What is the need of virtual interfaces ?

Virtual interfaces provide a mechanism for separating abstract models and test programs from the actual signals that make up the design. A virtual interface allows the same subprogram to operate on different portions of a design and to dynamically control the set of signals associated with the subprogram. Instead of referring to the actual set of signals directly, users are able to manipulate a set of virtual signals. Changes to the underlying design do not require the code using virtual interfaces to be rewritten. By abstracting the connectivity and functionality of a set of blocks, virtual interfaces promote code reuse.

Virtual interfaces can be declared as class properties, which can be initialized procedurally or by an argument to new(). This allows the same virtual interface to be used in different classes. The following example shows how the same transactor class can be used to interact with various different devices:

interface SBus; // A Simple bus interface
logic req, grant;
logic [7:0] addr, data;
endinterface

class SBusTransctor; // SBus transactor class
  virtual SBus bus; // virtual interface of type Sbus
  function new( virtual SBus s );
    bus = s; // initialize the virtual interface
  endfunction

  task request(); // request the bus
    bus.req <= 1'b1;
  endtask

  task wait_for_bus(); // wait for the bus to be granted
    @(posedge bus.grant);
  endtask
endclass

module devA( Sbus s ) ... endmodule // devices that use SBus
module devB( Sbus s ) ... endmodule

module top;
SBus s[1:4] (); // instantiate 4 interfaces

devA a1( s[1] ); // instantiate 4 devices
devB b1( s[2] );
devA a2( s[3] );
devB b2( s[4] );
initial begin

SbusTransactor t[1:4]; // create 4 bus-transactors and bind
t[1] = new( s[1] );
t[2] = new( s[2] );
t[3] = new( s[3] );
t[4] = new( s[4] );
// test t[1:4]

end
endmodule

 

In the preceding example, the transaction class SbusTransctor is a simple reusable component. It is written without any global or hierarchical references and is unaware of the particular device with which it will interact. Nevertheless, the class can interact with any number of devices (four in the example) that adhere to the interface’s protocol.

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