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U. Glaeser

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FIGURE 17.2 Throughput and energy/operation for three design approaches: (a) compute ASAP (b) clock frequency<br />

reduction, and (c) supply voltage reduction.<br />

FIGURE 17.3<br />

Supply Voltage Reduction<br />

When fCLK<br />

is reduced, the processor’s circuits have a longer cycle time to complete their computation in.<br />

In CMOS, the common fabrication technology for most processors today, the delay of the circuits<br />

increases as the supply voltage, VDD,<br />

decreases. Thus, with voltage scaling, which reduces VDD,<br />

the circuits<br />

can be slowed down until they just complete within the longer cycle time. This, in turn, will reduce the<br />

energy/operation, which is a quadratic function of VDD,<br />

as shown in Fig. 17.2(c).<br />

Figure 17.3 demonstrates that the throughput and energy/operation can vary more than tenfold over<br />

the range of VDD.<br />

The curves are derived from analytical sub-micron CMOS device models [6]. Because<br />

throughput and energy/operation roughly track each other, reducing VDD<br />

maintains approximately constant<br />

ETR, providing equivalent energy efficiency to the Compute ASAP approach. Thus, lower energy/<br />

operation can be achieved, but at the sacrifice of lower peak throughput.<br />

© 2002 by CRC Press LLC<br />

Delivered Throughput, Energy/operation<br />

Normalized Values<br />

1<br />

0.5<br />

(a) Compute ASAP<br />

Desired<br />

throughput<br />

(b) Clock frequency reduction<br />

(c) Supply voltage reduction<br />

Throughput<br />

Throughput and energy/operation vs. supply voltage.<br />

Delivered throughput:<br />

Energy/operation:<br />

Energy/operation<br />

Time<br />

Time<br />

Time<br />

0<br />

1 2 3 4<br />

VDD (VT )

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