Digital Systems and VLSI
1.1 Why Design Integrated Circuits?
This book describes design methods for integrated circuits. That may seem like a specialized topic. But, in fact, integrated circuit (IC) technology is the enabling technology for a whole host of innovative devices and systems that have changed the way we live. Jack Kilby and Robert Noyce received the 2000 Nobel Prize in Physics for their invention of the integrated circuit; without the integrated circuit, neither transistors nor computers would be as important as they are today. VLSI systems are much smaller and consume less power than the discrete components used to build electronic systems before the 1960s. Integration allows us to build systems with many more transistors, allowing much more computing power to be applied to solving a problem. Integrated circuits are also much easier to design and manufacture and are more reliable than discrete systems; that makes it possible to develop special-purpose systems that are more efficient than general-purpose computers for the task at hand.
applications of VLSI
Electronic systems now perform a wide variety of tasks in daily life. Electronic systems in some cases have replaced mechanisms that operated mechanically, hydraulically, or by other means; electronics are usually smaller, more flexible, and easier to service. In other cases electronic systems have created totally new applications. Electronic systems perform a variety of tasks, some of them visible, some more hidden:
- Personal entertainment systems such as portable MP3 players and DVD players perform sophisticated algorithms with remarkably little energy.
- Electronic systems in cars operate stereo systems and displays; they also control fuel injection systems, adjust suspensions to varying terrain, and perform the control functions required for anti-lock braking (ABS) systems.
- Digital electronics compress and decompress video, even at high-definition data rates, on-the-fly in consumer electronics.
- Low-cost terminals for Web browsing still require sophisticated electronics, despite their dedicated function.
- Personal computers and workstations provide word-processing, financial analysis, and games. Computers include both central processing units (CPUs) and special-purpose hardware for disk access, faster screen display, etc.
- Medical electronic systems measure bodily functions and perform complex processing algorithms to warn about unusual conditions. The availability of these complex systems, far from overwhelming consumers, only creates demand for even more complex systems.
The growing sophistication of applications continually pushes the design and manufacturing of integrated circuits and electronic systems to new levels of complexity. And perhaps the most amazing characteristic of this collection of systems is its variety—as systems become more complex, we build not a few general-purpose computers but an ever wider range of special-purpose systems. Our ability to do so is a testament to our growing mastery of both integrated circuit manufacturing and design, but the increasing demands of customers continue to test the limits of design and manufacturing.
advantages of VLSI
While we will concentrate on integrated circuits in this book, the properties of integrated circuits—what we can and cannot efficiently put in an integrated circuit—largely determine the architecture of the entire system. Integrated circuits improve system characteristics in several critical ways. ICs have three key advantages over digital circuits built from discrete components:
- Size. Integrated circuits are much smaller—both transistors and wires are shrunk to micrometer sizes, compared to the millimeter or centimeter scales of discrete components. Small size leads to advantages in speed and power consumption, since smaller components have smaller parasitic resistances, capacitances, and inductances.
- Speed. Signals can be switched between logic 0 and logic 1 much quicker within a chip than they can between chips. Communication within a chip can occur hundreds of times faster than communication between chips on a printed circuit board. The high speed of circuits on-chip is due to their small size—smaller components and wires have smaller parasitic capacitances to slow down the signal.
- Power consumption. Logic operations within a chip also take much less power. Once again, lower power consumption is largely due to the small size of circuits on the chip—smaller parasitic capacitances and resistances require less power to drive them.
VLSI and systems
These advantages of integrated circuits translate into advantages at the system level:
- Smaller physical size. Smallness is often an advantage in itself—consider portable televisions or handheld cellular telephones.
- Lower power consumption. Replacing a handful of standard parts with a single chip reduces total power consumption. Reducing power consumption has a ripple effect on the rest of the system: a smaller, cheaper power supply can be used; since less power consumption means less heat, a fan may no longer be necessary; a simpler cabinet with less shielding for electromagnetic shielding may be feasible, too.
- Reduced cost. Reducing the number of components, the power supply requirements, cabinet costs, and so on, will inevitably reduce system cost. The ripple effect of integration is such that the cost of a system built from custom ICs can be less, even though the individual ICs cost more than the standard parts they replace.
Understanding why integrated circuit technology has such profound influence on the design of digital systems requires understanding both the technology of IC manufacturing and the economics of ICs and digital systems.