Introductory Chapter: ASIC Technologies and Design Techniques

1. Introduction

This chapter aims to introduce the field of application specific integrated circuits (ASICs) and provide a basic reference list of crucial design techniques for the interested student. Modern designs using complementary metal oxide semiconductor (CMOS) technologies are at the forefront of nanoscale mass fabrication with recent CMOS processing nodes pushing towards 7 nm feature sizes. While the fabrication of structures on Silicon, Germanium, Gallium Arsenide and other substrates has advanced, so too have the systems we create [1, 2]. The rapidly increasing complexity inherent in these systems – be they digital or analogue in nature – offers its own challenges for engineers. This book therefore offers a small insight into this exciting field.

Modern silicon design plays a key part in our global, highly interconnected economy and has truly allowed society to advance on several fronts.

• We now have sensors and interfaces to the real world that digitise high-speed analog signals into easily manipulated digital signals at staggeringly high sample rates, well into the GHz range [3]. Modern analog-to-digital converters (ADCs) when combined with precision, low-noise operational amplifiers (op-amps) can accurately measure nano-volt amplitude signals and can separate signals from interfering background noise using a variety of signal processing techniques. The field’s prowess in analog ASICs is well exemplified by sensors and actuators that can interface directly with human brain tissue.

• Custom and general use microprocessors [4] have been a shining light of progress on complex integrated digital systems and now offer us continued growth for our increasingly complex computational needs. The improvements in super-computing are a testament to our ongoing, field-wide interest in increasing the floating-point operations per second, while recent work aims to tackle the energy required per computation. By combining high-performance, low-power ASICs and novel computational architectures (systolic arrays, array processing, graphics processing units, etc.), super-computers and data-centres are now able to provide enough resource for a variety of high-dimensional simulation problems. So powerful are modern central processing units (CPUs) that even embedded processors can run highly-nested (i.e. deep) neural networks of many thousands of complex artificial neurons, which has directly enabled the rise of machine learning (ML), deep learning (DL) and artificial intelligence (AI). Likewise, that same computation with low-energy and high-integration progress allows many of us to have a computer within our pocket that outstrips the military and commercial computers of the 80s.

• Integrated, solid-state memory circuits [5] continue to become smaller, of higher access speed, of higher long-term reliability and of lower power, now replacing traditional hard disk drives in many front-line computing tasks. This has allowed society to store vast quantities of data and has lead – with ease of access and computation – to the rise of big data analytics and an increased interest in statistical and data-guided adaptive signal processing.

• Rapid strides in digital image sensor designs [6] now allow individual pixels of less than 900 nm in width (very close to the diffraction limit), arrays of hundreds of megapixels, sensors able to be used in 645 medium format camera systems and even the counting and timing of single-photons with sub-10 pico-second timing resolutions. Digital image sensors using various semiconductors, substrates and readout methods can image at incredibly high frame rates – well into the millions of frames/s – and over a very large portion of the electromagnetic spectrum. Telescopes such as Hubble and James Webb use these image sensor ASICs to investigate the cosmos, expanding our scientific knowledge, detecting planets orbiting distant stars and showing the innate artistry of nature. And – if this wasn’t enough – digital image sensors for machine vision can be coupled with complex digital signal processing ASICs and CPUs/GPUs running neural networks to provide automatic pedestrian detection and avoidance for self-driving cars.

But the progress we have observed in each of the above sub-fields from the 1970s to present has required many difficult issues to be tackled and presents a highly interesting and challenging career for those that are interested in the design details and techniques that are required. In this chapter, Section 2 breaks the ASIC arena down into a set of sub-fields providing some key texts for each. While a little more emphasis is placed on digital circuits and CMOS optical detectors and image sensors, this is a consequence of this author’s interests. To this end, Section 3 discusses some of the digital ASIC and programmable logic device hardware description languages (HDLs) that are now routinely used to develop complex systems. Finally, Section 4 discusses three key industry standards – ISO-9001, ISO-26262and DO-254– which emphasise the robustness and design formalism required in the industry. We also discuss the universal design methodology (UDM), which alongside modern ASIC verification standards such as the open verification methodology (OVM) and the universal verification methodology (UVM), introduce some of the key ways in which designers of ASICs can handle increasing design complexity.

2. Suggested references and reading

In this section, a wide selection of texts is references that provide significant context to the design issues inherent within ASIC technologies. The introductory text by Huber [7] provides an insight into how systems were designed during the 1990s. While this does not cover the detail of theory required for design, it introduces us to the manner, methods and tools used within the field. Generally, the below set of resources is split into the overall themes of the field: (i) planar silicon processes and solid-state physics, (ii) analogue transistors and systems, (iii) digital circuit design, and finally, (iv) sensors and interfaces to the real macro world in which we utilise these devices.

3. Suggested hardware description languages

While the ASICs of the past were typically drawn by hand – a highly laborious task with a high-risk factor – modern ASICs are almost exclusively developed using complex computer aided design (CAD) packages. The advantages of this are clear, first, that designs can become significantly more complex in a scalable, well maintainable and modular manner and secondly that transient, temporal, thermal, frequency domain, power, parasitic circuit elements and complex second and third order effects can be added in a manner that would not be possible from a hand calculation perspective. CAD packages allow designs to be verified against multiple specifications. Likewise, simulations based upon reliable fitted models can be used to provide such a wide range of correct and erroneous stimuli that we can obtain 99.9% test coverage and a very high expectation of full functionality prior to silicon prototyping. It is possible to run a design against multiple design corners such as low/high power, low/high temperature and fast/slow transistors, allowing simulation to capture a high proportion of likely external factors and combinations thereof.

