As semiconductor technology advances, power consumption has emerged as one of the most critical design constraints in System on Chip (SoC) development. While functionality and performance remain essential, modern SoCs must also meet stringent power budgets, especially for applications in mobile devices, automotive systems, and high-performance computing.
Chapter 8 of the book highlights how power optimization is deeply intertwined with physical design, influencing decisions from floor planning to sign-off. This blog explores how low-power intent is implemented and preserved during SoC physical design.
Power in Modern SoCs
Power consumption directly affects battery life, thermal behavior, reliability, and overall system cost. Excessive power leads to heat generation, which can degrade performance and reduce silicon lifespan.
In advanced technology nodes, leakage power increases significantly due to reduced transistor dimensions, while dynamic power rises with higher switching activity and clock frequencies. Physical design engineers play a crucial role in managing both these components of power.
Power Components in Physical Design
SoC power consumption can broadly be divided into dynamic power and leakage power. Dynamic power is associated with signal switching and clock activity, while leakage power arises from transistors even when they are not actively switching.
Physical design techniques aim to minimize unnecessary switching, reduce capacitance, and limit leakage paths without compromising timing or functionality.
Power-Aware Floor planning and Placement
Low-power physical design begins at the floor planning stage. Strategic placement of power domains, voltage islands, and high-activity blocks help reduce long interconnects and unnecessary signal toggling.
During placement, grouping related logic and minimizing wire length reduces capacitive loading, which directly lowers dynamic power. Poor placement decisions can significantly increase switching power and make later optimization difficult.
Clock Power Optimization
The clock network is one of the largest contributors to dynamic power in the SoC. Clock tree synthesis must therefore be performed with power awareness.
Clock gating is widely used to disable clock propagation to inactive logic blocks. Physical design ensures that clock gating cells are placed optimally and that gated clocks are routed efficiently. Reducing clock buffer count, controlling clock tree depth, and balancing skew all contribute to lowering clock power.
Multi-Voltage Design and Power Domains
Advanced SoCs often operate with multiple voltage levels to optimize power consumption. High-performance blocks may require higher voltages, while peripheral or idle logic can function at lower voltages.
Physical design must correctly implement voltage islands, level shifters, and isolation cells. Proper placement and routing of these elements are critical to avoid timing and functional issues while achieving power savings.
Power Gating and Sleep Modes
Power gating is a key technique used to reduce leakage power by completely shutting off power to inactive blocks. This requires the integration of power switches, retention registers, and isolation logic.
From a physical design perspective, power gating introduces additional challenges in power routing, verification, and timing closure. Careful planning is required to ensure smooth transitions between active and sleep modes.
Routing and Power Optimization
Routing decisions have a direct impact on power consumption. Long routes and excessive vias increase resistance and capacitance, leading to higher dynamic power.
Power-aware routing prioritizes shorter paths for high-activity nets and optimizes metal layer usage to reduce interconnect parasitics. Power and ground routing must also be robust enough to handle peak currents without excessive IR drop.
Power Analysis and Sign-off
Low-power design is validated through detailed power analysis after routing. Using extracted parasitics, engineers analyze both average and peak power consumption across operating modes.
Static and dynamic power checks ensure that the design meets power budgets under worst-case scenarios. Any violations identified at this stage require targeted optimization before sign-off.
