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19.8.2 Virtual interfaces modports and clocking blocks</div>
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As shown in the example above, once a virtual interface is declared, its clocking block can be referenced using dot-notation. However, this only works for interfaces with no modports. Typically, a device under test and its testbench exhibit modport direction. This common case can be handled by including the clocking in the corresponding modport as described in Section 19.4.5. </p>
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The example below shows how modports used in conjunction with virtual interfaces facilitate the creation of abstract synchronous models. </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00858"><pre class="Element12">interface A_Bus( input bit clk );
wire req, gnt;
wire [7:0] addr, data;
clocking sb @(posedge clk);
input gnt;
output req, addr;
inout data;
property p1;
req ##[1:3] gnt;
endproperty
endclocking
modport DUT ( input clk, req, addr, // Device under test modport
output gnt,
inout data );
modport STB ( clocking sb ); // synchronous testbench modport
modport TB ( input gnt, // asynchronous testbench modport
output req, addr,
inout data );
endinterface</pre></div></div>
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The above interface A_Bus can then be instantiated as shown below: </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00859"><pre class="Element12">module dev1(A_Bus.DUT b); // Some device: Part of the design
...
endmodule
module dev2(A_Bus.DUT b); // Some device: Part of the design
...
endmodule
program T (A_Bus.STB b1, A_Bus.STB b2 ); // Testbench: 2 synchronous ports
...
endprogram
module top;
bit clk;
A_Bus b1( clk );
A_Bus b2( clk );
dev1 d1( b1 );
dev2 d2( b2 );
T tb( b1, b2 );
endmodule</pre></div></div>
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And, within the testbench program, the virtual interface can refer directly to the clocking block. </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00860"><pre class="Element12">program T (A_Bus.STB b1, A_Bus.STB b2 ); // Testbench: 2 synchronous ports
typedef virtual A_Bus.STB SYNCTB;
task request( SYNCTB s );
s.sb.req <= 1;
endtask
task wait_grant( SYNCTB s );
wait( s.sb.gnt == 1 );
endtask
task drive(SYNCTB s, logic [7:0] adr, data );
if( s.sb.gnt == 0 ) begin
request(s); // acquire bus if needed
wait_grant(s);
end
s.sb.addr = adr;
s.sb.data = data;
repeat(2) @s.sb;
s.sb.req = 0; //release bus
endtask
assert property (b1.p1); // assert property from within program
initial begin
drive( b1, $random, $random );
drive( b2, $random, $random );
end
endprogram</pre></div></div>
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The example above shows how the clocking block is referenced via the virtual interface by the tasks within the program block.</p></div>
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