Clock frequency

Also called: clock rate, clock speed

The number of clock cycles per second, in hertz. It equals 1 ÷ clock period; 1 GHz means a billion cycles each second.

The clock frequency f counts how many clock cycles happen each second. It's measured in hertz (Hz), usually MHz (10⁶ Hz) or GHz (10⁹ Hz), and it's the reciprocal of the clock period:

f = 1 / T

Shortcut: f in MHz = 1000 ÷ (T in ns). A 4 ns period gives 250 MHz.

A higher frequency means more cycles per second, but it doesn't by itself mean a faster program. Execution time also depends on how many cycles each instruction needs:

time = instructions × CPI ÷ f

So a 2 GHz CPU needing 3 cycles per instruction can lose to a 1.5 GHz CPU needing 1.2. Comparing CPUs by clock speed alone is a classic mistake.

In hardware design, the highest usable frequency is the maximum clock frequency, set by the slowest register-to-register path.

Worked example

Example

From period to frequency

A clock has a period of 2.5 ns. What's its frequency?

1.
f = 1000 ÷ 2.5 = 400 MHz.
2.
Long way: 1 ÷ (2.5 × 10⁻⁹ s) = 0.4 × 10⁹ Hz = 400 × 10⁶ Hz.
3.
Check: 2.5 ns × 400 MHz = 1.

Common mistakes

Dropping the prefix: 1 ÷ 2.5 ns is 0.4 GHz, not 0.4 Hz.
Judging performance by frequency alone. CPI and instruction count matter too.

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Clock frequency is taught in Basic CPU / Computer Architecture and Timing and Sequential Logic.

Basic CPU / Computer Architecture →Timing and Sequential Logic →

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