For analog, MEMs and photonic ICs, the user typically defines the lengths and widths of transistors, silicon features (resonant beams, masses, etc.) or waveguide structures using established theoretical techniques and formulae, and then iterates around this design as the tools iteratively increase the complexities added to the base models.

Digital design now exclusively uses hardware description languages. These were initially used to describe and model digital circuits, however modern ASIC design houses use a HDL to silicon synthesis process whereby the language describes a set of logical gates and sequential (clocked) registers that are synthesised – often via complex optimisation processes – into physical logic gates, registers and interconnections that are provided as standard cells by the CMOS foundry. This type of design entry is often called the register transfer level (RTL). The standard cells are themselves designed using the analog design flows and complex models to provide known digital performance (setup, hold and propagation times, etc.). There are two crucial HDLs used within the field. The first is Verilog [23], while the second is a very high speed integrated circuit (VHSIC) HDL, called VHDL [24]. Typically, Verilog is used for hardware descriptions that are synthesised to Silicon, while VHDL is typically synthesised to programmable devices such as FPGAs [21] and complex programmable logic devices (CPLDs). This split is however a rather grey area with a great deal of company preference. These description languages should therefore be treated as complementary.

While Verilog and VHDL have structures that comprehensively cover design concepts and the required complexity, languages such as SystemVerilog [25] and SystemC are used to provide functional verification of digital logic designs. The ASIC field would therefore write a SystemVerilog testbench that simulates a wide variety of stimuli for a digital module written in Verilog or VHDL. By doing so, we can be assured of the block’s functional or logical design before we progress to more complex timing verification steps. As designs have become more complex, so have the testbenches upon which we assess a design’s verifiable functionality and use case or test coverage. To handle such complexity, the digital ASIC field has shifted to a higher-level abstraction. It is here we have introduced the universal verification methodology (UVM) [26] which treats the verification of a block using a transaction-based system. The block or design-under-test (DUT) is provided with transactions that are monitored using a secondary process, with the DUT then being given a score based upon the successful completion of that transaction. For example, a two-input 16-bit adder could be given two series of random values. These would be passed through the DUT, while the monitor would compare the DUT output values with the deterministic, design-independent addition.

4. Standards and the universal design methodology (UDM)

As part of any introduction to design methods, practices and theory, we as designers must also pay attention to industry standards. While this is perhaps not given sufficient emphasis within academia, certain aspects of commercial engineering practice need to be used independent of a student’s career goals. We should emphasise that students should read up on industry standards such as ISO9001, ISO-26262or DO-254as these will be used by the clear majority of technology companies and do in fact benefit academic projects.

4.2. Universal design methodology

While the universal design methodology (UDM) [30] is often applied to FPGA-based hardware design, it is applicable to a wide range of electronics projects including both analog and digital ASIC designs. The power of the UDM model is its ability to fit well with the existing structures and processes of ISO-9001, while also enforcing a robust formalism in the design process. The pathway within UDM, shown in Figure 1, includes multiple opportunities for the design to be specified and constrained in a top down manner, while also providing suitable feedback loops to ensure designs meet all specifications without undue over-engineering. UDM also includes multiple signing off and review processes, each with the purpose of finalising, verifying and capturing issues as early as possible as the design moves towards implementation and shipping. In this way, once the design is passed to physical (gate-level) implementation, i.e. synthesis to Silicon or even custom analog layout, designers should never encounter a new requirement to add a separate reset line, a new state within an initialisation or control state machine or modify the block’s frequency domain specification.

UDM contains a significant emphasis on design verification at all stages. For example, the functional simulation of a HDL coded design is used with the design specification to ensure the design both captures all design intent within the specifications and is verifiably functionally correct and in agreement with the agreed (via review) list of specifications. Likewise, once the HDL design is synthesised to Silicon, it is formally verified via gate-level simulation or emulation on a programmable logic platform, to ensure that it meets all timing requirements and that no issues are present using a constrained random approach to testing. Part of the verification task is to demonstrate functional equivalence between the design simulated, verified and agreed upon in previous stages with the computer optimised design that was synthesised. The final review within UDM is crucial as it is at this point that all involved in the design, from marketing and product management right down to hardware and software engineers sign-off the design and prepare for manufacture and final testing of the product. UDM can therefore be instrumental in achieving agreement between professionals of highly disparate backgrounds and provides a mechanism whereby a bit-level hardware design has a proven track record and verifiable performance against a set of high-level requirements.

It should be noted that ASIC design tool vendors such as Cadence and Synopsis include many sign-off checks that complement the sign-off and review steps in UDM. For example, design rule checks (DRC) and layout-versus-schematic (LVS) checks were developed such that a design is not passed to manufacture or any other stage until verifiably passed and the “green light” given. Many CMOS foundries will not fabricate a design or require extensive waivers before allowing the tape-out of an IC with known DRC errors.

5. Conclusions

The design of digital, analog, and sensing ASICs is an exciting but complex field. The references and texts provided are intended as an introductory reading list. Of course, once a student has an idea of a sub-specialty they wish to pursue, they can delve far more deeply. Alongside introducing the different areas within ASIC design, some key industry standards are discussed. Students are encouraged to engage with ISO-9001 as early as possible. This is simply prudent planning for a career within robustly managed engineering projects. Finally, a design flow called the universal design methodology was briefly covered, which emphases the need for agreement as to requirements and scope, along with verification of the design throughout the lifecycle of a complex design.