Introduction to Digital Systems: Modeling, Synthesis, and Simulation Using VHDL

1.5 CUSTOM-DESIGNED LOGIC DEVICES

Chips designed to meet the specific requirements of an application are known as application-specific integrated circuits (ASICs) or custom-designed chips. The logic chip is designed from scratch. The logic circuitry is designed according to the specifications and then implemented in an appropriate technology. The main advantage of ASICs is that since they are optimized for a specific application, they perform better than do functionally equivalent circuits built from off-the-shelf ICs or programmable logic devices. They occupy very little area, as all of the logic can be built into one chip. Thus, less PCB area would be required, leading to some cost savings. The disadvantage of ASICs is that they can be justifiable economically only when there is bulk production of the ICs. Typically, hundreds of thousands of ASICs must be manufactured to recover the expenditures necessary in the design, manufacturing and testing stages. Another drawback of the custom-design approach is that it requires the work of highly skilled engineers in the design, manufacturing, and test stages. The design time needed for these chips is also high, as a lot of verification has to be carried out to check for correct functionality. The circuitry in the chip cannot be altered once it is fabricated.