The best way to start work on VLSI is to know VLSI designing tools. VLSI has been around for a long time, but as a side effect of advances in the world of computers, there has been a dramatic proliferation of tools that can be used to design VLSI circuits. Also, obeying Moore’s law, the capability of an IC has increased exponentially over the years in terms of computing power, utilisation of available area and yield. The combined effect of these two factors is that designers can now put diverse functionality into an IC, opening up new frontiers. Examples are embedded systems, where intelligent devices are placed inside everyday objects, and ubiquitous computing, where small computing devices proliferate to such an extent that even the shoes you wear may actually do something useful like monitoring your heartbeats!
How to deal with VLSI circuits?
Keep in mind that digital VLSI circuits are predominantly CMOS-based. So the way normal blocks like latches and gates are implemented, is different from what you have seen before. However, the behaviour remains the same. All the miniaturisation involves new things to consider. A lot of thought has to go into actual implementation as well as design.
Let us look at some of the external factors responsible for the functionality of the circuit—large complicated circuits running at very high frequencies have to tackle the delays in propagation of signals through gates and wires. This is a big problem even for areas a few micrometres across. The operational speed is so large that as the delays add up, these can actually become comparable to clock speeds.
Another effect of high operation frequencies is increased consumption of power. This has two-fold effect: devices drain batteries faster, and heat dissipation increases. Coupled with the fact that surface areas decrease, heat poses a major threat to the stability of the circuit itself.
Laying out the circuit components is a task common to all branches of electronics. The speciality in VLSI layout is number of possible ways available to do this; there can be multiple layers of different materials on the same silicon, there can be different arrangements of the smaller parts for the same component and so on.
The power dissipation and speed in a circuit present a trade-off; if you try to optimise on one, the other is affected. The choice between the two is determined by the way you choose to layout the circuit components. Layout can also affect the fabrication of VLSI chips, making it either easy or difficult to implement the components on the silicon.
How to proceed with designs?
Know the fundamentals of both analogue and digital design processes. You may start with a typical design flow of any one of them. For example, a digital design flow to get the end product is: specification→architecture→RTL cod-ing→RTL verification→synthesis→ backend→tape out to foundry. Here the end product is a wafer with repeated number of identical ICs.
Today, all digital designs start with a designer writing a hardware description of the IC (using hardware description language, or HDL) in Verilog/VHDL. A Verilog or VHDL program essentially describes the hardware (logic gates, flip-flops, counters, etc), the functionality and circuit block interconnects. Various CAD tools are available to synthesise a circuit based on the HDL. The most widely used synthesis tools come from two CAD companies: Synposys and Cadence.
Without going into details, you may consider VHDL as ‘C’ of the VLSI industry. VHDL stands for ‘VHSIC hardware definition language,’ where VHSIC stands for ‘very high-speed integrated circuit.’ This language is used to design the circuits at a high level, in two ways. It can either be a behavioural description, which describes what the circuit is supposed to do, or a struc tural description, which describes what the circuit is made of. There are other languages for describing circuits, such as Verilog, which work in a similar fashion. Both forms of description are then used to generate a very low-level description that actually spells out how all this is to be fabricated on the silicon chips. This results in the manufacture of the intended IC.
For an analogue design, the flow may vary to some extent, like specifications→architecture→circuit design→ SPICE simulation→layout→parametric extraction/back annotation→final design→tape out to foundry.
[stextbox id=”info”]There are a number of directions you can take while choosing a career in VLsi, and they are closely related to each other. so be a go-getter, dig all the possibilities to gain a practical exposure.[/stextbox]