10 Top Mistakes in FPGA Programming

Piyush Gupta

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FPGA (Field-Programmable Gate Array) programming is a complex and fascinating field that allows engineers to design and implement custom digital circuits. However, it’s not without its challenges. In the world of FPGA programming, even a small mistake can have significant consequences, resulting in design failures, performance issues, or even costly rework.

The top 10 programming errors for FPGAs will be covered in this blog post, along with advice on how to avoid them. Understanding these hazards will significantly increase the quality and dependability of your FPGA designs, whether you are an experienced programmer trying to advance your abilities or a beginner just starting started with FPGA development.

We’ll explore a variety of FPGA programming topics in this article, including design comprehension, planning, resource use, timing constraints, testing, clock domain control, error handling, coding standards, and documentation. You’ll be more prepared to construct reliable and effective FPGA designs by recognizing and correcting these typical errors.

So, let’s embark on a journey to uncover the top 10 mistakes in FPGA programming and learn how to steer clear of them, ultimately leading to successful FPGA projects and optimized performance

10 Mistakes in FPGA Programming

1. Having Asynchronous Logic

Even though asynchronous logic is synthesizable, it might result in timing errors in the design (metastability), which makes FPGA designs ideal for synchronous systems. Experience Asynchronous logic is something that FPGA engineers want to keep out of their designs. Latching all asynchronous functionality in your architecture that is driving a LUT synchronously is one way to solve this issue.

Asynchronous loops in your design could raise the FPGA’s power consumption, which would generate more heat and possibly cause unneeded thermal problems.

 The following are cases where having an asynchronous logic is beneficial:

  • In an AXI stream protocol, it is best to have “tready” as asynchronous, mitigating an asynchronous logic is possible here using a register slice but that increases the utilization for your entire design.
  • DSP slice-based calculations. Divisions, Multiplications using DSP slices can be done asynchronously with little to no effect on timing.
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2. Not Using Primitives Provided on Device

This is a typical rookie error in FPGA programming. For instance, Xilinx devices have primitives for clock buffers, CDC, DLL, clock dividers, etc. Utilizing the built-in primitives of devices will result in a more effective design and significantly shorter development times.

 For example:

  • Using xpm-cdc for a synchronizer will reduce the hassle of writing RTL for it as well as defining constraints for that signal.
  • Using a buffer for clock division will automatically place the generated clock on the clock tree, as well as infer the generated clock constraints.
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3. Under-utilizing DSP Slices

DSP slices can function as accumulators, adders, subtractors, divisions, and multipliers in Xilinx FPGAs. When these features are required, it is preferable to perform this operation with DSP slices rather than LUTs or Fabric. This will cut down on the amount of time needed for design optimization and timing closure at the conclusion of your design.

 In situations where certain designs need very high clock frequencies to function, DSP slices can significantly decrease timing problems.

4. Checking for Inferred Latches in Your Design

This is a case that lends itself well to an example-based explanation. One synchronous state and two asynchronous states are required when writing a state machine. 

The next_state signal will be assumed to be a produced clock in your design since it will be driving several LUTs in the asynchronous NEXT STATE LOGIC, which the tool will create if you fail to take into consideration all criteria. 

If such a situation is willfully ignored, the state machine will crash and won’t restart if there is a problem on board.

 5. Not Running DRC Checks at Each Stage

DRC checks are essential and ought to be performed both after synthesis and after implementation. Any important warning in the DRC should be properly studied and taken into account in the design. Failure to do so could lead to an unstable system that eventually fails.

6. Not following Proper Naming and Coding Conventions

 All RTL design engineers are expected to follow a standard naming convention for their designs such as.

  1. Adding i_ for all inputs
  2. Adding o_ for all outputs
  3. Adding s_ for all signals
  4. Adding c_ for all constants
  5. Adding g_ for all generic
  6. Clocks should be named as i_clk_<integer>m<decimal> : eg i_clk_122m88
  7. Resets should be named as i_rst_<polarity_<clock domain> : i_rst_n_122m88

Indentation guidelines such as 3 spaces for tabs are also expected.

All code should be properly documented, commented and tested (presynth as well as  postsynth)

7. Unnecessary Usage if Reset Signal

Since it significantly increases fanout in the design, driving all signals using a reset pin is not recommended in Xilinx devices. The gsr pin should be used for all intermediate signals instead.

I.e: when declaring signals initialize it there itself to the reset value.

8. Calculation of Utilization

The full utilization of all resources in FPGAs is not advised. Pick a device when you start the design that has an additional 10% to 20% empty space that can accommodate your design. This gives the router room to use its built-in logic to address all setup and hold time infractions.

In the event that you don’t, you’ll have to operate at a lower clock frequency in order to finish your design’s timing.

9. Using Block Designs for Large Designs

Block designs are an excellent technique to connect RTL easily, but for large projects, a pure RTL project is more predictable and less difficult to debug. In Xilinx Vivado, block designs also contain a number of intrinsic faults that result in a lot of pointless warnings in your design.

10. Not Checking to Log all Signals Options in Simulation Settings

It is best to first check the log all signal option in vivado while simulating a design. This will save you from having to restart the simulation from scratch only to include a new signal in the waveform. This holds true for any simulation program from a third party used in conjunction with vivado.

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Conclusion:

In conclusion, avoiding frequent programming errors in FPGAs is essential for producing effective and dependable designs. You can improve your FPGA programming abilities by comprehending the hardware, carefully planning, allocating resources, implementing precise timing constraints, carrying out thorough testing, managing clock domains, using robust error handling and coding techniques, and maintaining thorough documentation.

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