Understanding Mixed Non-blocking and Blocking Assignments in Verilog

Dive into the differences between mixed non-blocking and blocking assignments in Verilog. Learn how they affect simulation behavior and understand their applications in hardware design.
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Understanding Mixed Non-blocking and Blocking Assignments in Verilog

Verilog is a hardware description language (HDL) widely used in the design and modeling of electronic systems. One common area of confusion for many designers is the implementation of non-blocking and blocking assignments in Verilog code.

In this guide, we will explore a question regarding the use of mixed non-blocking and blocking assignments by examining a specific example. We’ll break down the solution into manageable sections to clarify how these assignments operate during simulation and how they impact the intended design behavior.

The Problem: Mixed Assignments

The question stems from a code snippet where the always block is split into two separate blocks: one for combinatorial logic and another for sequential logic. Here’s the original code snippet:

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When split into two, the code becomes:

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The concern raised is about the order of execution of the blocks and the potential for q to be assigned an outdated value of y.

Solution Breakdown

Understanding Blocking vs. Non-Blocking Assignments

Before diving in further, it's essential to understand what blocking and non-blocking assignments are:

Blocking Assignments (=): These are executed in the order they appear in the code, meaning the next statement will not execute until the current one is completed.

Non-blocking Assignments (<=): These allow the simulation to evaluate the statement but do not block the execution of subsequent statements.

Event-Driven Simulation

Verilog operates on an event-driven simulator model. It evaluates assignments in distinct buckets of simulation time. Key points to note are:

Each time a signal changes, the simulator checks the event-driven buckets.

Both the blocking and non-blocking assignments exist in these buckets, with blocking assignments executed first and non-blocking assignments executed subsequently.

Analyzing the Execution Order

Now, let’s analyze the execution based on the concerns raised about the potential issues in the split implementation:

Non-blocking Assignment for q:

When the clock edge occurs, and a new value for b is present, the bottom always block attempts to assign q the current value of y (which is a non-blocking assignment).

Blocking Assignment for y:

The top always block computes y with a ^ b. This will always occur before the non-blocking assignment for q executes because blocking assignments are processed in the first bucket of activity.

Clarifying the Flaw in Understanding

The initial worry was that the split version might assign q an outdated value of y. However:

The actual sequence ensures the new value of y computed from a and b will always be used for the update of q, thanks to the timing of the execution in their respective simulation buckets.

Conclusion: Best Practices

To avoid confusion in your designs:

Never mix blocking and non-blocking assignments in the same always block. This practice can lead to unpredictable behavior and confusion about which value is assigned.

Understand the nature of event-driven simulation and how blocking and non-blocking assignments affect the state and timing of your signals.

With these insights, you'll be better equipped to manage your Verilog designs with a clearer understanding of mixed assignments and their impacts on the hardware description.

Feel free to explore further into other coding styles and best practices to solidify your knowledge in Verilog design!

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