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U. Glaeser

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FIGURE 13.10 Effect of output inverter P:N size on delay.<br />

In a very short-pipeline, high-frequency processor design, the 5–15% performance lost to the ground<br />

interrupt device was unacceptable. Unfooted domino also lowers the load on the clock, which, in turn,<br />

may lower the area and power required for generating and distributing the clock.<br />

The delay of a domino gate can also be influenced by the sizing of the output inverter. The larger the<br />

ratio of PMOS to NMOS width, or beta ratio, the lower the delay for the output rising transition. The costs<br />

are an increase in the delay of the output reset operation and a decrease in noise-margin at the dynamic<br />

node.<br />

Figure 13.10 shows the effect of the beta ratio on the output evaluate (rising) delay and output resetting<br />

(falling) delay. Adjusting the beta ratio of the output inverter can improve the delay of domino gates by<br />

5–15% depending on the complexity of the gate. For the carry-merge gate used to derive Fig. 13.10, beta<br />

ratios of 3–6 provide most of the performance benefit without serious degradation of the delay of the<br />

reset operation or noise margins.<br />

In the design of microprocessors with millions of logic transistors, it is of dubious value to do full<br />

analysis and optimization of the device topologies and sizings. To control the complexity of the design<br />

generation of simple guidelines for acceptable topologies and sizing, rules based upon simple device size<br />

ratios suffice for much of the design.<br />

13.2 Noise, Robustness, and Reliability<br />

The techniques described previously to improve performance cannot be applied indiscriminately. Performance<br />

can only be realized if sufficient noise margins are maintained. Dynamic circuits are subject to several<br />

potential noise events: Precharge-evaluation collisions, coupled noise on the input of the gates or onto<br />

the dynamic node, AC-noise or DC-offset in theV dd or ground power supplies, subthreshold leakage in<br />

the pull-down network or through the PMOS pre-charge device, charge sharing between the dynamic<br />

node and the parasitic capacitances in the NMOS pull-down network, substrate charge collection at the<br />

dynamic node, tunnelling current through the gates of the output inverter, and SER events at the dynamic<br />

node. Dynamic circuits in partially-depleted SOI technologies have the additional challenge of bipolar<br />

leakage currents through the pull-down network but have improved charge-sharing characteristics.<br />

Dynamic circuits are more prone to these noise events primarily because of the undriven dynamic<br />

node. These noise events all tend to either remove charge from a precharged and undriven dynamic node<br />

or add charge to a discharged and undriven dynamic node. Increasing the capacitance of the dynamic<br />

node improves this problem, but generally slows the circuit. Alternative for most of these problems is to<br />

© 2002 by CRC Press LLC<br />

Delay (ps)<br />

150<br />

140<br />

130<br />

120<br />

110<br />

100<br />

90<br />

evaluation reset<br />

2 3 4 5 6 7 8<br />

Output inverter P:N ratio

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