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U. Glaeser

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FIGURE 21.4 An all-resonant, dual-rail LC oscillator used as a clock driver ([18] © 1996 IEEE).<br />

FIGURE 21.5 A scope trace of the almost nonoverlapping, two-phase clock waveforms ([28] © 1997 IEEE).<br />

The main advantage of resonant clock drivers is their high energy efficiency since, for all-resonant<br />

configurations, the energy dissipation for driving the clock loads can scale as the inverse of the switching<br />

time. Nevertheless, these all-resonant configurations pose design challenges when frequency stability is<br />

important. Their frequency and, therefore, the system frequency depends on their loads. For the all-resonant<br />

two-phase clock driver, two types of potential load imbalances occur: between the two phases and between<br />

different cycles for the same phase. First, loads should be approximately evenly distributed between the<br />

two phases. Otherwise, inductors with different inductance and/or two different supply voltages should<br />

be used so that ϕ 1 and ϕ 2 have the same width and amplitude. Second, for clock-powered microsystems,<br />

clock loads are data dependent. Therefore, the load may vary from cycle to cycle for the same phase, resulting<br />

in a data-dependent clock frequency. A simple solution for this problem is to use dual-rail clock-powered<br />

signaling, which ensures that half of clock-powered nodes switch per cycle. The drawback of such a clockpowered<br />

system is its high switching capacitance. For the purposes of this research, the all-resonant clock<br />

driver (Fig. 21.4) has been sufficient and highly energy efficient. Resonant clock drivers can also be<br />

designed with transmission lines [17] instead of inductors.<br />

21.4 Energy-Recovery Latch<br />

The E-R latch serves two purposes: to latch the input data, and, conditionally on the latched datum, to<br />

transfer charge from a clock line to a load capacitance C L and back again. Consequently, the E-R latch<br />

consists of two stages: the latch and the driver (Fig. 21.6(a)). The latch-stage design is not important for<br />

clock-powered logic and can be chosen to meet other system requirements. Suitable latch designs are the<br />

3-transistor dynamic latch consisting of a pass transistor and an inverter, and the doubled N-C 2 -MOS<br />

latch [19]. The driver stage is based on the bootstrapped clocked buffer (CB) [20] implemented in CMOS.<br />

The driver choice is discussed later in this section. The E-R latch operates from a two-phase, nonoverlapping<br />

clocking scheme (Fig. 21.6(b)): the input is latched on ϕ L and the output is driven during ϕ D.<br />

The two clock phases swing from 0 to voltage V ϕ. A symbol used to denote the clocked buffer part of<br />

the E-R latch is shown in Fig. 21.6(c).<br />

© 2002 by CRC Press LLC<br />

C ϕ<br />

ϕ 1<br />

ϕ 1<br />

ϕ 2<br />

L<br />

V dc<br />

L<br />

ϕ 2<br />

C ϕ

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