VLSI design flow

The VLSI design flow is the sequence of stages through which a chip is designed, from the first idea to tape-out, the moment the finished design is sent to the factory. VLSI stands for very large scale integration: putting a very large number of transistors on one chip. A transistor is a tiny electrical switch, and a modern chip holds billions of them. The VLSI design flow is also called the chip design flow. For a large chip, it takes two to three years, and hundreds of engineers.

Here is how it works in general. Architecture comes first. The architects decide what the chip will do and how its parts fit together: how many processor cores, how much memory, and how fast. Next comes design. Engineers describe every circuit in a hardware description language, such as Verilog. This is text that describes what the hardware does, in the way code describes what a program does. Then comes verification. Engineers run the design in software, with millions of tests, to prove that it behaves correctly. This is often the largest part of the work. Then comes physical design. Software turns the description into a network of logic gates, small groups of transistors that each make one simple decision, such as AND or OR. It places those gates on the chip and draws the wires between them. Last come the final checks of timing, power and whether the factory can make the design, called sign-off. Then the design is taped out. Meanwhile, other engineers write the drivers, compilers and libraries that programmers will need to use the chip.

Every stage runs in electronic design automation (EDA) software, because no one could place billions of transistors by hand.

Why so much checking? After tape-out, the factory makes masks, glass plates that carry the chip's pattern. It then prints the pattern onto wafers over about three months. A wafer is a thin disc of silicon on which hundreds of chips are made at once. If a mistake is found in the finished chip, the design must be corrected, new masks made and the wafers made again. That can cost months, and tens or even hundreds of crores of rupees.

Take, for example, chip design verification. A team writes tests that send millions of random inputs into the design of a processor core, and compares every output with the expected answer.

Consider physical design and sign-off. Engineers check that every signal can cross its part of the chip within one tick of the chip's clock, often less than a billionth of a second, before the design is released.

This whole flow is the main work of a fabless chip company, which designs chips and pays a foundry to make them.

How is the VLSI design flow different from building software? A software team can correct a bug and release an update the same day, once its automated tests pass. A chip cannot be changed after it is made. So chip teams spend a far larger share of their time on verification before tape-out.