What Is Integrated Circuit Design?

Integrated circuit design (IC design), also known as VLSI design, refers to the design process aimed at integrated circuits and very large-scale integrated circuits. The design of integrated circuits involves the establishment of models of electronic devices (such as transistors, resistors, capacitors, etc.) and interconnection lines between devices. All devices and interconnects need to be placed on a piece of semiconductor substrate material. These components are placed on a single silicon substrate through semiconductor device manufacturing processes (such as photolithography) to form circuits.

The most commonly used substrate materials for integrated circuit design are

Integrated circuit design is usually based on "modules". For example, for a multi-bit full adder, its secondary module is a one-bit adder, and the adder is composed of the next-level AND gate and NOT gate modules. The AND and NOT gates can finally be decomposed into lower abstractions. CMOS devices.

From the abstraction level, digital integrated circuit design can be top-down, that is, first define the functional modules of the highest logical level of the system, define sub-modules according to the needs of the top-level modules, and then continue to decompose layer by layer; Bottom-up, that is, first design the most specific modules separately, and then use these bottom modules as the building blocks to implement the upper modules, and finally reach the highest level. In many designs, top-down and bottom-up design methodologies are mixed. System-level designers plan the overall architecture and divide the sub-modules, while the underlying circuit designers design upwards layer by layer. Optimize individual modules. Finally, designers from both directions meet at some level of abstraction in the middle to complete the entire design.

For different design requirements, engineers can choose to use semi-custom design approaches, such as using programmable logic devices (field programmable logic gate arrays, etc.) or ASICs based on standard cell libraries to implement hardware circuits; full custom designs can also be used , Control the transistor layout to the full details of the system structure.

Full custom design

This design method requires the designer to use the layout editor to complete the layout design, parameter extraction, and cell characterization, and then use these self-designed cells to complete the circuit construction. In general, a fully custom design is designed to maximize circuit performance. If a required unit is missing from the standard cell library, a fully custom design approach is also required to complete the required unit design. However, this design method usually takes a long time.

Semi-custom design

The design method opposite to full custom design is semi-custom design. In short, semi-custom integrated circuit design is based on some pre-designed logic cells. For example, a designer can design an application-specific integrated circuit based on a standard component library (which can usually be purchased from a third party) and select the required logic units (such as various basic logic gates, flip-flops, etc.) to build the required circuit. . They can also use programmable logic devices to complete the design. Almost all the physical structures of such devices have been fixed in the chip, and only some of the wires can be programmed by the user to determine their connection mode. The performance parameters related to these pre-designed logic cells are also usually provided by their suppliers to facilitate designers' timing and power analysis. The advantages of implementing a design on a semi-custom field programmable logic gate array (FPGA) are short development cycles and low cost.

Programmable logic device

Programmable logic devices are usually provided by semiconductor manufacturers with commercial chips. These chips can be connected to the computer through JTAG and other methods, so designers can use electronic design automation tools to complete the design, and then use the design code to program the logic chip. The programmable logic array chip has an array of logic gates defined in advance before leaving the factory, and the connection lines between the logic gates can be controlled to connect and disconnect through programming. With the development of technology, the programming of the connection line can be realized through EPROM (programming with higher piezoelectricity, erasing with ultraviolet radiation), EEPROM (programming and erasing using electrical signals multiple times), SRAM, flash memory, etc. The field programmable logic gate array is a special type of programmable logic device. Its physical basis is a configurable logic unit, which is composed of a look-up table, a programmable multiplexer, and a register. Lookup tables can be used to implement logical functions. For example, a lookup table at three inputs can implement all three variable logical functions.

ASIC

The advantages of application-specific integrated circuits (ASICs) designed for special applications are that area, power consumption, and timing can be maximized. Application-specific integrated circuits can only be manufactured after the entire integrated circuit design is completed, and the participation of a professional semiconductor factory is required. Application-specific integrated circuits can be based on standard cell libraries or fully custom designs. In the latter approach, designers have more control over the location and connection of components on the wafer. Unlike the programmable logic device approach, they can only choose to use some of these hardware resources, causing some resources to be wasted. The area, power consumption, and timing characteristics of ASICs can usually be better optimized. However, the design of ASICs is more complicated and requires a dedicated process manufacturing department (or outsourcing to a foundry) to manufacture the GDSII file into a circuit. Once the application-specific integrated circuit chip is manufactured, the logic functions of the circuit cannot be reconfigured like a programmable logic device. For a single product, the economic and time cost of implementing an integrated circuit on an application-specific integrated circuit is higher than that of a programmable logic device. Therefore, in the early design and debugging process, programmable logic devices, especially field-programmable logic gate arrays; If the integrated circuit is designed to be put into mass production at a later stage, mass production of application-specific integrated circuits will be more economical.

Integrated circuit design can be roughly divided into two categories: digital integrated circuit design and analog integrated circuit design. However, the actual integrated circuit may also be a mixed-signal integrated circuit, so many circuit designs use both processes.

