Interfaces provide a way to specify communication between blocks. An interface is a group of nets and variables that are grouped together for the purpose of making connections between modules easier to write. The syntax for a basic interface is:
interface interface_name;
parameters and ports;
items;
endinterface : interface_name
The interface_name at the end is optional but makes the code easier to read. For an example, see the following code:
module bottom1 (
input clk,
input [9:0] d1,d2,
input s1,
input [9:0] result,
output logic sel,
output logic [9:0] data1, data2,
output logic equal);
//logic//
endmodule
module bottom2 (
input clk,
input sel,
input [9:0] data1, data2,
output logic [9:0] result);
//logic//
endmodule
module top (
input clk,
input s1,
input [9:0] d1, d2,
output equal);
logic [9:0] data1, data2, result;
logic sel;
bottom1 u0 (clk, d1, d2, s1, result, sel, data1, data2, equal);
bottom2 u1 (clk, sel, data1, data2, result);
endmodule
The previous code snippet instantiates two lower-level modules with some signals that are common to both.
These common signals can all be specified with an interface:
interface my_int
logic sel;
logic [9:0] data1, data2, result;
endinterface : my_int
Then, in the two bottom-level modules, you can change to:
module bottom1 (
my_int int1,
input clk,
input [9:0] d1, d2,
input s1,
output logic equal);
and
module bottom2 (
my_int int1,
input clk);
Inside the modules, you can also change how you access sel, data1, data2, and result. This is because, according to the module, there are no ports of these names. Instead, there is a port called my_int. This requires the following change:
if (sel)
result <= data1;
to:
if (int1.sel)
int1.result <= int1.data1;
Finally, in the top-level module, the interface must be instantiated, and the instances reference the interface:
module top(
input clk,
input s1,
input [9:0] d1, d2,
output equal);
my_int int3(); //instantiation
bottom1 u0 (int3, clk, d1, d2, s1, equal);
bottom2 u1 (int3, clk);
endmodule