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U. Glaeser

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21.6 Driver Comparison<br />

In clock-powered logic, combinational logic blocks begin to switch as soon as clock-powered nodes are<br />

charged to V th. For example, assume that a clock-powered signal drives a pulse-to-level converter. The<br />

converter starts operating as soon as the clock-powered signal voltage passes the threshold voltage V th.<br />

Therefore, the switching time for charging the loads of clock-powered nodes to V ϕ is not as important<br />

as the delay for converting pulses to levels. With clock-powered logic, it is possible to overlap the time<br />

required for charge (and energy) recovery time with the computation time of the logic block. It is also<br />

possible, to a lesser degree, to overlap some of the charging time. The latter depends on many factors<br />

including clock and dc voltage levels, logic styles, and the CMOS technology.<br />

In other E-R approaches (e.g., reversible logic [3,9], retractile cascade logic [27], and partially adiabatic<br />

logic families [10–16]), the signal switching time is important because the inputs of a logic block must<br />

be fully charged before the block starts operating. Furthermore, as in conventional CMOS circuits, voltage<br />

scaling is possible for clock-powered nodes at the expense of increased circuit latencies.<br />

To investigate the effectiveness and the scalability of clock-powered logic, a simulation experiment was<br />

conducted to compare the driver stage of the E-R latch, i.e., the clocked buffer to a conventional driver.<br />

The two circuits were evaluated for energy versus delay and energy-delay product (EDP) versus voltage<br />

scaling.<br />

Experimental Setup<br />

The goal of the experiment was to compare the clock-powered approach for driving high-capacitance<br />

nodes with a conventional, low-power approach. Because it is impractical to compare clock-powered logic<br />

against all low-power conventional techniques, a dual-supply-voltage approach in which high-capacitance<br />

nodes are charged to a lower voltage V ddL than the rest of the nodes was chosen. The dual-supply-voltage<br />

approach is similar to the clock-powered approach in that it attempts to reduce power dissipation in<br />

the high-capacitance nodes. Furthermore, like the clock-powered approach, the dual-supply-voltage<br />

approach requires that low-supply-voltage signals be converted to high-supply-voltage signals before they<br />

are fed to high-supply-voltage gates. Otherwise, these gates would suffer from short-circuit current, or<br />

may not work at all, depending on the two supply voltage levels, the logic style, and the technology process.<br />

The dual-rail-input, static pulse-to-level converter (DS P2LC—Fig. 21.11(b)) is a converter circuit that<br />

operates simply with both approaches (Fig. 21.16). Two 150 fF capacitive loads were added to the converter<br />

FIGURE 21.16 Clocked buffer (a) and conventional drivers (b) connected to 150 fF capacitance loads and a DD<br />

P2LC.<br />

© 2002 by CRC Press LLC<br />

D in<br />

ϕ D<br />

CB<br />

x p ∧ϕ D<br />

VddL Din D<br />

x l<br />

150fF<br />

x p<br />

x l<br />

150fF<br />

x l<br />

150fF<br />

(a)<br />

(b)<br />

x p ∧ϕ D<br />

x l<br />

150fF<br />

x p<br />

V ϕ<br />

0<br />

ϕ D<br />

VddL Din D<br />

T s<br />

T h<br />

T w<br />

T s

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