Verilog: proper way of connecting ports

Question

Assume there are two different modules (first_module, second_module). Both of the modules are synchronized with clock signal. first_module has the following structure:

module first_module(
input clk,
input reset_n,
input in1,
output reg out1,
output reg out2
);

//******** some verilog codes *********

endmodule

And second_module has similar structure:

module second_module(
input clk,
input reset_n,
input in1,
input in2,
output reg out1
);

//******** some verilog codes *********

endmodule

And then there is a module called top_module which uses instances of both modules:

module top_module(
input clk,
input reset_n,
input insignal1,
input insignal2,
output outsignal1,
output outsignal2
);

first_module fm1(
  .clk(clk),
  .reset_n(reset_n),
  .in1(insignal1),
  .out1(outsignal1),
  .out2(<connection1>) // to be connected to the connection2 
);

second_module sm1(
  .clk(clk),
  .reset_n(reset_n),
  .in1(insignal2),
  .in2(<connection2>), // to be connected to the connection1
  .out1(outsignal2)
);    

endmodule

The aim is to connect connection1 to connection2. According to my knowledge (if it is correct), we can either declare a single wire (let its name be connection) and replace both <connection1> and <connection2> with it, or we can declare two distinct wires connection1 and connection2, then:

assign connection2 = connection1;

And connect them accordingly.

Are those two methods synthesized differently? If the answer is yes, I would be glad if you could explain how they are synthesized.
If the answer is no, can one of the methods be better than the other in different conditions? Not in terms of lines of code or simplicity, but in terms of synthesis.

Oldfart Oldfart · Accepted Answer · 2019-05-24T07:53:39

Yes there is a difference. But not in your specific case.

Using a connection directly makes that is can be uni-directional or bi-directional depending on what the underlying ports in the module are.

But assign connection2 = connection1; is only uni-directional.

Thus between bi-directional ports should use direct connections or you should only use bi-directional Verilog constructs between them. The assign ... is not one of them.

But in you case the signal is uni-directional so it does not matter.

Note that modern FPGAs no longer have on-chip bi-directional buses. (At least I don't know one that has). Also in chip design on-chip buses are strongly discourage or outright forbidden by the manufacturers.
Therefore bi-directional signals are normally only present in the test-bench. As that does not get synthesized your question does not apply there.

Last but not least: In HDL design I would strongly discourage from changing the name of a signal for no apparent reason. Having the same name throughout the design makes it easier to debug and trace your signals post-synthesis.

Verilog: proper way of connecting ports

1 Answers