HDL simulation and synthesis tools are essential for electronics hardware design, but they can also produce errors that can affect the functionality and performance of your design. In this article, we will discuss some of the common HDL synthesis errors and how to fix them.
Syntax errors are the most basic and easy to spot errors in your HDL code. They occur when you use incorrect or missing keywords, punctuation, operators, or identifiers. Syntax errors can prevent your code from being compiled or simulated, and they can usually be fixed by correcting the spelling, grammar, or syntax of your code. For example, if you forget to end a statement with a semicolon or a block with an end keyword, you can add them to fix the syntax error.
Logic errors are more subtle and difficult to detect errors in your HDL code. They occur when your code does not behave as you intended or expected, or when it produces incorrect or unexpected results. Logic errors can cause your design to fail or malfunction, and they can be caused by various factors, such as incorrect assignment, wrong comparison, faulty arithmetic, or improper timing. Logic errors can be fixed by debugging your code, using testbenches, waveforms, or assertions to verify the functionality and correctness of your design.
Create a proper testbench and exercise your circuit in simulation before synthesizing it. This is the most efficient way to determine logic errors, since you can probe every signal in your design on the screen with little effort.
Prior to diving into a synthesis tool, a quick look at an RTL diagram can show where faulty logic signals have been removed (optimized out) from the module to module interconnect scheme. It's a quick way to check for proper signal flow.
Synthesis errors are errors that occur during the process of converting your HDL code into a hardware implementation, such as a netlist or a bitstream. Synthesis errors can arise from incompatible or unsupported features, constructs, or libraries in your code, or from constraints or settings that are not met or violated by your design. Synthesis errors can prevent your design from being implemented or optimized, and they can be fixed by modifying your code, using synthesis directives, or adjusting the constraints or settings of your synthesis tool.
Be sure to check all warnings produced during synthesis. Sometimes the tool will complete optimizations that you do not want but will produce a design that it considers complete. For example the MAXII CPLD allows you to have both an asynchronous preset and clear per DFF, but MachXO2 only allows for one or the other and will roll one of them back into the synchronous logic with only a warning. This can drastically change the behavior of a design.
Simulation errors are errors that occur during the process of simulating your HDL code or your synthesized design. Simulation errors can stem from discrepancies or mismatches between your code and your design, or between your design and your target device or environment. Simulation errors can affect the accuracy or validity of your simulation results, and they can be fixed by checking the consistency and compatibility of your code, design, and simulation tool, or by using simulation models, libraries, or commands that match your target device or environment.
Optimization errors are errors that occur during the process of optimizing your HDL code or your synthesized design for performance, power, or area. Optimization errors can result from trade-offs or conflicts between different optimization goals, criteria, or methods, or from over-optimization or under-optimization of your design. Optimization errors can degrade the quality or efficiency of your design, and they can be fixed by balancing or prioritizing your optimization objectives, using optimization directives or strategies, or tuning the parameters or options of your optimization tool.
Verification errors are errors that occur during the process of verifying your HDL code or your synthesized design against your specifications, requirements, or standards. Verification errors can indicate that your design does not meet or comply with your expectations or constraints, or that it contains bugs, faults, or defects. Verification errors can compromise the reliability or safety of your design, and they can be fixed by reviewing your specifications, requirements, or standards, using verification techniques, tools, or methodologies, or by testing, debugging, or correcting your design.