VHDL Introduction

VHDL has a rich and interesting history. But since knowing this history is probably not going to help you write better VHDL code, it will only be briefly mentioned here. Consulting other, lengthier texts or search engines will provide more information for those who are interested. 

Regarding the 

VHDL acronym, 

the V is short for yet another acronym:

VHSIC or Very High-Speed Integrated Circuit

The HDL stands for Hardware Description Language

Clearly, the state of technical affairs these days has done away with the need for nested acronyms. VHDL is a true computer language with the accompanying set of syntax and usage rules. But, as opposed to higher-level computer languages, VHDL is primarily used to describe hardware. The tendency for most people familiar with a higher-level computer language such as C or Java is to view VHDL as just another computer language. This is not altogether a bad approach if such a view facilitates the understanding and memorization of the language syntax and structure. The common mistake made by someone with this approach is to attempt to program in VHDL as they would program a higher-level computer language. Higher-level computer languages are sequential in nature; 

VHDL is not. VHDL was invented to describe the hardware and in fact, VHDL is a concurrent language. What this means is that, normally, VHDL instructions are all executed at the same time (concurrently), regardless of the size of your implementation. Another way of looking at this is that higher-level computer languages are used to describe algorithms (sequential execution) and VHDL is used to describe hardware (parallel execution). This inherent difference should necessarily encourage you to re-think how you write your VHDL code. Attempts to write VHDL code with a high-level language style generally result in VHDL code that no one understands. Moreover, the tools used to synthesize this type of code has a tendency to generate circuits that generally do not work correctly and have bugs that are nearly impossible to trace.

Introduction To VHDL And if the circuit does actually work, it will most likely be inefficient because the resulting hardware was unnecessarily large and overly complex. This problem is compounded as the size and complexity of your circuits becomes greater. There are two primary purposes for hardware description languages such as VHDL. First, VHDL can be used to model digital circuits and systems. Although the word “model” is one of those overly used words in engineering, in this context it simply refers to a description of something that presents a certain level of detail. The nice thing about VHDL is that the level of detail is unambiguous due to the rich syntax rules associated with it. In other words, VHDL provides everything necessary to describe any digital circuit. 

Likewise, a digital circuit/system is any circuit that processes or stores digital information. Second, having some type of circuit model allows for the subsequent simulation and/or testing of the circuit. The VHDL model can also be translated into a form that can be used to generate actual working circuits. The VHDL model is magically interpreted by software tools in such a way as to create actual digital circuits in a process known as synthesis. 

There are other logical languages available to model the behavior of digital circuit designs that are easy to use because they provide a graphical method to model circuits. For them, the tendency is to prefer the graphical approach because it has such a comfortable learning curve. But, as you can easily imagine, your growing knowledge of digital concepts is accompanied by the ever-increasing complexity of digital circuits you are dealing with. 

The act of graphically connecting a bunch of lines on the computer screen quickly becomes tedious. The more intelligent approach to digital circuit design is to start with a system that can describe exactly how your digital circuit works (in other words, modeling it) without having to worry about the details of connecting massive quantities of signal lines. Having a working knowledge of VHDL will provide you with the tools to model digital circuits in a much more intelligent manner. 

Finally, you will be able to use your VHDL code to create actual functioning circuits. This allows you to implement relatively complex circuits in a relatively short period. The design methodology you will be using allows you to dedicate more time to designing your circuits and less time “constructing” them. The days of placing, wiring, and troubleshooting multiple integrated circuits on a proto-board are gone. VHDL is a very exciting language that can allow the design and implementation of functions capable of processing an enormous amount of data by employing relatively low-cost and low-power hardware. 

Moreover, what is really impressive is that, via simple VHDL modules, you can have direct access to basic ns-level logic events as well as communicate using a USB port or drive a VGA monitor to visualize graphics of modest complexity. Modeling digital circuits with VHDL is a form of modern digital design distinct from It is not really magic. There is actually a well-defined science behind it. Golden Rules of VHDL schematic-based approaches. The programmer writes a loose description of what the final logic circuit should do and a language compiler, in this case, called a synthesizer, attempts to “infer” what the actual final physical logic circuit should be. Novice programmers are not always able to convince the synthesizer to implement something that seems very clear in their minds. A somehow old-fashioned alternative to a descriptive language such as VHDL is one in which the programmer simply interconnects a finite number of digital blocks that he has pooled from a library in an attempt to reach the same objective. This approach is not only very time consuming but also inherently limiting and very error-prone. Modern digital design is more about appropriately modeling digital circuits and maintaining a quality description of the circuit. All that is left now is to learn how to properly use VHDL to describe what you want to implement.

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