Скачать или смотреть How to Properly Set a Signal at Both posedge and negedge of a Clock in Verilog

Discover how to implement a controller in Verilog that manages clock signals effectively without synthesis errors. Learn about using a gated clock approach for better results!
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This video is based on the question https://stackoverflow.com/q/63274426/ asked by the user 'Tommytml' ( https://stackoverflow.com/u/13972656/ ) and on the answer https://stackoverflow.com/a/63284458/ 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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Understanding the Problem: Clock Signal Management in Verilog

When designing digital circuits, especially in environments like Xilinx ISE, managing clock signals can become one of the more challenging aspects of ensuring that your controller behaves as expected. In this guide, we tackle a common issue: how to set a signal at both the posedge (positive edge) and negedge (negative edge) of a clock input while avoiding synthesis errors.

The Original Scenario

Imagine you are developing a controller that is meant to send out the same clock signal it receives as input. However, this controller needs to have the flexibility to halt the output signal when required. The intention was to:

Set clk_out to 0 on the negedge of clk_in.

Set clk_out to 1 on the posedge of clk_in unless the output is to be halted, in which case it remains 0.

Here's a brief look at the problematic design you might have implemented:

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

Upon synthesis, an error indicates that clk_out is driven by multiple sources. Let’s dive into resolving this issue.

The Solution: Avoiding Multiple Drivers

The synthesis error occurs because you have two different always blocks trying to drive the same clk_out signal. In Verilog, an output signal must only be driven from a single source to maintain clarity and prevent conflicts during synthesis.

Recommended Approach: Using a Gated Clock

Instead of trying to control the clock signal based on both rising and falling edges through separate blocks, consider using a gated clock approach. This simplifies the design considerably. Here’s how you can implement this:

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

Steps to Implement the Solution

Define Enable Logic:

You need to determine how enable is generated. This signals whether the output clock should be active (1) or inactive (0).

Combine Control Logic:

Integrate any logic needed to set or clear enable based on your specific design requirements, ensuring it's clear and concise.

Avoid Race Conditions:

As a best practice, never use non-blocking assignments (<=) when generating clock signals to prevent potential clock/data race conditions.

Conclusion

By using a gated clock approach, you can effectively manage clock signals in your Verilog designs without running into synthesis errors due to multiple drivers. This method not only simplifies the control of your output signal but also aligns with best practices in digital design.

Feel free to adapt this strategy for your specific controller needs and explore how it can improve the reliability of your designs in environments like Xilinx ISE!

If you have further questions or need assistance with your Verilog designs, feel free to reach out in the comments below!