Entity instantiation and component instantiation

VHDL instantiation

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.

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RC servo controller using PWM from an FPGA pin

The Lattice iCEstick FPGA board controlling a TowerPro SG90 servo

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 to implement PWM with the precise timing that an FPGA can exert on its output pins. In this article,

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How to create a Tcl-driven testbench for a VHDL code lock module

Digital code lock

Most VHDL simulators use the Tool Command Language (Tcl) as their scripting language. When you type a command in the console of the simulator, 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 used Tcl instead of VHDL to verify that a VHDL module behaves correctly.

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How to stop simulation in a VHDL testbench

How to Stop Simulation in a VHDL Testbench

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 Tcl, I will list the commands for the ModelSim and Vivado simulators.

You can download an example project with the four different testbenches from this article if you have ModelSim installed.

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Clk’event vs rising_edge

(clk'event and clk = '1') vs rising_edge(clk)

When you have worked with VHDL code written by many other FPGA engineers, you are bound to notice that there are two common ways to model an edge detector in VHDL. There’s the rising_edge(clk) statement, and there’s the old style clk’event and clk = ‘1’ method.

The two if-statements do the same thing once the code is on the FPGA. They postulate that whatever is inside of the if-statement happens on the rising edge of the trigger signal,

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Make Lattice iCEcube2 work on Ubuntu 20.04 and program the iCEstick FPGA board

Lattice iCEcube2 running in Ubuntu 20.04

This tutorial shows how to install the Lattice iCEcube2 FPGA design software on Ubuntu 20.04. Instead of the Lattice Diamond Programmer, we will use the alternative programmer from Project IceStorm that works flawlessly on Ubuntu Linux.

The Lattice iCEcube2 FPGA design software only works on Red Hat-based Linux distributions out of the box. Fortunately, we can make it run perfectly on Ubuntu Linux by installing dependencies and making a few changes to our system.

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How to create a breathing LED effect using a sine wave stored in block RAM

Sine wave

I’ve noticed that many of the gadgets that I’ve bought the last couple of years have shifted away from LED blinking to led breathing. Most electronic gizmos contain a status LED whose behavior gives away indications of what’s going on inside of the device.

My electric toothbrush flashes an LED when it’s time to recharge it, and my mobile phone uses the LED to call my attention for a wide variety of reasons.

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How to create a PWM controller in VHDL

PWM simulation in ModelSim waveform

Pulse-width modulation (PWM) is an efficient way to control analog electronics from purely digital FPGA pins. Instead of attempting to regulate the analog voltage, PWM rapidly switches on and off the supply current at full power to the analog device. This method gives us precise control over the moving average of energy provided to the consumer device.

Examples of use cases that are good candidates for PWM are audio modulation (speakers), light intensity control (lamps or LEDs),

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How to make ModelSim from Quartus Prime Lite work on Ubuntu 20.04

ModelSim-Intel FPGA Edition running in Ubuntu 20.04

The ModelSim version that comes with Intel Quartus Prime Lite Edition is a good alternative if you want to try out VHDL simulation on your home computer. The software is available for both Windows and Linux, but Intel only supports Red Hat-based distros like CentOS Linux.

Fortunately, you are just a few hacks away from ModelSim working nicely on your Ubuntu box.

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