Tuning Fork Crystal 32.768 kHz

Why do tuning fork crystals always have a frequency of 32.768 kHz? This post answers that common question!

The frequency of a tuning fork crystal commonly found in quartz watches is always 32768 Hz or 32.768 kHz.

OK but where does the standard frequency for a tuning fork crystal of 32.768 kHz spring up?

Simply, a quartz with a natural frequency of only 1Hz would be so large, and obviously, that would be rather impractical in terms of production and application.

But watch crystals with a frequency of 32.768kHz are relatively easy to produce. Thus real time clock (RTC) applications have primarily been leaning on quartz crystals with a frequency of 32.768 kHz.

Since the 32768 Hz specific frequency can be divided down to 1 Hz (equivalent to one second in frequency) making it the base for all day, date, and timekeeping functions for any electronic device.

Since it has a power of 2^15, dividing down to a 1 Hz stable and accurate signal ideal for timekeeping is an easy task.

Today, oscillator circuits are commonly merged into integrated circuits (ICs) so that only a few more external components are needed to setup a tuning fork crystal oscillator.

Below figure shows the typical circuit of a Pierce Oscillator commonly used in digital processor designs.

In practice, the original frequency of 32768 Hz can be split using T Flip Flop or Ripple Counters. Since each T Flip Flop can halve the frequency, 15 of the series-connected T Flip Flop circuit finally delivers the exact 1 Hz output frequency.

Note at this point that a T Flip Flop (Toggle Flip Flop) toggles its output depending upon on the input. The T Flip Flop will toggle its output every time the clock signal transitions from High to Low or Low to High, hence it divides the frequency of the input clock signal by 2. 

By following the below shown idea (click image to enlarge), you can setup a quick and easy Crystal Controlled 1 Hz Time Base (Source http://www.hackersbench.com/Projects/1Hz/).

As you can see, this circuit uses a standard 32.768 kHz wrist watch crystal and a couple of commonly available ICs as the core components.

Admittedly, we have not tested this particular circuit yet. So, let us know how it turns out. See you next week!