Modport lists with directions are defined in an interface to impose certain restrictions on interface access within a module. The keyword
modport indicates that the directions are declared as if inside the module.
modport [identifier] ( input [port_list], output [port_list] );
Shown below is the definition of an interface myInterface which has a few signals and two
modport declarations. The modport dut0 essentially states that the signals ack and sel are inputs and gnt and irq0 are outputs to whatever module uses this particular modport.
Similarly, another modport called dut1 is declared which states that gnt and irq0 are inputs and the other two are outputs for any module that uses modport dut1.
interface myInterface; logic ack; logic gnt; logic sel; logic irq0; // ack and sel are inputs to the dut0, while gnt and irq0 are outputs modport dut0 ( input ack, sel, output gnt, irq0 ); // ack and sel are outputs from dut1, while gnt and irq0 are inputs modport dut1 ( input gnt, irq0, output ack, sel ); endinterface
Example of named port bundle
In this style, the design will take the required correct modport definition from the interface object as mentioned in its port list. The testbench only needs to provide the whole interface object to the design.
module dut0 ( myinterface.dut0 _if); ... endmodule module dut1 ( myInterface.dut1 _if); ... endmodule module tb; myInterface _if; dut0 d0 ( .* ); dut1 d1 ( .* ); endmodule
Example of connecting port bundle
In this style, the design simply accepts whatever directional information is given to it. Hence testbench is responsible to provide the correct modport values to the design.
module dut0 ( myinterface _if); ... endmodule module dut1 ( myInterface _if); ... endmodule module tb; myInterface _if; dut0 d0 ( ._if (_if.dut0)); dut1 d1 ( ._if (_if.dut1)); endmodule
What is the need for a modport ?
Nets declared within a simple interface is
inout by default and hence any module connected to the same net, can either drive values or take values from it. In simple words, there are no restrictions on direction of value propagation. You could end up with an X on the net because both the testbench and the design are driving two different values to the same interface net. Special care should be taken by the testbench writer to ensure that such a situation does not happen. This can be inherently avoided by the use of modports.
Example of connecting to generic interface
module can also have a generic interface as the portlist. The generic handle can accept any modport passed to it from the hierarchy above.
module dut0 ( interface _if); ... endmodule module dut1 ( interface _if); ... endmodule module tb; myInterface _if; dut0 d0 ( ._if (_if.dut0)); dut1 d1 ( ._if (_if.dut1)); endmodule
Lets consider two modules master and slave connected by a very simple bus structure. Assume that the bus is capable of sending an address and data which the slave is expected to capture and update the information in its internal registers. So the master always has to initiate the transfer and the slave is capable of indicating to the master whether it is ready to accept the data by its sready signal.
Shown below is an
interface definition that is shared between the master and slave modules.
interface ms_if (input clk); logic sready; // Indicates if slave is ready to accept data logic rstn; // Active low reset logic [1:0] addr; // Address logic [7:0] data; // Data modport slave ( input addr, data, rstn, clk, output sready); modport master ( output addr, data, input clk, sready, rstn); endinterface
Assume that the master simply iterates the address from 0 to 3 and sends data equal to the address multiplied by 4. The master should only send when the slave is ready to accept and is indicated by the sready signal.
// This module accepts an interface with modport "master" // Master sends transactions in a pipelined format // CLK 1 2 3 4 5 6 // ADDR A0 A1 A2 A3 A0 A1 // DATA D0 D1 D2 D3 D4 module master ( ms_if.master mif); always @ (posedge mif.clk) begin // If reset is applied, set addr and data to default values if (! mif.rstn) begin mif.addr <= 0; mif.data <= 0; // Else increment addr, and assign data accordingly if slave is ready end else begin // Send new addr and data only if slave is ready if (mif.sready) begin mif.addr <= mif.addr + 1; mif.data <= (mif.addr * 4); // Else maintain current addr and data end else begin mif.addr <= mif.addr; mif.data <= mif.data; end end end endmodule
Assume that the slave accepts data for every addr and assigns them to internal registers. When the address wraps from 3 to 0, the slave requires 1 additional clock to become ready.
module slave (ms_if.slave sif); reg [7:0] reg_a; reg [7:0] reg_b; reg reg_c; reg [3:0] reg_d; reg dly; reg [3:0] addr_dly; always @ (posedge sif.clk) begin if (! sif.rstn) begin addr_dly <= 0; end else begin addr_dly <= sif.addr; end end always @ (posedge sif.clk) begin if (! sif.rstn) begin reg_a <= 0; reg_b <= 0; reg_c <= 0; reg_d <= 0; end else begin case (addr_dly) 0 : reg_a <= sif.data; 1 : reg_b <= sif.data; 2 : reg_c <= sif.data; 3 : reg_d <= sif.data; endcase end end assign sif.sready = ~(sif.addr & sif.addr) | ~dly; always @ (posedge sif.clk) begin if (! sif.rstn) dly <= 1; else dly <= sif.sready; end endmodule
The two design modules are tied together at a top level.
module d_top (ms_if tif); // Pass the "master" modport to master master m0 (tif.master); // Pass the "slave" modport to slave slave s0 (tif.slave); endmodule
The testbench will pass the interface handle to the design, which will then assign master and slave modports to its sub-modules.
module tb; reg clk; always #10 clk = ~clk; ms_if if0 (clk); d_top d0 (if0); // Let the stimulus run for 20 clocks and stop initial begin clk <= 0; if0.rstn <= 0; repeat (5) @ (posedge clk); if0.rstn <= 1; repeat (20) @ (posedge clk); $finish; end endmodule
Remember that the master initiates bus transactions and the slave captures data and stores it in its internal registers reg_* for the corresponding address.