ClockGating

Clock gating is a power management technique used in digital integrated circuits to reduce dynamic power by selectively turning off the clock signal to idle portions of a circuit. When a block is gated, it does not toggle, lowering switching activity and energy per clock cycle. It is widely used in CPUs, GPUs, and system-on-chips to save power in portable devices.

A typical clock gating cell sits in the clock distribution network and consists of an enable input, a latch, and a gating element such as an AND gate. The gated clock is produced by logically ANDing the original clock with a latched version of the enable signal. The enable is captured by the latch during the clock’s low phase, ensuring that changes to the enable do not cause glitches when the clock is high. This arrangement helps preserve correct timing and synchronous operation across clock domains. Proper use requires that the gated block does not rely on continuous clock activity for correct behavior.

Designers weigh timing, area, and power trade-offs. Incorrect gating can introduce clock glitches, skew, or functional faults if the enable signal is not properly synchronized with the clock. Timing analysis must account for the altered clock path, and gating is typically limited to well-defined, idle blocks in synchronous logic.

Benefits and limitations: Clock gating reduces dynamic power but not static (leakage) power and adds design complexity. Its effectiveness depends on how often the gated blocks are idle. In practice, modern processors employ multiple levels of clock gating within the clock distribution network, controlled by power-management units and microarchitectural state to maximize energy savings while preserving correctness.