLI_BYTE8 *tfname;  // First character must be `$' 

 5   PLI_INT32 (*calltf)(PLI_BYTE8 *);

 6   PLI_INT32 (*compiletf)(PLI_BYTE8 *);

 7   PLI_INT32 (*sizetf)(PLI_BYTE8 *); // For sized function callbacks only 

 8   PLI_BYTE8 *user_data;

 9 } s_vpi_systf_data, *p_vpi_systf_data;

Example for hello routine

 1 #include "hello_vpi.c"

  2 

  3 // Associate C Function with a New System Task

  4 void registerHelloSystfs() {

  5   s_vpi_systf_data task_data_s;

  6   p_vpi_systf_data task_data_p = &task_data_s;

  7   task_data_p->type = vpiSysTask;

  8   task_data_p->tfname = "$hello";

  9   task_data_p->calltf = hello;

 10   task_data_p->compiletf = 0;

 11 

 12   vpi_register_systf(task_data_p);

 13 }

 14 

 15 // Register the new system task here

 16 void (*vlog_startup_routines[ ] ) () = {

 17    registerHelloSystfs,

 18    0  // last entry must be 0 

 19 }; 

Invoking System Tasks

Like PLI1.0 and PLI2.0, each simulator has its own way of linking VPI routines into simulator. We will see following simulators as examples:

• VCS
• Modelsim
• NCSim

VCS

With VCS simulator you need to create a tab file. For our example the tab file looks like:

Modelsim

Like VCS, modelsim simulator has its own way for communicating with PLI. We need to create a function listing all user defined functions that will be referred in Verilog and the corresponding C function to call. Unlike VCS, we need to do it in a C file as shown below. We discussed it already in "Associating C Functions with a New System Task".

  1 #include "hello_vpi.c"

  2 

  3 // Associate C Function with a New System Task

  4 void registerHelloSystfs() {

  5   s_vpi_systf_data task_data_s;

  6   p_vpi_systf_data task_data_p = &task_data_s;

  7   task_data_p->type = vpiSysTask;

  8   task_data_p->tfname = "$hello";

  9   task_data_p->calltf = hello;

 10   task_data_p->compiletf = 0;

 11 

 12   vpi_register_systf(task_data_p);

 13 }

 14 

 15 // Register the new system task here

 16 void (*vlog_startup_routines[ ] ) () = {

 17    registerHelloSystfs,

 18    0  // last entry must be 0 

 19 };