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Designing a VLSI chip with the help of computer programs isknown as CAD, or Computer Aided Design.
Design Automation (DA), on the other hand, refers to entirelycomputerized design process with no or very little humanintervention.
Some of the earliest CAD software dealt with placement of logicmodules on printed circuit boards (PCBs) and finding shortelectrical paths to wire the interconnections.
Logic minimization was also an important facet of electronicdesign, since eliminating even a handful of logic gates resulted insignificant cost savings.
The rapidly changing technology has also radicallytransformed design issues.
For instance, in the LSI/VLSI technologies, it is notvery important to save on transistors; the costreduction through logic minimization is unlikely to besignificant when the total number of transistors is inthe order of a million.
On the other hand, it is important to save oninterconnection costs, since wires are far moreexpensive in VLSI than transistors.
Once the system architecture is defined, it is necessary to carry out twothings:
(a) Detailed logic design of individual circuit modules.(b) Derive the control signals necessary to activate and deactivate thecircuit modules
.The first step is known as data path design. The second step is called control
path design. The data path of a circuit includes the various functional blocks,
storage elements, and hardware components to allow transfer of data amongfunctional blocks and storage elements. Examples of functional blocks are adders,
multipliers, and other arithmetic/logic units. Examples of storage elements are shift
registers, random access memories, buffers, stacks, and queues. Data transfer is
achieved using tristate busses or a combination of multiplexers and
demultiplexers.
The control path of a circuit generates the various control signals necessary tooperate the circuit. Control signals are necessary to initialize the storage elements
in the circuit, to initiate data transfers among functional blocks and storage
elements, and so on. The control path may be implemented using hardwired
control (random logic) or through microprogrammed control.
If the circuit must be implemented on a printed-circuit boardusing off-the shelf components, then the next stage in design is
to select the components so as to minimize the total cost and atthe same time maximize the performance.
Following the selection procedure, the IC chips are placed onone or more circuit boards and the necessary interconnectionsare established using one or more layers of metal deposits. Asimilar procedure may be used in case the circuit must be
implemented on a VLSI chip using pre-designed circuitcomponents from a module library . The pre-designed modulesare also known as macro-cells. The cells must be placed on thelayout surface and wired together using metal and polysilicon(poly) interconnections.