Скачать или смотреть How to Effectively Handle 64-Bit Addition with Carry-Out in Verilog Adder Module

Learn how to implement a 64-bit adder module in Verilog that correctly handles carry-out from addition operations. A guide for FPGA developers.
---
This video is based on the question https://stackoverflow.com/q/66606021/ asked by the user 'RhinoECE' ( https://stackoverflow.com/u/15204047/ ) and on the answer https://stackoverflow.com/a/66606184/ provided by the user 'toolic' ( https://stackoverflow.com/u/197758/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Behavioral module for Adder that adds two 64-bit inputs and carry in Input. How to assign carry-out to MSB of the sum?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding the 64-Bit Adder with Carry-Out in Verilog

When working with digital circuits and hardware description languages like Verilog, designing an adder that can handle very large inputs such as 64-bit numbers can pose unique challenges. One common problem encountered is managing the carry-out during addition. In this guide, we'll break down how to create a simple yet effective behavioral module that adds two 64-bit inputs along with a carry-in input, correctly managing carry-out using Verilog.

The Problem

You may want to add two 64-bit inputs, represented as A and B, along with a 1-bit carry-in (cin). This addition could potentially result in a value that exceeds 64 bits, necessitating an effective way to separate the sum from the carry-out. The expected outcome is:

A sum (the result) that is 64 bits.

A carry-out (cout) that captures the most significant bit of the actual result, which is 65 bits in the case of an overflow.

However, many programmers face errors when attempting to assign cout within procedural blocks in their original implementations.

The Solution

The original code attempts to calculate the addition but leads to compile errors due to the incorrect assignment of the cout within an always block. Here's a breakdown of the most effective way to handle this in Verilog, as well as how to rectify potential issues from the original design.

Original Code Review

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

Issues Identified:

The cout is defined as a wire, but it's assigned within an always block, which is not allowable for wire types.

Use of nonblocking assignments (<=) for combinational logic, while blocking assignments (=) are generally preferred for clarity in straightforward combinational designs.

Simplified Implementation

Now, let’s address how to correctly implement this by using continuous assignment, which will eliminate complications with procedural blocks and make the design cleaner. Here’s a more optimal version of the code:

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

Explanation of the Simplified Code

Input and Output Definitions: The module takes two 64-bit inputs and one 1-bit carry-in, returning a 64-bit sum and a carry-out.

Continuous Assignment: The assign statement is used to perform the addition. It bundles the carry-out and sum together directly, allowing the implementation to handle overflow elegantly. The curly braces {} enable the result to be assigned in a way that captures both the carry-out bit and the 64-bit sum in one step.

Advantages of This Approach

Simplicity: This design is intuitive and easier to understand for anyone looking at the code.

Efficiency: It performs the addition in a single line, maintaining efficient resource usage in FPGA designs.

Error Reduction: Avoids potential errors associated with procedural assignments altogether.

Conclusion

Creating a behavioral Adder module in Verilog that adds two 64-bit numbers and manages carry-out correctly involves understanding the basic principles of combinational logic and appropriate data assignment. By utilizing the streamlined approach outlined, you can successfully implement effective 64-bit additions suitable for FPGA designs.

Now that you're equipped to handle 64-bit additions in Verilog, you'll be better prepared to tackle larger digital design challenges effectively! Feel free to ask questions or leave comments if you have any issues or want to discuss further improvements.