14-02-2013, 12:30 PM
INTRODUCTION TO VLSI
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INTRODUCTION
The first digital circuit was designed by using electronic components like vacuum tubes and transistors. Later Integrated Circuits (ICs) were invented, where a designer can be able to place digital circuits on a chip consists of less than 10 gates for an IC called SSI (Small Scale Integration) scale. With the advent of new fabrication techniques designer can place more than 100 gates on an IC called MSI (Medium Scale Integration). Using design at this level, one can create digital sub blocks (adders, multiplexes, counters, registers, and etc.) on an IC. This level is LSI (Large Scale Integration), using this scale of integration people succeeded to make digital subsystems (Microprocessor, I/O peripheral devices and etc.) on a chip.
At this point design process started getting very complicated. i.e., manually conversion from schematic level to gate level or gate level to layout level was becoming somewhat lengthy process and verifying the functionality of digital circuits at various levels became critical. This created new challenges to digital designers as well as circuit designers. Designers felt need to automate these processes. In this process, Rapid advances in Software Technology and development of new higher level programming languages taken place. People could able to develop CAD/CAE (Computer Aided Design/Computer Aided Engineering) tools, for design electronics circuits with assistance of software programs. Functional verification and Logic verification of design can be done using CAD simulation tools with greater efficiency. It became very easy to a designer to verify functionality of design at various levels.
With advent of new technology, i.e., CMOS (Complementary Metal Oxide Semiconductor) process technology. One can fabricate a chip contains more than Million of gates. At this point design process still became critical, because of manual converting the design from one level to other. Using latest CAD tools could solve the problem. Existence of logic synthesis tools design engineer can easily translate to higher-level design description to lower levels. This way of designing (using CAD tools) is certainly a revolution in electronic industry. This may be leading to development of sophisticated electronic products for both consumer as well as business. Designing Systems using Hardware always gives best results when compared to software (like Speed Reliability, performance and etc.,) Using CMOS VLSI Design methodology designer could design and fabricate ICs without spending much time when compared to traditional way of designing.
HISTORY:
The requirements for the language were first generated in 1988 under the VHSIC chips for the department of Defence (DOD). Reprocurement and reuse was also a big issue. Thus, a need for a standardized hardware description language for the design, documentation, and verification of the digital systems was generated. The IEEE in the December 1987 standardized VHDL language; this version of the language is known as the IEEE STD 1076-1987. The official language description appears in the IEEE standard VHDL language Reference manual, available from IEEE. The language has also been recognized as an American National Standards Institute (ANSI) standard.
According to IEEE rules, an IEEE standard has to be reballoted every 5 years so that it may remain a standard so that it may remain a standard. Consequently, the language was upgraded with new features, the syntax of many constructs was made more uniform, and many ambiguities present in the 1987 version of the language were resolved. This new version of the language is known as the IEEE STD 1076-1993
HARDWARE ABSTRACTION:
VHDL is used to describe a model for a digital hardware device. This model specifies the external view of the device and one or more internal views. The internal view of the device specifies functionality or structure, while the external view specifies the interface of the device through which it communicates with the other modules in the environment.
In VHDL each device model is treated as a distinct representation of a unique device, called an Entity. The Entity is thus a hardware abstraction of the actual hardware device. Each Entity is described using one model, which contains one external view and one or more internal views.
Basic terminology:
VHDL is a hardware description language that can be used to model a digital system. A hardware abstraction of this digital system is called an entity. An entity X, when used in another entity Y, becomes a component for the entity Y.