Many people turn to me for advice regarding their FPGA projects. Often, my answer is not what they expected: have you considered using a microcontroller instead of an FPGA?

I’m running VHDLwhiz, the website dedicated to FPGA design using VHDL. My life revolves around FPGAs, and I’m the biggest proponent of VHDL there is. Why would I say such a thing?

It’s because, for some tasks, an FPGA is overkill.

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In this article, I will present a VHDL module that can display a two-digit number on the Pmod SSD: Seven-segment Display from Digilent. The dual 7-segment display is compatible with the Pmod interface, meaning that you can use it without any soldering. It fits into the Pmod connector, which is standard on many FPGA development boards.


VHDL has a built-in pseudo-random generator, but it can only generate floating-point numbers between 0 and 1. Fortunately, you can derive from this any other kind of random data format you should need. Continue reading this article to find out how to produce real or integer values of any range, as well as random std_logic_vector sequences and time values.

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Converting the image file to a bitmap format makes for the easiest way to read a picture using VHDL. Support for the BMP raster graphics image file format is built into the Microsoft Windows operating system. That makes BMP a suitable image format for storing photos for use in VHDL testbenches.

In this article, you will learn how to read a binary image file like BMP and store the data in dynamic memory in the simulator.

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Reading signal values from  file is an alternative way of generating stimuli for the device on test (DUT). The testbench sequence and timing is hard-coded in a stimulus file that is read by the VHDL testbench, line by line. This allows you to easily change the pattern of the waveform that you want to feed to the test object.

Sometimes you have a very specific test pattern or sequence of events that you want to put your DUT through.

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A convenient way to populate block RAM with initial values is to read binary or hexadecimal literals from an ASCII file. This is also a good way to create a ROM (read-only memory) in VHDL. After all, RAM and ROM are the same thing in FPGAs, ROM is a RAM that you only read from.

The examples throughout this article will assume that the following constants and RAM type have been declared at the start of the declarative region of the VHDL file.

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I was a little annoyed by the peculiarities of the AXI interface the first time I had to create logic to interface an AXI module. Instead of the regular busy/valid, full/valid, or empty/valid control signals, the AXI interface uses two control signals named “ready” and “valid”. My frustration soon changed to awe.

The AXI interface has built-in flow control without using additional control signals. The rules are easy enough to understand, but there are a few pitfalls one has to account for when implementing the AXI interface on an FPGA.

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I know that I have googled this at least a hundred times throughout my career as an FPGA engineer; how to check if all bits in a std_logic_vector signal are ‘0’ or ‘1’. Of course, you know a few ways to do it already, but you want to find the most elegant code that will work with vectors of any length. Right?

Let’s have a look at the best methods for checking that all the bits in a vector are set or unset.

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