What are Application Specific ICs?

The electronics industry has always focused on miniaturization. This miniaturization was driven by the desire to integrate maximum logic in minimum dimensions. Still, in 2021, the industry is not prepared for the end of Moore’s law, despite a gradual rise in the R&D costs to continue along this path.

The ability of transistor logic to implement computing in micro-scale chips very first lead to the development of CPUs. It was not late that CPUs got bundled with other peripherals as MPUs and microcontrollers. While MPUs continue to cater the general-purpose computing, microcontrollers were focused on application-specific and industry-oriented market segments. These market segments forced the semiconductor manufacturers to develop more specific and performant designs in the form of Application Specific Integrated Circuits (ASICs) and FPGAs among the digital ICs.

According to the latest statistics, general-purpose computing accounts for only 21% of the semiconductor market share. In contrast, the other market segments like wireless communication, industrial electronics, consumer electronics, automotive electronics, and others account for the rest. While the memory and logic ICs market saw the highest revenues around $170,000 million and $100,000 million respectively in 2018, application specific logic stands at the third spot with revenues around $95,000 million. The microprocessor’s revenue was half of that, and of the microcontrollers, not even one-third compared to ASICs.

Apart from embedded control, ASICs have newly found use in applications like AI inference, high-speed search, and vision & image processing. In embedded systems, CPUs and MCUs lead in low-cost, low-power mobile applications. ASICs are still dominant in non-consumer applications. The ASICs are particularly finding a spot in high-volume vision and image-processing applications.

With a market share greater than microprocessors and microcontrollers, ASICs seem to be really important. Let us see what ASICs actually are!

What is an ASIC?

Integrated circuits can be designed to either cater to many applications or to serve a single application. The digital integrated circuits designed to fulfill a particular application are called Application-Specific Integrated Circuits (ASICs). From the root level, these ICs are tailored to implement a particular use case. These chips usually accommodate many functional units like processors, communication peripherals, memory, etc., all densely packed on a single chip. These types of ASICs are called System on Chip (SoC). The ASICs have a high cost and are best suitable for large-scale applications. Usually, the ASICs are designed to fit into a particular product.

Different types of ASICs

In terms of computing, ASICs are actually not programmable chips. However, these can be customized by the user to best suit the respective use case. The customization is done using an Electronic Design Automation (EDA) tool, and the chip design is specified by the user using a Hardware Description Language (HDL). The level of customization determines the type of ASIC. The ASICs can be one of the following types –

Full custom
Semi-custom
Programmable

Full-custom ASICs

All the logic cells are designed for the specific application with a fixed circuit layout in a full-custom ASIC. A programmer cannot change the interconnections of the chip. Such designs are useful for applications like memory chips, sensors and transducers, power management ICs, analog processors, and microprocessors. The advantage of using a full-custom ASIC is the highest possible performance in the smallest chip size. However, it is all in place of complex design, longer time-to-market, and high cost. That is why these ASICs are designed for only high-level and high-volume applications.

Semi-custom ASICs

In these types of ASICs, logic cells are not fully customized. They are taken from standard libraries, and interconnections between them are both customized as well as standard. Compared to full-custom ASIC, where the design may have been done at the transistor level, the lowest design involved is a logic or gate in semi-custom ASIC. This approach is useful in reducing the time and cost involved in designing a full-custom ASIC. Depending upon the design approach, semi-custom ASICs can be of two types:

Standard Cell-based ASIC – These ASICs are designed using standard cell libraries. The logic cells like logic gates, multiplexers, demultiplexer, registers, etc., are predesigned taken from a cell library. The logic cells can be even microprocessors or microcontrollers. The logic cells of this size are called mega cells. All the logic cells are arranged in rows where the programmer can customize the position of the blocks on customized mask layers.
Gate Array-based ASIC – In these ASICs, the logic cells have a fixed placement on the chip, and the designer can change interconnections between the blocks using masked layers. The base cells are transistors that are repeated to form a base array. This defines a pattern of gate arrays interconnected by the designer to complete the IC design. Gate Array ICs are of three types – Channeled Gate Array, Channel-less Gate Array, and Structured Gate Array.

Programmable ASICs

These ASICs are generally used for prototyping designs. A designer can fully customize the design programmatically. The programmable ASICs are of two types:

Programmable Logic Devices (PLD) – These ASICs have no customized logic cell or interconnects. There is a regular matrix of logic cells that are programmable using the software. The designer can customize the blocks as well as define interconnects. This allows producing a design specific to a particular application. PLDs themselves serve as standard cells and help in a fast design. Some good examples of PLD are PROM and EPROM.
Field Programmable Gate Array (FPGA) – FPGA consists of programmable gate arrays surrounded by programmable interconnects. The gate arrays serve as programmable logic cells that can be customized to have combinational or sequential logic. The surrounding interconnects can also be programmed using some methods. FPGAs are useful for an unbelievable fast design that may complete in just a few hours.

Types of Gate Array-based ASICs

There are three types of Gate Array-based ASICs:

Channeled Gate Array – In channeled gate arrays, the logic blocks of fixed height are arranged in arrays with space between them for interconnections.
Channel-less Gate Array – In channel-less gate arrays, there is no space left between the rows of logic blocks. Instead, routing is done over the logic cells by placing the transistors in the path unused by the interconnects.
Structured Gate Array – In structured gate arrays, some space on die is left reserved for fixed, embedded blocks, and the rest of the gate array blocks are available for routing. The use of embedded blocks reduces the time to design, though it may not result in the best-optimized design.

Conclusion

ASICs usually involve chip design reusing high-level components. These components have gate-level sophistication of registers, counters, multiplexers, floating-point operation units, and arithmetic logic units. The high-level components are usually available as Intellectual Property (IP) Cores. The design company holds the copyright for IP cores involved in designing an ASIC. Various blocks are interconnected according to the customer’s design, and the customer has to pay a royalty to the design company for every chip shipped.

ASICs have a large market share in the semiconductor industry. Even if not general-purpose computing, ASICs serve various other market segments along with high-level components in embedded control. ASICs are designed, tailored, and produced for a specific use case and fit directly to a specific end product.