Clock Gating and Multi-Bit Register Clustering for Low-Power Design

Clock networks consume up to 40% of an ASIC’s dynamic power budget. Clock gating is the most effective logical optimization technique to minimize dynamic power by shutting down clock branches when registers are idle.

Gating Logic Setup Time and Local Clock Skew

Inserting Clock Gating Cells (ICGs) can introduce set-up timing violations on the gating path. Furthermore, inserting gated cells in clock trees creates local delay variations, introducing clock skew that degrades overall clock network synchronization.

Integrated Clock Gating (ICG) Cells, Multi-Bit Registers, and Activity Synthesis

Physical designers optimize dynamic power during logical synthesis and clock tree construction:

• Integrated Clock Gating (ICG) Cells: Utilizing dedicated, glitch-free ICG cells (latch + AND gate) to secure timing-safe clock shut-off.
• Multi-Bit Register Clustering: Swapping individual registers with multi-bit registers to share clock buffers, saving area and power.
• RTL Activity-Aware Gating: Analyzing register write-enable signals in RTL to maximize the depth of gating logic branches.
• Clock Tree Power-Aware Routing: Restricting clock tree routing depth to avoid unnecessary clock tree power dissipation.

Low-Power Synthesis and Clock Tree Signoff

ASIC designers drive low-power synthesis using Synopsys Power Compiler or Cadence Joules, and analyze clock trees via PrimeTime clock tree analysis.

Conclusion

Clock gating is a primary driver of low-power silicon. Combining RTL activity analysis with physical register clustering yields high dynamic power savings without Timing degradation.