Analog integrated circuit

Another major branch of integrated circuit design is analog integrated circuit design. This branch usually focuses on power integrated circuits, radio frequency integrated circuits, and so on. Because the real-world signals are analog, integrated circuits for analog-to-digital and digital-to-analog conversion are also widely used in electronic products. Analog integrated circuits include operational amplifiers, linear rectifiers, phase-locked loops, oscillator circuits, and active filters. Compared with digital integrated circuit design, analog integrated circuit design is more related to the physical properties of semiconductor devices, such as its gain, circuit matching, power dissipation, and impedance. Amplification and filtering of analog signals require a certain degree of fidelity to the signal, so analog integrated circuits use more large-area devices than digital integrated circuits, and their integration is relatively low.

Before the advent of microprocessors and computer-aided design methods, analog integrated circuits were completely artificially designed. Due to the limited ability of people to deal with complex problems, analog integrated circuits at that time were usually more basic circuits, and operational amplifier integrated circuits were a typical example. At the time, such integrated circuits might involve more than a dozen transistors and interconnections between them. In order to enable the design of analog integrated circuits to reach the level of industrial production, engineers need to take multiple iterations to test and troubleshoot. Reusing already designed and verified designs can further form more complex integrated circuits. Since the 1970s, the price of computers has gradually decreased. More and more engineers can use this modern tool to aid design. For example, if they use a compiled computer program to simulate, they can obtain higher calculations and designs than before Accuracy. SPICE is the first software for simulation of analog integrated circuits (in fact, the design of standard cells in digital integrated circuits also requires SPICE for parameter testing), which literally means "simulation programs focused on integrated circuits" (English: Simulation Program with Integrated Circuit Emphasis) Computer-aided design-based circuit simulation tools can adapt to more complex modern integrated circuits, especially application-specific integrated circuits. Using a computer for simulation can also enable some errors in project design to be discovered before the hardware is manufactured, thereby reducing the large cost caused by repeated testing and troubleshooting. In addition, computers are often capable of performing extremely complex and cumbersome tasks that humans are not capable of, making it possible such as Monte Carlo methods. The actual hardware circuit will encounter deviations that are inconsistent with the ideal situation, such as temperature deviations and deviations of semiconductor doping concentrations in the device. Computer simulation tools can also be used for simulation and processing. In short, computerized circuit design and simulation can make circuit design performance better, and its manufacturability can be more guaranteed. Nevertheless, compared to digital integrated circuits, the design of analog integrated circuits places stricter requirements on engineers' experience and ability to weigh contradictions.

Digital circuits

Roughly speaking, digital integrated circuits can be divided into the following basic steps: system definition, register transfer level design, physical design. According to the level of logic abstraction, the design is divided into system behavior level, register transfer level, and logic gate level. Designers need to properly write functional code, set up synthesis tools, verify logical timing performance, plan physical design strategies, and more. At a specific point in the design process, multiple inspections and debugs of logic functions, timing constraints, and design rules are required to ensure that the final design result meets the original design convergence goals.

System definition

System definition is the initial planning for integrated circuit design. At this stage, the designer needs to consider the macro functions of the system. Designers may use some high-level modeling languages and tools to complete the description of the hardware, such as C-level, C ++, SystemC, SystemVerilog and other transaction-level modeling languages, as well as tools such as Simulink and MATLAB to model signals. Although the mainstream is the register transfer level design as the center, there are some advanced synthesis (or behavioral level synthesis) and advanced verification tools that are directly converted from system level descriptions to low abstraction level descriptions (such as logic gate level structure descriptions) and advanced verification tools. progression stage. During the system definition phase, the designer also planned the performance indicators such as the process, power consumption, clock frequency, and operating temperature of the chip. ^[2]

With the ever-increasing complexity of VLSI circuits, the time and economic costs required for testing after circuit fabrication have also increased. In the past, most of the energy was devoted to the design itself, without considering the subsequent tests, because the tests at that time were relatively simple. In recent years, testing itself has gradually become a huge subject.

For example, it is relatively easy to control certain internal signals from outside the circuit so that they present a specific logic value, while some internal signals rely on a large number of other internal signals and it is difficult to directly change their values from the outside. In addition, the change of the internal signal can not be observed at the main output many times (sometimes the signal output of the main output seems to be correct, in fact, the internal state is wrong, just observing the output of the main output is not enough to judge whether the circuit is working properly). The above two types of problems, namely controllability and observability, are the two major components of testability.

People have gradually discovered that adding special structures (such as scan chains and built-in self-tests) to the circuit during the design of the circuit can greatly facilitate subsequent circuit testing. Such designs are known as testability designs. They make the circuit more complex, but they can reduce the cost of the entire project with simpler testing.

With the increasing integration of VLSI circuits and increasing market competition pressures, reusable design methodologies are gradually introduced in integrated circuit design. The main significance of reusable design methodology is that suppliers who provide IP cores (Intellectual Property Cores) can provide some pre-completed designs to the designer in the form of goods, which can use the IP core as a complete module in Use in your own design projects. Therefore, when implementing similar functions, each company does not need to repeatedly design similar modules. Although this will increase business costs, it also significantly reduces the complexity of the design, thereby shortening the cycle required for the company to design large circuits, thereby improving market competitiveness. The products provided by the IP core supplier may be verified hardware description language codes. In order to protect the supplier's intellectual property, these codes are often encrypted. The IP core itself is also designed as an integrated circuit, but in order to be used in different design projects, it will focus on enhancing its portability, so its design code specifications are more stringent. Some chip companies specialize in the development and sales of IP cores. ARM is a typical example. These companies make profits through the authorization of intellectual property rights.