Formal derivation of optimal active shielding for low-power on-chip buses

Passive shielding has been used to reduce the capacitive coupling effects of adjacent bus lines by inserting passive ground or power lines (shields) between them. Active shielding is another shielding technique in which the shield is allowed to switch depending on the switching pattern of its adjacent bus lines. This paper formally derives the optimal active shielding logic function for minimum power dissipation. It is also shown that this optimal active shielding architecture depends on the ratio of coupling to ground capacitance (γ=C_c/C_g). Optimal active shielding is shown to provide up to 25% reduction in bus power dissipation compared to conventional passive shielding. A suboptimal active shielding architecture with simpler hardware is also proposed. Theoretically, using the suboptimal shielding architecture leads to less than 6% bus power penalty compared to the optimal active shielding logic circuit. However, due to the simpler shield encoding circuitry, simulation results show that the suboptimal active shielding architecture leads to higher overall energy savings compared to the optimal active shielding architectures.