Question

This interface example shows how to use modports to control signal directions. It shows the use of the interface keyword in the module definition. The actual interface and modport are specified in the module instantiation.

interface simple_bus (input bit clk); // Define the interface
    logic req, gnt;
    logic [7:0] addr, data;
    logic [1:0] mode;
    logic start, rdy;
    modport slave (input req, addr, mode, start, clk,
                   output gnt, rdy,
                   ref data);
    modport master(input gnt, rdy, clk,
                   output req, addr, mode, start,
                   ref data);
endinterface: simple_bus

module memMod(interface a); // Uses just the interface
    logic avail;
    always @(posedge a.clk) // the clk signal from the interface
        a.gnt <= a.req & avail; // the gnt and req signal in the interface
endmodule

module cpuMod(interface b);
    ...
endmodule

module top;
    logic clk = 0;
    simple_bus sb_intf(clk); // Instantiate the interface
    memMod mem(sb_intf.slave); // Connect the modport to the module instance
    cpuMod cpu(sb_intf.master);
endmodule