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U. Glaeser

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FIGURE 19.11<br />

FIGURE 19.12<br />

Figure 19.11 describes the gated-clock scheme, which is very popular, and is the basic scheme to reduce<br />

power consumption. Activation of clock for target flip-flops is controlled by enable signal that is asserted<br />

only when needed. The latch of Fig. 19.4 prevents clock from glitch. This scheme is used to inactivate<br />

blocks or units when they are not used. Unless clocks are controlled on demand, all clock lines and inside<br />

of flip-flops are toggled and also unnecessary data propagate into circuit units through flip-flops, which<br />

causes large waste in power all over the chip. The gated-clock used to be handled manually by designer;<br />

today, however, it can be generated automatically in gate compilation and also static timing analysis can<br />

be applied without special care at latch by EDA tool. This means the gated-clock has become a very<br />

common and important scheme.<br />

The operating voltage is conventionally fixed at the standard voltage like 5 V or 3.3 V. But when the<br />

system runs at multiple performance requirements, frequency can be varied to meet each performance.<br />

At this time, the operating voltage can be also changed to the minimum to attain that frequency. The<br />

power consumption is a quadratic function of voltage, therefore to control voltage has a very big impact<br />

and is quite an effective method of power reduction. Figure 19.12 shows an effect of scaling with frequency<br />

and voltage. Scaling with only frequency reduces power consumption in just proportion to frequency.<br />

On the other hand, scaling with frequency and voltage achieves drastic power saving because of quadratic<br />

effect of voltage reduction. It is really important to handle the voltage as a design parameter and not as<br />

© 2002 by CRC Press LLC<br />

enable<br />

clock<br />

Gated-clock.<br />

Normalized Power<br />

clock<br />

enable<br />

gated-clock<br />

1.0<br />

0.9<br />

0.8<br />

0.7<br />

0.6<br />

0.5<br />

0.4<br />

0.3<br />

0.2<br />

0.1<br />

0.0<br />

Frequency and voltage scaling.<br />

D<br />

G<br />

latch<br />

Q<br />

gated-clock<br />

activated deactivated<br />

with frequency<br />

scaling only<br />

target flip-flops<br />

DFF<br />

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0<br />

Performance (Frequency)<br />

with frequency and<br />

voltage scaling

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