How does a CPU change frequency?

Question

From what I've found on the internet, CPU clocks work with piezoelectric material to produce a very stable oscillating signal but those materials must change in shape or size to get a different frequency. So how does a CPU change frequency?
Do CPUs use some other kind of less stable clocks then fix them with a crystal oscillator or ...?

(I know very little about electronics, please stay simple, thanks)

Please note that I'm not asking how it is decided to change the frequency (like this question), I'm asking how the frequency change actually happens in the circuit.

@Ignacio Vazquez-Abrams, no I've read that. That question asks how it is decided to change the frequency, I'm asking how the frequency change can be implemented in circuits because from what I know crystal oscillators don't change frequency that simply. — kptlronyttcna, Sep 25 '15 at 06:57
And yet you've failed to mention how that doesn't answer your question. — Ignacio Vazquez-Abrams, Sep 25 '15 at 06:58
google PLL. IIRC there was a question about PLLs last week or so. — Wouter van Ooijen, Sep 25 '15 at 07:13
@Ignacio Vazquez-Abrams, OK I've read FakeMoustache's answer and now I can see it is somewhat a duplicate, should I leave the question or delete? (sorry) — kptlronyttcna, Sep 25 '15 at 07:27
There should be a way for you to accept that this is a duplicate in the options there I think. — Ignacio Vazquez-Abrams, Sep 25 '15 at 07:27

score 2 · Accepted Answer · answered Sep 25 '15 at 07:14

The clock that the CPU uses does not come directly from the crystal oscillator in the motherboard ! The (fixed frequency) clock from the crystal oscillator is first fed to a system called a Phase Locked Loop or PLL. This PLL can be a separate chip on the motherboard but also inside any of the other chips or both.

The simplest PLLs can only multiply the fixed frequency clock from the crystal oscillator so if the crystal oscillator runs at 40 MHz then such a PLL can generate 80 MHz, 120 MHz, 160 Mhz, etc. Basically n times the crystal clock frequency.

More advanced PLLs can also make almost any frequency in between by using a fracional divider and sigma-delta modulation techniques.

So a CPU gets its frequency by multiplying a fixed frequency clock by some number? — kptlronyttcna, Sep 25 '15 at 07:24

How does a CPU change frequency?

1 Answers1