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U. Glaeser

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The voltage converter required for DVS is fundamentally different from a standard voltage regulator<br />

because in addition to regulating voltage for a given fCLK,<br />

it must also change the operating voltage when<br />

a new fCLK<br />

is requested. To minimize the speed and energy consumption of this voltage transition, a small<br />

output capacitor on the converter is desirable, in contrast to the supply ripple requirements.<br />

Thus, the fundamental trade-off in a DVS system is between good voltage regulation and fast/efficient<br />

dynamic voltage conversion. As will be shown in the “Voltage Converter” section, it is possible to optimize<br />

the size of this capacitor to balance the requirements for good voltage regulation with the requirements<br />

for a good dynamic voltage conversion.<br />

Scalability with Technology<br />

Although the prototype system described next demonstrates DVS in a 3.3 V, 0.6 µ m process technology,<br />

DVS is a viable technique for improving processor system energy efficiency well into deep sub-micron<br />

process technologies. Maximum VDD<br />

decreases with advancing process technology, seeming to reduce the<br />

potential of DVS, but this decrease is alleviated by decreases in the device threshold voltage, VT.<br />

While the<br />

maximum VDD<br />

may be only 1.2 V in a 0.10 µ m process technology, the VT<br />

will be approximately 0.35 V<br />

2 2<br />

yielding an achievable energy efficiency improvement, , still in excess of a tenfold increase.<br />

17.4 A Custom DVS Processor System<br />

DVS has been demonstrated on a complete embedded processor system, consisting of a microprocessor,<br />

external SRAM chips, and an I/O interface chip [9]. Running on the hardware is a preemptive, multitasking,<br />

real-time operating system, which supports DVS via a modular component called the voltage<br />

scheduler. Benchmark programs, typical of software that runs on portable devices, were then used to<br />

quantify the improvement in energy efficiency possible with DVS on real programs.<br />

System Architecture<br />

As shown in Fig. 17.6, this prototype system contains four custom chips in a 0.6 µ m 3-metal VT<br />

≈ 1 V<br />

CMOS process: a battery-powered DC-DC voltage converter, a microprocessor, SRAM memory chips, and<br />

an interface chip for connecting to commercial I/O devices. The entire system can operate at 1.2–3.8 V<br />

and 5–80 MHz, while the energy/operation varies from 0.54 to 5.6 mW/MIP.<br />

The prototype processor, which contains a custom implementation of an ARM8 processor core [10],<br />

is a fully functional microprocessor for portable systems. The design contains a multitude of different<br />

FIGURE 17.6<br />

© 2002 by CRC Press LLC<br />

F DES<br />

ARM8<br />

Core<br />

System<br />

Co-processor<br />

f CLK<br />

VCO<br />

Converter<br />

16kB<br />

Cache<br />

Write<br />

Buffer<br />

VDD<br />

CPU<br />

Bus Interface<br />

V BAT<br />

Prototype DVS processor system architecture.<br />

VDD/V T<br />

System<br />

Bus<br />

64 kB<br />

SRAM<br />

1<br />

0.5 MB<br />

2<br />

8<br />

I/O<br />

Chip<br />

3.3 V<br />

Commercial<br />

Peripheral Devices

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