Verilog Always Block: The Heart of Sequential Logic

The always block is a fundamental building block in Verilog for describing sequential logic. Unlike the assign statement, which defines combinational logic, the always block allows you to implement sequential behavior where the output depends not only on the current input values but also on the state of the system in previous simulation cycles.

Key aspects of always blocks:

• Sequential execution: Statements inside an always block are executed sequentially, one after the other. This allows you to model complex state machines and control the flow of your design.
• Sensitivity list: An optional sensitivity list specifies the signals whose changes trigger the execution of the always block. This ensures that the logic inside the block reacts only to relevant changes in the input values.
• Edge-triggered vs. level-triggered: You can specify the triggering behavior using keywords like posedge, negedge, or level within the sensitivity list. This determines whether the block is triggered by the rising or falling edge of a specific signal or by any change in its level.
• State registers: You can use variables of data types like reg or integer within the always block to store state information and implement memory elements. These variables retain their values between simulation cycles, allowing you to model sequential behavior.
• Wide range of applications: Always blocks are used to implement various functionalities like state machines, counters, timers, FSMs, and control logic.

Examples:

1. Basic D Flip-Flop:

always @(posedge clk) begin
  if (rst) begin
    q <= 1'b0;
  end else begin
    q <= d;
  end
end

This always block implements a D flip-flop where the output q is assigned the value of the input d on the rising edge of the clock clk. The rst signal resets the flip-flop to 0.

2. Counter with enable:

reg [3:0] count;

always @(posedge clk or posedge rst) begin
  if (rst) begin
    count <= 4'd0;
  end else if (enable) begin
    if (count == 4'd15) begin
      count <= 4'd0;
    end else begin
      count <= count + 1;
    end
  end
end

This always block implements a four-bit counter with an enable signal. The counter increments on the rising edge of the clock clk if the enable signal is high. It resets to 0 on the rising edge of the reset signal rst.

3. FSM with multiple states:

reg [2:0] state;

always @(posedge clk or posedge rst) begin
  if (rst) begin
    state <= 3'b000; // Initial state
  end else begin
    case (state)
      3'b000: begin
        state <= 3'b001;
      end
      3'b001: begin
        if (condition) begin
          state <= 3'b010;
        end
      end
      3'b010: begin
        // ... further state transitions ...
      end
    endcase
  end
end

This always block implements a finite state machine (FSM) with three states. The state variable state transitions between different states based on the conditions and input signals. This demonstrates the flexibility of always blocks in modeling complex sequential behavior.

Benefits of using always blocks:

• Structured approach: Always blocks provide a structured way of describing sequential logic, making your code more organized and maintainable.
• Flexibility: They offer a versatile tool for implementing a wide range of functionalities, from simple state elements to complex control systems.
• Modularization: You can combine multiple always blocks to build larger and more complex designs, promoting modularity and code reuse.

Combinational Logic with Always Block in Verilog

In Verilog, combinational logic refers to circuits whose output depends solely on the current values of their inputs. While the assign statement is the primary tool for defining combinational logic, the always block can also be used in certain situations. Here’s an overview of using always blocks for combinational logic:

When to Use Always Block for Combinational Logic:

• Multiple outputs: If your combinational logic needs to generate multiple outputs based on a single set of inputs, an always block can be more concise and organized than using multiple assign statements.
• Complex expressions: If your combinational logic involves complex expressions with nested functions or conditional statements, an always block can improve code readability and maintainability.
• Mixing combinational and sequential logic: An always block can be particularly useful when you need to combine combinational logic with sequential logic in the same block.

Example 1: Multi-bit AND operation:

always @(a or b) begin
  out = a & b;
end

This always block performs a bitwise AND operation on two signals a and b and assigns the result to the output signal out. This is a simple example where an assign statement would be equally effective.

Example 2: Multiplexer with case statement:

always @(sel or data0 or data1) begin
  case (sel)
    2'b00: out = data0;
    2'b01: out = data1;
    default: out = 4'bx;
  endcase
end

This always block implements a 2-to-1 multiplexer based on the select signal sel. It uses a case statement to choose one of the input signals data0 or data1 based on the value of sel. This example showcases the advantage of using an always block for complex logic with conditional statements.

Example 3: Mixing combinational and sequential logic:

always @(posedge clk or posedge rst) begin
  if (rst) begin
    count <= 4'd0;
  end else begin
    count <= count + 1'b1;
  end
end

always @(count) begin
  out = count & data;
end

This code snippet first defines a counter using an always block triggered by the clock and reset signals. The second block uses the counter value as an input to implement combinational logic with the data signal. This demonstrates how always blocks can be used to combine both sequential and combinational logic within the same design.

Benefits of Using Always Block for Combinational Logic:

• Improved readability: For complex logic, always blocks can lead to more concise and organized code compared to multiple assign statements.
• Flexibility: They offer the ability to combine combinational logic with sequential logic within the same block, simplifying design structure.
• Maintainability: Easier to understand and modify the logic due to a more structured and organized code.

Drawbacks of Using Always Block for Combinational Logic:

• Synthesis overhead: Always blocks may introduce additional logic gates during synthesis compared to using assign statements.
• Simulation performance: Combinational logic defined in always blocks may lead to slightly slower simulation performance due to sequential execution.

Conclusion:

While the assign statement remains the primary tool for defining combinational logic, always blocks offer a valuable alternative in specific situations. Understanding when and how to use always blocks effectively can enhance your Verilog design skills and lead to more efficient and maintainable digital circuits.

always @(posedge clk) begin
  if (rst) q <= 1'b0;
  else q <= d;
end

reg [3:0] count;

always @(posedge clk) begin
  if (rst) count <= 4'b0;
  else if (enable) count <= count + 1'b1;
end

reg [2:0] state;

always @(posedge clk or posedge rst) begin
  if (rst) state <= 3'b000; // Initial state
  else begin
    case (state)
      3'b000: if (start) state <= 3'b001; // Transition based on condition
      3'b001: // ... further state transitions and behavior ...
    endcase
  end
end