Clock period

Also called: cycle time, clock cycle time

The time for one full clock cycle, written T. It equals 1 ÷ clock frequency, so a faster clock has a shorter period.

The clock period T is the length of one clock cycle: from one rising edge to the next, high time plus low time. It's the reciprocal of the clock frequency:

T = 1 / f

Handy pairs:

1 GHz ↔ 1 ns
500 MHz ↔ 2 ns
100 MHz ↔ 10 ns

Shortcut: T in ns = 1000 ÷ (f in MHz).

Why it matters:

In a CPU, the period turns a cycle count into time: cpu execution time = instructions × CPI × T.
In hardware design, the period must be long enough for data to travel between registers. That's the setup constraint, and the shortest period that works is the minimum clock period.
A multi cycle processor can use a short period because each cycle does one small step; a single cycle processor needs a period long enough for the slowest whole instruction.

Worked examples

Example

Converting frequency to period

Find the clock period at 125 MHz and at 2.5 GHz.

1.
125 MHz: T = 1000 ÷ 125 = 8 ns.
2.
2.5 GHz = 2500 MHz: T = 1000 ÷ 2500 = 0.4 ns = 400 ps.
3.
The faster clock has the shorter period, as it must.

Example

Time for one Tiny8 instruction

Tiny8 runs at 50 MHz. How long does one LOAD take?

1.
T = 1000 ÷ 50 = 20 ns.
2.
LOAD takes 6 cycles.
3.
6 × 20 = 120 ns.

Common mistakes

Using the MHz number as nanoseconds. 50 MHz is a 20 ns period, not 50 ns.
Multiplying by the frequency to get a time. Multiply by the period, or divide by the frequency.

Practice Clock period

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Learn it step by step

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

Basic CPU / Computer Architecture →Timing and Sequential Logic →

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