Скачать или смотреть How to Correctly Implement a 2^n to n Priority Encoder in Verilog

Discover how to fix issues in your `2^n` to `n` priority encoder code in Verilog. Learn about proper assignment in generate blocks and utilize functions effectively.
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This video is based on the question https://stackoverflow.com/q/64407297/ asked by the user 'blindside044' ( https://stackoverflow.com/u/5156790/ ) and on the answer https://stackoverflow.com/a/64408811/ provided by the user 'Serge' ( https://stackoverflow.com/u/1143850/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Fixing Your 2^n to n Priority Encoder in Verilog

If you're working on digital design using Verilog, you might run into problems while developing a priority encoder. Specifically, you may encounter issues when implementing a 2^n to n priority encoder with continuous assignments. This guide will help you understand common issues with your implementation and guide you on how to correct them.

The Problem

You might start with a code snippet that is supposed to convert 2^n inputs to an n output value, but when it doesn't work as expected in your test bench, confusion can arise. Here’s the main issue you’re facing when running a test bench for the encoder:

Inconsistent Output

The output from your encoder does not match the expected results. Instead of correctly identifying the highest priority signal in your inputs, your encoder seems to return incorrect values based on your test inputs. Here’s a brief recap of your output issues:

Input with value 0000000000000001 should output 1, but it outputs 0.

Input with value 0000000000000100 should output 2, but again it returns 0.

The Solution

To solve the problem of your priority encoder not functioning correctly, we will address two main areas: generate blocks and function implementation.

Understanding the Generate Block Issue

Static Evaluation: The generate block in Verilog is evaluated at a pre-processing step, meaning it cannot condition dynamically based on variable input.

Direct Assignments: The line causing the issue, assign encoder_out[i] = (encoder_in[i]) ? i : 0;, is not setting up the encoder’s output correctly. Each encoder_out[i] will always be evaluated without a proper condition across the input.

Suggested Code Adjustments

Instead of using a generate block for encoding, consider using an algorithmic approach that utilizes functions. Here's how you can adjust your code to make it more effective:

Use a Function

Using a function allows you to iterate through the input values and identify the highest bit set. Here's an example of how you can implement this in your design:

[[See Video to Reveal this Text or Code Snippet]]

Testing the Correct Implementation

Once you have made these adjustments, run your test bench again. You should now see results consistent with the expected outcomes:

0000000000000000 outputs 0

0000000000000001 outputs 1

0000000000000100 outputs 2

This approach not only resolves the immediate issue but also improves the readability of your code.

Conclusion

By understanding the limitations of generate blocks in Verilog and effectively utilizing functions, you can overcome the challenges presented when building a 2^n to n priority encoder. This correction will ensure that your encoder works correctly across all input cases and provides accurate outputs in your digital designs.

Feel free to explore these coding strategies further and enhance your Verilog expertise. Happy coding!