Description of Hardware Description Languages

Before discussing the more complex terms, let’s understand first what is HDL. We know that digital circuits are generally made up of linked transistors as discussed in the previous blog. We use a hierarchical structure to design and analyse these circuits: we could theoretically interpret a central processing unit (CPU) as a vast set of transistors, but it is much easier to organise transistors into logic gates, logic gates into adders, registers, or timing modules, registers into memory banks, and so on.

Because of its hierarchical nature, we may effectively represent a digital circuit using linked diagrams. This is referred to as a schematic. Another method for describing digital circuits is to utilise a textual language designed expressly to capture the distinguishing qualities clearly and simply and such type of language is known as hardware description languages (HDLs).

Description of Hardware Description Languages

Verilog and VHDL are the most widely used hardware description languages. They are frequently used in combination with FPGAs, which are digital devices particularly intended to make it easier to create customised digital circuits.

Hardware description languages enable you to describe a circuit using words and symbols, and debugging tools may subsequently turn that textual description into configuration data that is put into the FPGA to achieve the necessary performance.

Programming language vs Hardware Description Language

Many people are already familiar with programming languages when they start learning about hardware description languages. HDLs mimic high-level programming languages such as C or Python, but there is a key distinction: statements in HDL code entail parallel action, whereas programming languages reflect sequential execution, i.e., when we design a computer programme or firmware module, we understand that the processor will execute lines of code one at a time, in the same order as we read text on a page. Whereas in HDL code, even though the matching lines of code are written in a top-to-bottom arrangement, we are describing digital hardware and various pieces of this hardware can run simultaneously.

Evolution of Computer-Aided Digital Design

Over the past 25 years, digital circuit design has evolved tremendously. The very first digital circuits were developed using vacuum tubes and transistors. Logic gates were then integrated on a single chip to create integrated circuits. The very first integrated circuit (IC) chips were SSI (Small Scale Integration) chips with a relatively low gate count. As technology advanced, designers were able to build circuits with dozens of gates on a single chip. These have been known as MSI (Medium Scale Integration) chips. With the development of LSI (Large Scale Integration), designers were able to fit thousands of gates onto a single chip. At this point, design procedures had become extremely complex, and designers saw the necessity to automate these processes.

Computer-Aided Design (CAD) methods have begun to expand. Chip designers started to use circuit and logic simulation techniques to test the operation of the building blocks of the hundreds of transistors. The circuits were still tested on the breadboard, and the layout was done on sheets or by hand on a graphic computer terminal.

With the introduction of VLSI (Very Large Scale Integration) technology, designers were able to create single chips with over a million transistors. Because of the intricacy of these circuits, they could not be verified on a breadboard. Computer-aided approaches have become essential for VLSI digital circuit verification and design. Computer programmes for automated placement and routing of circuit layouts have gained popularity as well. The designers were now manually constructing gate-level digital circuits on graphic terminals. They would construct little construction pieces before deriving higher-level blocks from them. This method would be repeated until the top-level block was completed. Logic simulators were created to test the operation of these circuits before they were built on chips. Logic simulation became increasingly significant in the design process as designs grew larger and more complicated. Before the chip was further created, designers could straighten out functional problems in the structure.

Fig 1 Evolution

Emergence of HDL’s

Over a long period, programming languages like FORTRAN, Pascal, and C were used to define sequential computer programmes. Similarly, designers in the field of digital design realised the need for a consistent vocabulary to explain digital circuitry. As a result, Hardware Description Languages (HDLs) were created. Designers were able to represent the concurrency of processes found in hardware pieces using HDLs. Verilog HDL and VHDL have become prominent hardware description languages. Gateway Design Automation created Verilog HDL in 1983. VHDL was later developed under a DARPA grant. Designers readily accepted Verilog and VHDL simulators for simulating massive digital circuitry.

Despite the fact that HDLs were widely used for logic verification, designers had to manually convert the HDL-based design into a schematic circuit with gate linkages. The introduction of logic synthesis in the late 1980s fundamentally altered design methods. An HDL might be used to define digital circuitry at the register transfer level (RTL).

As a result, logic synthesis propelled the HDLs to the forefront of digital design. Designers are no longer required to manually position gates in order to construct digital circuits. By building such circuits in HDLs, they were able to define complicated circuits at an abstract level in terms of functionality and data flow. The requested functionality would be implemented using logic synthesis tools in the form of gates and gate linkages.

HDLs were also employed for system-level design. System boards, interconnect buses, FPGAs (Field Programmable Gate Arrays), and PALS were all simulated using HDLs (Programmable Array Logic). A popular strategy is to use an HDL to build each IC chip and then simulate the system’s functioning.

Importance of HDL’s

HDLs are unquestionably a future trend. With the rising complexity of digital circuits and the sophistication of CAD tools, HDLs will most likely be the sole solution for large-scale digital design. As opposed to traditional schematic-based design, HDLs have several advantages.

  1. The use of HDLs allows for highly abstract design descriptions.
  2. By expressing the design in HDLs, functional verification may be performed early in the design cycle. Designers may optimise and change the RTL description at the RTL level since they operate at the level.
  3. Designing using HDLs is similar to computer programming. A written description with comments is a more convenient method of developing and debugging circuits.

After reading this, we can easily understand the basic terms related to HDLs, but let’s recall what we have grasped while reading this interesting blog.

  1. What do you mean by HDL?
  2. What are the functionalities of HDLs?
  3. What is the difference between programing language and HDL?
  4. How does HDL emerge?
  5. What is the importance of HDL over other languages?
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