Using wire or reg with input or output in Verilog

Question

When you declare something as input or output, how do you know if you have to also declare it as a reg or a wire?

Answer 1

reg and wire specify how the object will be assigned and are therefore only meaningful for outputs.

If you plan to assign your output in sequential code,such as within an always block, declare it as a reg (which really is a misnomer for "variable" in Verilog). Otherwise, it should be a wire, which is also the default.

Answer 2

An output reg foo is just shorthand for output foo_wire; reg foo; assign foo_wire = foo. It's handy when you plan to register that output anyway. I don't think input reg is meaningful for module (perhaps task). input wire and output wire are the same as input and output: it's just more explicit.

Answer 3

The Verilog code compiler you use will dictate what you have to do. If you use illegal syntax, you will get a compile error.

An output must also be declared as a reg only if it is assigned using a "procedural assignment". For example:

output reg a;
always @* a = b;

There is no need to declare an output as a wire.

There is no need to declare an input as a wire or reg.

Answer 4

seeing it in digital circuit domain

1. A Wire will create a wire output which can only be assigned any input by using assign statement as assign statement creates a port/pin connection and wire can be joined to the port/pin
2. A reg will create a register(D FLIP FLOP ) which gets or recieve inputs on basis of sensitivity list either it can be clock (rising or falling ) or combinational edge .

so it completely depends on your use whether you need to create a register and tick it according to sensitivity list or you want to create a port/pin assignment

Answer 5

basically reg is used to store values.For example if you want a counter(which will count and thus will have some value for each count),we will use a reg. On the other hand,if we just have a plain signal with 2 values 0 and 1,we will declare it as wire.Wire can't hold values.So assigning values to wire leads to problems....