VLSI DESIGN STYLES

While the standard-cells-based design is often referred to as full custom design , it is less so in a strict sense because the cells are pre-designed for general use and the same cells are used in a variety of chip designs. In a more comprehensive custom design, the whole mask is created from scratch, without the use of any libraries.However, the expense of developing a design style like this is becoming prohibitively expensive. As a result, the idea of design reuse is gaining attention and becoming popular  as a way to cut down on design cycle time and production costs. The design of a memory cell, whether static or dynamic, may be the most rigorous complete custom design.There will be no alternative to high density memory chip design , since the same layout design  is repeated.Using a combination of different design types on the same chip, such as regular cells, data-path cells, and PLAs, can achieve a good compromise in logic chip design. Design productivity is normally ver...

One of the most common full custom design types that necessitates the creation of a full custom mask package is the standard-cells based design. The polycell is another name for the standard cell. All of the commonly used logic cells are generated, characterised, and stored in a standard cell library in this design style.Inverters, NAND gates, NOR gates, complex AOI, OAI gates, D-latches, and flip-flops are among the cells found in a typical library. To provide adequate driving capability for different fanouts, each gate form may have several implementations.For example, the inverter gate may have standard, double, and quadruple size transistors, allowing the chip designer to choose the appropriate size to achieve high circuit speed and layout density. Each cell is characterised for a number of different categories. . It consists of delay time vs. load capacitance circuit simulation model timing simulation model fault simulation model cell data for place-and-route mask data Each cell l...

The gate array (GA) comes after the FPGA because of the quick prototyping capability. As user programming is used to implement the design of the FPGA chip, metal mask design and processing is used to implement the design of the gate array. A two-step manufacturing process is needed for gate array implementation: The first phase, which uses generic (standard) masks, leaves each GA chip with an array of uncommitted transistors. These uncommitted chips can be saved for later customization after the metal interconnects between the array's transistors are defined (Fig. 1). Since the patterning of metallic interconnects is done at the end of the chip fabrication, the turn-around time can be still short, a few days to a few weeks. Figure 2 depicts a gate array chip's corner, which includes bonding pads on the left and bottom sides, diodes for I/O protection, nMOS and pMOS transistors for chip output driver circuits in the neighbouring areas of bonding pads, arrays of nMOS and pMOS tra...

 For chip implementation of specified algorithms or logic functions, a variety of design styles can be considered. Each design style has its own advantages and disadvantages, so designers must make an informed decision in order to provide functionality at a low cost. Field Programmable Gate Array (FPGA) Users may order fully fabricated FPGA chips with thousands of logic gates or even more, as well as programmable interconnects, for custom hardware programming to achieve desired functionality. This design style allows for fast prototyping as well as cost-effective chip design, which is particularly useful for low-volume applications. I/O buffers, an array of configurable logic blocks (CLBs), and programmable interconnect structures make up a typical field programmable gate array (FPGA) chip. Programming of RAM cells whose output terminals are attached to the gates of MOS pass transistors is used to programme the interconnects.Figure 1 depicts the general architecture of an XILI...

 By splitting the large structure into many sub-modules, the hierarchical design approach eliminates design complexity. To make the process easier, other design principles and approaches are usually needed. Regularity ensures that a large system's hierarchical decomposition can produce as many simple and identical blocks as possible.The design of array structures made up of similar cells, such as a parallel multiplication array, is a good example of regularity. Regularity can be seen at all levels of abstraction: uniformly sized transistors simplify the design at the transistor stage. Identical gate structures can be used at the logic level, and so on.A 2-1 MUX (multiplexer), a D-type edge-triggered flip flop, and a one-bit full adder are shown in Figure 7 as standard circuit-level designs. All of these circuits were created solely with inverters and tri-state buffers. This theory can be used to create a variety of different functions if the designer has a limited library of well-d...

  Design Hierarchy The hierarchy, or 'divide and conquer' strategy, entails breaking down a subsystem into sub-modules and then repeating the process on the sub-modules until the smaller parts' complexity is manageable. This method is similar to how large programmes are broken down into smaller and smaller parts before simple subroutines with well-defined functions and interfaces can be written in software. The architecture of a VLSI chip can be expressed in three domains, as we've seen.As a result, each domain's hierarchy structure can be represented separately. However, it is important for design simplicity that the hierarchies in different domains can be easily mapped into one another. Figure 1 demonstrates the structural decomposition of a CMOS four-bit adder into its components as an example of structural hierarchy. Decomposing the adder into one-bit adders, separate carry and sum circuits, and finally individual logic gates is possible.The design of a simple c...