Define register in verilog

Understanding and Using reg in Verilog for Digital Design

Explore the fundamental reg keyword in Verilog, its role in sequential logic, and how it differs from wire for effective hardware description.

In Verilog, a Hardware Description Language (HDL), understanding how to declare and use different data types is crucial for accurately modeling digital circuits. The reg keyword is one of the most fundamental data types, primarily used to represent storage elements within sequential logic blocks. This article will delve into what reg signifies, its characteristics, and how it's employed in Verilog designs, contrasting it with other data types like wire.

What is a reg in Verilog?

The reg (register) data type in Verilog is used to declare variables that can store a value. Unlike wires, which represent continuous connections and cannot store values, regs are capable of holding their value until a new value is assigned. This characteristic makes them ideal for modeling memory elements such as flip-flops, latches, and registers in sequential circuits.

When a reg is declared, it doesn't inherently imply a physical register in the synthesized hardware. Instead, it indicates that the variable will be assigned a value within a procedural block (like always or initial). The synthesis tool then infers the appropriate hardware (e.g., flip-flop, latch, or combinational logic) based on the assignment behavior within that block.

module simple_register (
  input wire clk,
  input wire reset,
  input wire data_in,
  output reg data_out
);

  always @(posedge clk or posedge reset) begin
    if (reset) begin
      data_out <= 1'b0; // Asynchronous reset
    end else begin
      data_out <= data_in; // Data transfer on clock edge
    end
  end

endmodule

Example of a D-flip-flop using reg for data_out

Key Characteristics of reg

Understanding the properties of reg is essential for correct Verilog coding:

• Value Storage: reg variables can hold a value. They retain their last assigned value until explicitly updated.
• Procedural Assignment: reg variables can only be assigned values within procedural blocks (always, initial, task, function). They cannot be assigned values using continuous assignments (assign).
• Bit-width: reg can be declared as single bits or as vectors (e.g., reg [7:0] my_byte;).
• Initialization: reg variables are typically initialized to an unknown (x) state at the beginning of simulation unless explicitly initialized in an initial block or reset logic.
• Synthesizable vs. Non-synthesizable: While reg is used for both, its usage within always @(posedge clk) blocks often leads to synthesizable flip-flops, whereas regs used in initial blocks or for simulation-only purposes might not synthesize to physical hardware.

flowchart TD
    A[Declare `reg` variable] --> B{Assigned in procedural block?}
    B -- Yes --> C{Assignment type?}
    C -- Blocking (`=`) --> D[Combinational logic or Latch]
    C -- Non-blocking (`<=`) --> E[Sequential logic (Flip-flop)]
    B -- No --> F[Error: `reg` cannot be continuously assigned]
    D --> G[Synthesized Hardware]
    E --> G

Decision flow for reg synthesis inference

Distinguishing reg from wire

The distinction between reg and wire is a common point of confusion for Verilog newcomers. Here's a clear breakdown:

• wire: Represents a physical connection, like a wire on a breadboard. It does not store a value; its output is continuously driven by its input. wires are assigned using continuous assignments (assign) or by the output of a module instance.
• reg: Represents a storage element. It holds its value until a new value is assigned within a procedural block. It's used to model memory elements or variables within procedural logic.

In essence, if a signal's value is determined by a combinational logic expression or the output of another module, it's likely a wire. If a signal needs to retain its value over time, especially in response to a clock edge or an event, it's a reg.

module wire_vs_reg (
  input wire a,
  input wire b,
  input wire clk,
  output wire sum_wire,
  output reg sum_reg
);

  // Continuous assignment for wire
  assign sum_wire = a ^ b; // sum_wire continuously reflects a XOR b

  // Procedural assignment for reg
  always @(posedge clk) begin
    sum_reg <= a + b; // sum_reg updates only on clock edge
  end

endmodule

Comparing wire (combinational) and reg (sequential) assignments