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.

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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As most hardware engineers, I started off my computer science career by learning a sequential programming language. The first language I learned at the University of Oslo was Java. While it’s not considered to be the most exciting language today, at the time, Java was at the pinnacle of its popularity.

The engineers who built Java were trying to solve a number of issues which earlier languages were lacking in one blow. Perhaps a wise decision to do a fresh start instead of continuing down the C path and creating C+++.

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VHDL includes few built-in types, but offers a number of additional types through extension packages. Two of the most widely used types are the std_logic and std_ulogic. The difference between them is that the former is resolved, while the latter isn’t.

Before we go on to investigate what it means that a type is resolved, let’s first look at the traits that the two types share in common.

Bit and boolean are part of the standard package,

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It is possible to drive external signals from a procedure. As long as the signal is within the scope of the procedure, it can be accessed for reading or writing, even if it isn’t listed in the parameter list.

Procedures that are declared in the declarative region of the architecture, cannot drive any external signals. This is simply because there are no signals in its scope at compile time. A procedure declared within a process,

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An impure function can read or write any signal within its scope, also those that are not on the parameter list. We say that the function has side effects.

What we mean by side effects is that it is not guaranteed that the function will return the same value every time it is called with the same parameters. If the function can read signals that are not on the parameter list,

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Functions are subprograms in VHDL which can be used for implementing frequently used algorithms. A function takes zero or more input values, and it always returns a value. In addition to the return value, what sets a function apart from a procedure, is that it cannot contain Wait-statements. This means that functions always consume zero simulation time.

If you are familiar with functions or methods from other programming languages, VHDL functions should be easy to grasp.

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A finite-state machine (FSM) is a mechanism whose output is dependent not only on the current state of the input, but also on past input and output values.

Whenever you need to create some sort of time-dependent algorithm in VHDL, or if you are faced with the problem of implementing a computer program in an FPGA, it can usually be solved by using an FSM.

State-machines in VHDL are clocked processes whose outputs are controlled by the value of a state signal.

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