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Next: Different output network topologies Up: Reducing Spurious Switching Activity Previous: Tri-state buffer based technique

Overhead for gating

When applying gating proposed in the above subsection, we introduce overheads in the controller for generating gating signals and introduce extra control wires. Such overheads should be taken into consideration when examining the benefits of gating a signal to suppress SSA. We adopt a method from [62] for overhead computation and make similar assumptions for this purpose. We assume a typical gating control signal requires 10 gates to generate, a typical one of such gates has three fanouts, the gate is four times the minimum gate size, and the switching activity is 0.5. The switched capacitance, $SC_{cl}$, and area overhead, $A_{oh}$, in the controller are estimated as

\begin{eqnarray*}
SC_{cl}=0.5N_c{\cdot}10{\cdot}4{\cdot}C_{in}^{min}\cdot3=60N_cC_{in}^{min}\\
A_{oh}=4N_cA^{min}{\cdot}1.2=4.8N_cA^{min}
\end{eqnarray*}



where $N_c$ is the number of gating control signals, $C_{in}^{min}$ the input capacitance for a minimum sized gate, $A^{min}$ the minimum sized gate area, and the scaling factor $1.2$ for area overhead is to account for spacing between gates and the wiring space. Moreover, the switched capacitance overhead of the gating control signal wires is estimated as

\begin{displaymath}
SC_{cw}=2\sqrt{Area}C^{min}_wN_{sw}
\end{displaymath}

where $Area$ is the final circuit area, $\sqrt{Area}$ is used as the average gating control signal wire length, $C^{min}_w$ is the unit-length capacitance for a minimum-width metal-1 wire, $N_{sw}$ is the frequency of transitions on the gating control signal wires, and the scaling factor $2$ is to account for the fact that control signal wires are not usually routed as metal-1 or at minimum width. $N_{sw}$ can be estimated using the profiling and scheduling information. All the overheads are included in the results we report later.
next up previous
Next: Different output network topologies Up: Reducing Spurious Switching Activity Previous: Tri-state buffer based technique
Lin Zhong 2003-10-11