What is a clock signal?

A clock is the chip's drumbeat. Each tick tells the circuits to take one step; faster ticks mean faster work and more power.

1. A steady beat
A chip's clock signal does not tell the time: it switches on and off, again and again. On each tick, every circuit on that clock takes one step together, like workers who move on each drumbeat.
2. Counting the ticks
Frequency is the number of ticks each second. It is counted in megahertz (MHz, millions of ticks) or gigahertz (GHz, billions).
3. Making a fast clock
A tiny quartz crystal on the board, like the one in a watch, gives a slow, steady beat. A circuit called a PLL (phase-locked loop) multiplies it into a fast clock, and dividers make slower copies.
4. Each block keeps its own beat
Each block sits in its own clock domain. So the CPU, graphics and memory can each tick at a different speed.
5. Slow down or stop to save power
A slower clock uses less power. Stopping a block's clock is called clock gating: it saves even more, and the block keeps its data until the clock restarts.

Words to know

Clock signal: A steady on-off beat that tells circuits when to take their next step.
Tick: One beat of the clock. Also called a cycle.
Frequency: How many ticks happen each second.
MHz and GHz: Megahertz and gigahertz: millions and billions of ticks a second.
Crystal: A tiny piece of quartz that shakes at one very steady rate.
PLL (phase-locked loop): A circuit that turns a slow, steady beat into a fast one.
Divider: A circuit that makes a slower copy of a clock.
Clock domain: A group of circuits that share one clock.
Clock gating: Stopping a block's clock so it pauses and saves power.

Testers often give a block some work and check that its clock speeds up. When the work stops, they check that the clock slows down again.

What happens when the clock ticks faster?Think first, then tap to see the answer.

The chip does more work each second, but uses more power.