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…
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…
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. Even worse, you must decide on the strings’…
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,…
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…
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.
Radio-controlled (RC) model servos are tiny actuators typically used in hobbyist model planes, cars, and boats. They allow the operator to control the vehicle via a radio link remotely. Because RC models have been around for a long time, the de-facto standard interface is pulse-width modulation (PWM), rather than a digital scheme. Fortunately, it’s easy…
Most VHDL simulators use the Tool Command Language (Tcl) as their scripting language. When you type a command in the simulator console, you are using Tcl. Furthermore, you can create scripts with Tcl that run in the simulator and interact with your VHDL code. In this article, we will create a self-checking testbench that uses…
How do you stop the VHDL simulator when the simulation is complete? There are several ways to do that. In this article, we will examine the most common ways to end a successful testbench run. The VHDL code presented here is universal, and it should work in any capable VHDL simulator. For the methods involving…
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