What is Computer-Aided Design (CAD) Tools ? How it Help Us in the Integrated Circuit Production.

 Hello Dear Readers, 

Today in this post I will provide some deep insight into Computer-Aided Design (CAD) Tools. So let's start. 

Computer-aided design (CAD) tools have advanced significantly during the past decade, and nowadays digital design is performed using a variety of software tools. Prototypes or even final plans can be created without discrete components and interconnection wires. Fig. 1 illustrates the steps in modern digital system design. Like any engineering design, the first step in the design flow is formulating the problem, stating the design requirements, and arriving at the design specification. The next step is to develop the design at a conceptual level, either at a block diagram level or at an algorithmic level.

Fig. 1: Design Flow in Modern Digital System Design 

Design entry is the next step in the design flow. Previously, this would have been a hand-drawn schematic or blueprint. Now with CAD tools, the design conceptualized in the previous step needs to be entered into the CAD system in an appropriate manner. Designs can be entered in multiple forms. A few years ago, CAD tools were used to provide a graphical method to enter designs. This was called schematic capture. The schematic editors typically were supplemented with a library of standard digital building blocks such as gates, flip-flops, multiplexers, decoders, counters, registers, and so forth. ORCAD (a company that produced design automation tools) provided a very popular schematic editor. Nowadays, hardware description languages (HDLs) are used to enter designs in textual form. Two popular HDLs are VHDL and Verilog. 

A hardware description language (HDL) allows a digital system to be designed and debugged at a higher level of abstraction than schematic capture. In schematic capture, a designer inputs a schematic with gates, flip-flops, and standard MSI building blocks. However, with HDLs, the details of the gates and flip-flops do not need to be handled during early phases of design. A design can be entered in what is called a behavioral description of the design. In a behavioral HDL description, one specifies only the general working of the design at a flow-chart or algorithmic level without associating to any specific physical parts, components, or implementations. Another method to enter a design in VHDL and Verilog is the structural description entry. In structural design, specific components or specific implementations of components are associated with the design. A structural VHDL or Verilog model of a design can be considered as a textual description of a schematic diagram that you would have drawn interconnecting specific gates, flip-flops, and other modules. 

Once the design has been entered, it is important to simulate it to confirm that the conceptualized design does function correctly. Initially, one should perform the simulation at the high-level behavioral model. This early simulation unveils problems in the initial design. If problems are discovered, the designer goes back and alters the design to meet the requirements. 

Once the functionality of the design has been verified through simulation, the next step is synthesis. Synthesis means the conversion of the higher-level abstract description of the design to actual components at the gate and flip-flop levels. The use of computer-aided design tools to do this conversion, also called synthesis, is standard practice in the industry now. The output of the synthesis tool, consisting of a list of gates and a list of interconnections, specifying how to interconnect them, is often referred to as a netlist. Synthesis is analogous to writing software programs in a high-level language such as C and then using a compiler to convert the programs to machine language. Just as a C compiler can generate optimized or unoptimized machine code, a synthesis tool can generate optimized or unoptimized hardware. The synthesis software generates different hardware implementations, depending on algorithms embedded in the software to perform the translation and optimization. A synthesis tool is nothing but a compiler to convert design descriptions to hardware, and it is not unusual to name synthesis packages with phrases such as “design compiler,” “silicon compiler,” and the like. 

The next step in the design flow is post-synthesis simulation. The earlier simulation at a higher level of abstraction does not take into account specific implementations of the hardware components that the design is using. If post-synthesis simulation unveils problems, one should go back and modify the design to meet timing requirements. Arriving at a proper design implementation is an iterative process. 

Next, a designer moves into specific realizations of the design. A design can be implemented in several different target technologies. It could be a completely custom IC, or it could be implemented in a standard part that is easily available from 

Fig. 2: Spectrum of Design Technologies

a vendor. The target technologies that are commonly available now are illustrated in Fig. 2. 

At the lowest level of sophistication and density is an old-fashioned printed circuit board with off-the-shelf gates, flip-flops, and other standard logic-building blocks. Slightly higher in density are programmable logic arrays (PLAs), programmable array logic (PALs), and simple programmable logic devices (SPLDs). PLDs with higher density and gate count are called complex programmable logic devices (CPLDs). In addition, there are the popular field programmable gate arrays (FPGAs) and mask programmable gate arrays (MPGAs), or simply gate arrays. The highest level of density and performance is a fully custom application-specific integrated circuit (ASIC). 

The two most common target technologies currently are FPGAs and ASICs. The initial steps in the design flow are largely the same for either realization. Towards the final stages in the design flow, different operations are performed depending on the target technology. This is indicated in Fig. 1. The design is mapped into specific target technology and placed into specific parts in the target ASIC or FPGA. The paths taken by the connections between components are decided during the routing. If an ASIC is being designed, the routed design is used to generate a photomask that will be used in the integrated circuit (IC) manufacturing process. If a design is to be implemented in an FPGA, the design is translated to a format specifying what is to be done to various programmable points in the FPGA. In modern FPGAs, programming simply involves writing a sequence of 0s and 1s into the programmable cells in the FPGA, and no specific programming unit other than a personal computer (PC) is required to program an FPGA. 

See the below articles also,

What is Simulation? How Simulation Can Guide Real Engineering Design Decisions

How FPGA is Programmable ASIC and Its Building Blocks

What is High-Level Synthesis(HLS)

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