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Explore how to eliminate unwanted latches in your System Verilog multiplication code. We break down the problem, provide a clear solution, and guide you through the implementation for better synthesis results.
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Understanding System Verilog Loops: Fixing Latches in Multiplication Code

In this guide, we will tackle a common issue encountered while coding in System Verilog—specifically, the unintentional creation of latches during synthesis. Our focus will be a Shift-Add Algorithm used for 32x32 bit multiplication.

The Problem

As you develop hardware projects using System Verilog, you may encounter situations where your code synthesizes incorrectly, resulting in new unexpected latch components that you don't want in your design. An example of such a situation arises in the multiplication circuit defined in the following code snippet:

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

According to your synthesis tool (Yosys), the following latches were generated:

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

Latches are typically added when there are variables in the code that are not assigned a new value in all conditional branches. As a result, a latch is created to hold the variable's last value.

Identifying the Issue

In the provided code, several output variables (fin_res, finished_o, etc.) might not be assigned in all branches of the control statement. This can lead to unwanted latches during synthesis. Here are key points to consider:

Conditional Outputs: If you modify an output variable in only one branch of a conditional, and leave it unmodified in another, the synthesis tool creates a latch to hold its value.

State Tracking: The variable rst_flag is set only in STATE_A and is used in STATE_B, making it necessary to retain its value across states—this requires a latch behavior.

Implementing the Solution

Step-by-Step Fix

To eliminate latches, follow these best practices:

Set Default Values: Always assign default values to your variables at the beginning of your always block. This ensures that these variables have defined values irrespective of which state or condition is active.

Reorganize Your Case Statements: Use a well-structured case statement that assigns all necessary values across all conditions.

Here’s a revised version of your code to avoid generating unwanted latches:

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

Important Considerations

While the above changes will help eliminate latches, be aware of a few requirements:

State Management: rst_flag must be allowed to retain its value across states, necessitating some form of latch behavior.

Output Control: Carefully consider the conditions under which finished_o and other outputs should retain their previous values or be set anew. These decisions will impact the overall functionality of your circuit.

Conclusion

In System Verilog coding, especially when implementing algorithms like multiplication, it is crucial to manage your variables carefully to prevent unwanted synthesis artifacts such as latches. By setting default values at the start of your always blocks and reviewing your control structures, you can streamline your designs for better performance.

For further questions or detailed insights into System Verilog, feel free to share your thoughts and experiences in the comments below!