blocking vs nonblocking verilog

Guri Bee

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22-10-2024

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Blocking vs. Non-Blocking Assignments in Verilog: A Clear Explanation

Verilog, a hardware description language, offers two primary assignment operators: blocking (=) and non-blocking (<=). Understanding the difference between these operators is crucial for writing efficient and accurate Verilog code, especially when modeling complex digital circuits.

Blocking Assignments (=)

What is it? Blocking assignments execute sequentially within a single time step. This means the right-hand side of the assignment is evaluated and then the result is assigned to the left-hand side variable. If multiple blocking assignments are used in a single procedural block, they are executed one after another in the order they appear.

Example:

always @(posedge clk) begin
  a = b; // Blocking assignment
  b = c; // Blocking assignment
end 

In this example, b is assigned the value of c after a is assigned the value of b. This sequential execution is similar to how traditional programming languages work.

When to use it?

• Sequential code: When the order of assignments matters, like in arithmetic operations where the result of one assignment depends on the previous one.
• Simple assignments: For straightforward assignments where there is no dependence on other variables.

Non-Blocking Assignments (<=)

What is it? Non-blocking assignments execute concurrently within a single time step. This means that all the right-hand side expressions are evaluated first, and then the values are assigned to the left-hand side variables simultaneously at the end of the time step.

Example:

always @(posedge clk) begin
  a <= b; // Non-blocking assignment
  b <= c; // Non-blocking assignment
end

In this example, both a and b are assigned their values simultaneously. The values of b and c are read at the beginning of the time step, and the assignments happen at the end of the time step.

When to use it?

• Concurrent code: When modeling concurrent behavior like a flip-flop where the output depends on the input from the previous clock cycle.
• Combinational logic: When the output of a block of code depends on multiple inputs, and all those inputs need to be considered at the same time.

Important Considerations:

• Timing: Non-blocking assignments are better suited for simulating the behavior of real hardware where the output is updated after a certain delay.
• Dataflow: Non-blocking assignments represent dataflow, where the values are assigned to the target variables based on the values of other variables at a particular point in time.
• Combinational Logic: While blocking assignments can be used for combinational logic, they can create race conditions if the order of assignments is not carefully managed. Non-blocking assignments avoid this issue.

Example scenario:

Imagine a simple flip-flop circuit. The output of the flip-flop should be updated only when the clock edge arrives. Using a non-blocking assignment ensures that the output is updated only at the end of the time step when the clock edge occurs.

Here is an example of a D flip-flop implemented using a non-blocking assignment:

always @(posedge clk) begin
  q <= d; // Non-blocking assignment to update the output 'q'
end

Choosing the right assignment:

The choice between blocking and non-blocking assignments depends on the specific behavior you want to model.

• Blocking assignments are ideal for sequential operations and simple assignments.
• Non-blocking assignments are preferred for concurrent operations, modeling dataflow, and representing real hardware behavior.

Additional Information:

• You can find a detailed discussion on blocking and non-blocking assignments in the Verilog language reference manual.
• Numerous online resources like Stack Overflow and Verilog.com provide in-depth explanations and examples.

By understanding the differences between blocking and non-blocking assignments, you can write accurate and efficient Verilog code that effectively represents your hardware design.