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U. Glaeser

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FIGURE 10.74 Normalized delay (FO4 inverter) of various latches and flip-flops: (a) unbuffered transmission gate<br />

latch, (b) buffered transmission gate latch, (c) C 2 MOS latch, (d) TSPC latch, (e) master-slave flip-flop, (f) C 2 MOS<br />

flip-flop, (g) TSPC flip-flop, (h) sense-amplifier flip-flop, (i) hybrid latch flip-flop, (j) semi-dynamic flip-flop, and<br />

(k) pulsed transmission gate flip-flop.<br />

driver to compute delay. For instance, a buffered transmission where CK is switching (Fig. 10.74(b)) is<br />

considered as a two-stack structure switching from top. If D switches instead, it is considered as a twostack<br />

structure switching from bottom. For each design, the worst-case switching delay is assumed. In<br />

cases where the high-to-low and low-to-high delays are unbalanced, further speed optimization could be<br />

accomplished by equalizing both delays. A diamond is used to indicate the transistor in the stack that is<br />

switching. In estimating the total delay of each design, the following assumptions are made. Precharged<br />

stages (e.g., sense-amplifier flip-flop, TSPC flip-flop, SDFF) are skewed and therefore faster (see Table 10.7,<br />

skewed logic). Output inverters are complementary static in all cases. The input inverter in the senseamplifier<br />

flip-flop (Fig. 10.74(h)) is skewed, favoring the low-to-high transition, because its speed is<br />

critical in that direction only. The SR latch is complementary static. In the case of HLFF (Fig. 10.74(i)),<br />

© 2002 by CRC Press LLC<br />

D<br />

D D<br />

D<br />

CK<br />

CK<br />

CK<br />

CK<br />

D<br />

0.30 + 1.20 = 1.50<br />

CK<br />

CK<br />

(a)<br />

(b)<br />

CK (c)<br />

1.20 + 1.30 + 1.20 = 3.70<br />

1.20 + 0.35 + 1.00 + 1.15 + 1.20 = 4.90<br />

D<br />

(d)<br />

1.55 + 1.15 + 1.20 = 3.90<br />

(h)<br />

CK<br />

D<br />

(f)<br />

0.60 + 0.95 + 1.20 = 3.90<br />

+ 1.15<br />

D<br />

1.20 + 0.35 + 1.00 = 2.55<br />

CK<br />

D<br />

CK<br />

(e)<br />

1.55 + 0.70 + 0.60 + 1.00 = 3.85<br />

D<br />

(g)<br />

0.70 + 1.20 + 1.00=2.90<br />

D<br />

(i)<br />

1.55 + 1.00 = 2.55<br />

0.95 + 0.60+ 1.00 = 2.55<br />

1.20 + 0.35 + 1.00 + 0.30 + 1.20 = 4.05<br />

(j) (k)<br />

CK

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