U. Glaeser

FIGURE 14.3
today’s design trends are such that the variability in the amount of current drawn from cycle to cycle is
also increasing. These factors combine to make resistive and inductive noise a big problem. Dealing with
this is becoming increasingly costly.
Figure 14.3 gives an idea of the range of dollar amounts associated with the above costs for different
system components in a personal computer. As can be seen, when the CPU power is in the 35–40 W
range, the cost of each additional watt goes above $1 per watt per chip. An interesting observation is that
the power cost of the other system components (DRAM, chipsets, graphics) is on a steeper curve than
the CPU. This is because the spatial layout of today’s system chassis designs is such that these components
are harder to cool. This is likely to change with new designs, further increasing the relative importance
of the CPU power cost. Given the above trends, there is a clear need to analyze and optimize power
consumption for all components of a given system.
This chapter is organized as follows. The need for reducing power consumption in systems is motivated
in greater detail in the section on “The Need for Low Power.” The sources of power consumption are
reviewed in “Sources of Power Consumption,” where a basic power model for complementary metal
oxide semiconductor (CMOS) circuits is presented. “Reducing Power Consumption” provides a overview
of basic power reduction methods. Section 14.2 deals with power estimation, which is a prerequisite for
power aware design. Section 14.3 describes power reduction techniques at various levels of the design
process in greater detail.
The Need for Low Power
The different factors that have led to the increasing importance of the power metric are described in the
following subsections.
Heat Dissipation
The power consumed in an IC is dissipated as heat. Unless this heat is removed, the IC gets hot. The
electrical properties of the devices on the chip are rated for specific temperature ranges, and exceeding
the ranges shifts the parameters and the behavior of circuits. In addition, as an IC gets hot, catastrophic
failure mechanisms become more likely. These include silicon interconnect and junction fatigue, package
failure, thermal runaway, and gate dielectric breakdown. It has been estimated that each 10°
C rise in
operating temperature roughly doubles the component failure rate.
The power consumption in today’s ICs has reached the point that expensive packaging and cooling
mechanisms are needed to keep the operating temperature in check. The peak sustained power consumption
in recent high-performance microprocessors is in excess of 70 W, as shown in Fig 14.1 and Table 14.1.
Packaging and cooling costs are of direct concern even in the domain of lower performance and low
power microprocessors and microcontrollers. The high volume nature of the market for these devices
means that even slight reductions in cost can translate into large revenue increases for the manufacturer.
© 2002 by CRC Press LLC
Total Integration Cost ($)
40
30
20
DRAM
10
Chipset
Graphics
$1/W
0 10 20 30 40
Power (Watts)
Cost of delivering power and cooling in a PC system.
CPU