Theory

Semi custom VLSI design can be cell based or array based. In cell based design, the designer reuses the cells that have already been designed and stored in the library as a part of the current design. In array based design, the designer is provided with a chip consisting of configurable logic blocks as generic building blocks. The designer either needs to finish the topmost metallic interconnect routing to connect the different configurable logic blocks or needs to program the switches on interconnect through electric fields. In the former case, the array is called a mask programmable gate array and in the latter case, the array is called a field programmable gate array. With semi custom VLSI chip, there may be a slight degradation in performance with respect to an equivalent full custom VLSI chip. But the minimization of the design cycle time due to reusability of components from the library makes it outperform its full custom counterpart and has earned a huge popularity amongst the designers over the years. Full custom designs are nowadays limited to only performance critical applications and in case a new technology library is adopted for designing a circuit.