VHDL and Programmable Logic

VHDL is a hardware description language used to design and model digital electronic systems. Its name stands for VHSIC Hardware Description Language, where VHSIC refers to a United States programme on Very High Speed Integrated Circuits. Rather than drawing a circuit gate by gate, an engineer describes the desired behaviour and structure of digital logic in text, which design tools then translate into an actual circuit implementation.

VHDL allows designs to be expressed at different levels of abstraction, from the flow of data and the behaviour of logic over time down to the interconnection of specific components. A description can be simulated to verify that it behaves correctly before any hardware is built, and the same description can then be synthesised into the logic needed to realise it. This separation of design from a particular technology makes VHDL portable and reusable across projects and devices.

The language is closely associated with programmable logic devices, which are chips whose internal logic can be configured by the user rather than being fixed at manufacture. These include simple devices such as PALs (Programmable Array Logic) and PLAs (Programmable Logic Arrays), as well as more complex field-programmable gate arrays (FPGAs). VHDL is also used to design application-specific integrated circuits (ASICs), which are custom chips manufactured for a particular function. By describing logic functions such as AND, OR, NAND, NOR and NOT in a structured way, VHDL lets designers build and test complex digital systems efficiently. Verilog is another widely used hardware description language serving similar purposes.

Frequently asked questions

What is VHDL used for?

It is a hardware description language used to design, simulate and synthesise digital circuits, describing their behaviour and structure in text rather than as a schematic.

What is a programmable logic device?

It is a chip, such as a PAL, PLA or FPGA, whose internal logic can be configured by the user after manufacture to implement a desired digital function.

What is the difference between an FPGA and an ASIC?

An FPGA is a reconfigurable device programmed by the user, while an ASIC is a custom chip manufactured for a specific fixed function. VHDL can be used to design both.