Many people struggle to understand the ModelSim/QuestaSim VHDL simulator’s workflow. I think that’s unnecessary because the basic workflow isn’t hard to learn.
I regard the graphical user interface (GUI) as a front-end for the commands listed in this article. For example, when you click the Compile button in the ModelSim GUI, it runs the vcom command back-end. The screenshot below shows the command echoed to the transcript window after we click the Compile button.
VUnit is one of the most popular open-source VHDL verification frameworks available today. It combines a Python test suite runner with a dedicated VHDL library to automate your testbenches.
When designing VHDL for safety-critical FPGA applications, it’s not enough to write testbenches at best-effort. You have to present proof that the module works as intended and without undesirable side-effects.
Formal verification techniques can help you map a requirement to a test, proving that your VHDL module conforms to the specification. It’s an instrumental tool for verifying healthcare applications or gaining DO-254 certification for airborne FPGA solutions.
This tutorial covers using the Integrated Logic Analyzer (ILA) and Virtual Input/Output (VIO) cores to debug and monitor your VHDL design in the Xilinx Vivado IDE.
In many cases, designers are in need to perform on-chip verification. That is, gaining access to an internal signal’s behavior in their FPGA design for verification purposes.
One option is to bring these signals to the FPGA pins and connect them to LEDs to see their behavior visually.
Professional VHDL simulators are more expensive than most individuals can afford. But fortunately, there are many free and legal alternatives that you can download and install.
Microsoft Windows is the easiest to install on, but many simulators also have a Linux version. FPGA vendors usually only support the Red Hat Linux distribution. But with a little effort, you can make it work in other distros, including Ubuntu.
Mac OS is the most challenging platform because there’s only one open-source simulator that works natively.
Text strings in VHDL are generally limited to fixed-length character arrays. That makes sense because VHDL describes hardware, and generic-length strings require dynamic memory.
To define an array of strings, you have to allocate space at compile-time for the highest number of strings you want to store. And even worse, you must decide on the strings’ max length and pad every occurrence to that number of characters. The code below shows an example usage of such a construct.
A latch is a logic element that can sample and hold a binary value, much like a flip-flop (register). But unlike a flip-flop, which is edge-triggered, the latch is level-triggered.
This article examines how we can use configuration constructs to create variants of a module or testbench without maintaining multiple versions of the file. We will also look at other use cases for configuration declarations in VHDL design.
Configurations have been part of the VHDL standard since the first version of the language. But still, many FPGA designers never use them, perhaps because few people understand how configurations work.
I find that unfortunate because it’s really not that complicated.
There are two ways to instantiate a module in VHDL: component instantiation and entity instantiation. Some people refer to the latter as direct instantiation.
Entity instantiation didn’t exist in the first revisions VHDL, but it has been available since VHDL’93. This is the method that I recommend unless you have specific reasons to use component instantiation.
To use the component instantiation method, you first have to declare the component in the declarative scope of where you want the module instance.
The generate statement in VHDL can automatically duplicate a block of code to closures with identical signals, processes, and instances. It’s a for loop for the architecture region that can create chained processes or module instances.
