How does a device like a phone or tablet accurately keep track of time?

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For context, let us consider something that would be disconnected from a network to prevent syncing with the internet.

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2 Answers

Anonymous 0 Comments

A small quartz tuning fork is placed inside of the device. This fork, powered by electricity, creates a vibrating signal. This signal can then be literally counted, and if you know how many counts make a second then keeping time is trivial.

Anonymous 0 Comments

Let’s think of an old-timey pendulum clock first.

If you have a pendulum swinging back and forth, the amount of time it takes to complete a single back-and-forth swing depends on one thing and one thing only: its length.

Using this knowledge, it’s possible to build a pendulum of a very specific length such that each back-and-forth swing lasts exactly 1 second. You can then use a clever gear-ratcheting mechanism to count up the number of swings that go by, and use that to drive a clock.

In electronics, they actually use a somewhat similar mechanism. Instead of a pendulum swinging due to gravity, they use a tiny little tuning fork that, when you whack it, vibrates back-and-forth with a known, very specific frequency. All you need to turn this into a clock is a way to do the whacking, and a way to count the wiggles.

Quartz has a very peculiar property that is useful for both of these requirements. If you bend a piece of quartz in the right way, it will, just for a moment, emit a weak pulse of electricity. The reverse is also true: if you zap the quartz with a pulse of electricity, it will bend a little bit.

With a piece of quartz cut into the shape of an itty bitty tuning fork, we have the mechanism we are looking for. We can zap it with a pulse of electricity to set it vibrating. Then, once it’s set vibrating, we can count the wiggles by sensing the electrical pulses that are created as it flexes back and forth. An electronic device can simply keep track of that wiggle count to tell how much time has gone by.

[Here is what one of the more common varieties of quartz tuning forks looks like.](https://cdn.hackaday.io/images/2175501472442901850.jpg) The object on bottom is the naked fork itself. Its length is about half the width of your pinkie finger nail. The object on top is what it usually looks like, stuffed inside a protective cover.

These simple tuning forks are good enough for common wall clocks. But they’re not good enough for ultra-precise computers, like phones and tablets. Those require a much more complicated solution that are outside the realm of ELI5. [That solution, in addition to a visualization of the quartz tuning fork solution, is explored in this Branch Education video if you want to try to understand it.](https://www.youtube.com/watch?v=oEC5fIw0bL0)