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VLSI Programming Using Hardware Descriptive Languages

Key Takeaways

  • In front end design, VLSI programming is done using hardware descriptive languages such as Very High-Speed Integrated Circuit Hardware Description Language (VHDL), Verilog, and System Verilog.

  • The design process of VLSI involves three stages: behavioral representation, logic circuit representation, and layout representation.

  • Hardware descriptive language is used to describe, simulate, and create hardware electronic circuits.  

VLSI technology

VLSI technology enhances the operating speed of ICs

VLSI technology transformed electronic circuit applications in such a broad way that we are now able to reduce a complete electronic system or processor into a silicon chip. Apart from size reduction, VLSI technology has enhanced operating speeds, power consumption, reliability, and cost of electronic circuits. 

The process of IC fabrication using VLSI technology involves a long procedure, with steps such as problem specification, architecture definition, functional design, logic design, circuit design, physical design, wafer processing, lithography, etching, ion implantation, metallization, assembly, and packaging. In front end design, VLSI programming is done using hardware descriptive languages (HDLs) such as Very High-speed Integrated Circuit Hardware Description Language  (VHDL), Verilog, and System Verilog.  

VLSI Programming Using HDL

The design process of VLSI involves three stages: behavioral representation, logic circuit representation, and layout representation. 

Behavioral Representation

HDL is an integral part of behavioral representation. The VLSI design flow starts with behavioral representation, where the functionalities of the IC that is being fabricated are specified. The way in which the IC interacts with the exterior circuits is also defined at this stage. 

The VLSI programming language plays an important role in defining the behavior of the IC. Using HDL, the behavioral description is created to analyze functionalities, performance, compliance to standards, and other IC specifications. Concurrent algorithms are used in the behavioral description stage to describe the system, which is executed sequentially. At this stage, there is not much description of the structure realization of the IC.

Register-Transfer Level Description

The register-transfer level (RTL) description is also completed using HDLs and is simulated to test the functionalities. The characteristics of the system are specified using the operation and transfer of data between the registers at this stage. Once the RTL description is verified, it is converted into a gate-level netlist. The gate-level netlist describes the circuit using gates and interconnections and verifies speed, power, and size specifications. In this stage, all the signals are discrete and use the basic logic operations. After the verification of each stage, a physical layout is made for fabricating the IC. 

Verilog being used

Verilog is an HDL used in VLSI programming

Hardware Descriptive Languages 

HDLs are used to describe, simulate, and create hardware electronic circuits. VHDL, Verilog, and System Verilog are the most popular HDLs in use. VHDL is a self-explanatory HDL, Verilog is somewhat like C language, with similar syntax, and System Verilog is the most advanced HDL. 

The number of instructions for defining a function is more often in VHDL than in Verilog. However, all the HDLs are effective in encapsulating the concepts of the entity, connectivity, concurrency, and timing related to IC design. HDLs express the dimensions of timing and concurrency using algorithms in VLSI behavioral representation and describe the hardware structure at the VLSI RTL stage. 


HDL used as a VLSI programming language is independent of technology—you can implement the design using any technology. It is easy to design and troubleshoot any bugs in HDL. HDLs are so user-friendly to design large complex ICs that schematics are not drawn for such designs. Large and complex designs involving more than ten lakh gates can be expressed accurately in an HDL program. 

With this much complexity, modeling capabilities are flexible with HDL. Design changes can be easily implemented in the HDL environment. Design optimization is an important advantage of HDL in VLSI design. The design description, including both very abstract and structural details, can be efficiently encrypted in HDL. The trade-off decision between speed and footprint can also be made at the HDL design stage and the designer can explore alternative designs with the help of HDL. 

It is also possible for a designer to logic synthesis around twenty thousand gates in a day with the use of HDL. This increases the productivity of semiconductor IC manufacturing industries. 

VLSI programming using HDL helps the designer to capture the characteristics of the IC and bring it to hardware design. It gives a perspective about the structure as well as the behavior of the IC in the design stage itself. HDL languages such as VHDL and Verilog satisfy industrial standards and are compatible with most software tools. If you are looking to design a digital circuit using VLSI technology, learning an HDL makes the process easier. 

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