What Is a Clock Cycle?

A clock cycle, clock signal, or logic beat as they used to be called, is a sort of metronome that helps coordinate actions and timings in a computer. A clock generator generates a clock signal. Unlike a standard wall clock isn’t used to tell the time. Instead, it continuously switches between a ‘high’ and a ‘low’ state. The actual form of the clock is variable – different systems use different types to accomplish the same goal.

The most typical form of a clock cycle would be a square wave with a fixed, constant frequency. The clock is always in one of two states – rising towards the peak or falling towards the trough. Circuits that use the clock cycle for their timing and actions become active on the signal’s relevant rising or falling edge.

Tip: The exception is systems with a double data rate, such as DDR RAM. In the case of dual data rate systems, the circuits would become active in both the rising and falling edges of the cycle.

Types of Clock

There are different types of clocks. The single-phase clock is the most common type used in systems that rely on clock cycles. Here, all clock signals are essentially transmitted via one single wire. Naturally, there are alternatives to this.

A system that sends the clock signal on two wires, with non-overlapping pulses of the clock, is called a two-phase clock. The two wires are referred to as phases one and two, respectively. The two different phases are always guaranteed to be inversely synchronized. This provides additional ways for circuits to take their timing from them – and in effect, it allows processors to make do with fewer gates, leading to a smaller overall design. Creating a processor with this setup is more complex and sometimes not as high-performance as a single-phase clock.

The third option for clocks is a 4-phase clock. Early on in processor development, four-phase logic was found in integrated circuits. This logic featured four separate, non-overlapping clock signals. This was more complicated than single-clock circuits. While early Texas Instruments and Western Digital microprocessors relied on this, it was phased out in favor of the simpler single-phase clock cycles.

Clocks for Modern CPUs

Most modern microprocessors use a low-frequency clock instead of a high-frequency one. That clock is then run through a multiplier to adjust the clock speed to the appropriate rate the processor needs. This makes it possible for the computer to adjust the clock. A slower clock speed is more power efficient, while a higher clock speed allows the CPU to perform better.

Most devices don’t need the clock to stay at a constant speed all the time. The only necessity is that it maintains a steady interval at all times. Many devices have a variable clock speed that moves between a set minimum and maximum point as needed.

This means less energy is consumed as the device idles during low-processing requests. The clock speed, and power draw, only ramp up when the device becomes busy. This may happen, for example, when a gaming PC launches a game rather than just idling with Facebook open. What are your thoughts on the subject? Share your thoughts in the comments below.