# EEVblog Electronics Community Forum

## General => General Chat => Topic started by: XOIIO on May 15, 2014, 06:02:34 am

Title: Quick question, why are watch crystals 32kHz?
Post by: XOIIO on May 15, 2014, 06:02:34 am
Just a quick thing that crossed my mind, and found nothing with a Google search, why are watch crystals set at 32kHz (or rather 32.768 kHz). I'm just curious as to why that frequency in particular was chosen.
Title: Re: Quick question, why are watch crystals 32kHz?
Post by: Richard Crowley on May 15, 2014, 06:13:25 am
32768 is 2^15  So it is easy to divide down to 1Hz with a series of flip-flops.
And I believe they optimized the physical design to be long-term stable in wrist-watch use.

Title: Re: Quick question, why are watch crystals 32kHz?
Post by: AndyC_772 on May 15, 2014, 06:17:16 am
The function of a watch is to count exact, whole seconds, for as long as possible using as little power as possible. The frequency chosen should be one which can be divided down to 1Hz by a chain of D-type latches, which are simple to construct and require very little power to operate.

The chosen frequency should be as low as possible, because the slower it oscillates the less power is required. 32k is a practical lower limit on what can be inexpensively manufactured.
Title: Re: Quick question, why are watch crystals 32kHz?
Post by: Dr. Frank on May 15, 2014, 06:24:16 am
32768Hz can be easily divided to 1pps by a binary counter chain.

A low clock frequency like this gives very low power consumption in the control logic, i.e. 0.15µW ( see µEM Micro Electro marin http://www.emmicroelectronic.com/Products.asp?IdProduct=255 (http://www.emmicroelectronic.com/Products.asp?IdProduct=255))

The tuning fork type XTAL is quite susceptible to temperature changes (parabolic shape, up to 160ppm/K in the automotive temperature range: http://cfm.citizen.co.jp/english/product/cvo_character.html (http://cfm.citizen.co.jp/english/product/cvo_character.html)) , but the body temperature stabilizes the frequency, if the zero gradient point of the XTALs parabola is precisely set around 30°C.

Frank
Title: Re: Quick question, why are watch crystals 32kHz?
Post by: amyk on May 15, 2014, 01:17:29 pm
Not only watches, but many RTCs have also settled on the 32kHz frequency for probably much of the same reasons.
Title: Re: Quick question, why are watch crystals 32kHz?
Post by: pipe on May 16, 2014, 12:14:44 pm
Not only watches, but many RTCs have also settled on the 32kHz frequency for probably much of the same reasons.

Another major reason for this would probably be that there now was a lot of cheap 32 kHz crystals on the market... :)
Title: Re: Quick question, why are watch crystals 32kHz?
Post by: SeanB on May 17, 2014, 04:36:55 pm
32kHz was chosen for being able to make a crystal at a low frequency that was strong enough to survive in a watc yet small, and as the first stage of the oscillator divider uses half the power of the divider chain, making as few divide stages as possible made sense from a power consumption from a single small cell angle. Each extra stage doubles the power use by having parasitic capacitance that has to be charged with each cycle and dumped as heat when the clock goes low. Seems strange but a CMOS divider has a pretty close to linear frequency  to current relationship, and this has been used to make a few frequency to voltage converters.
Title: Re: Quick question, why are watch crystals 32kHz?
Post by: tom66 on May 17, 2014, 08:48:15 pm
Some early watches used 10kHz, 10.24kHz and 100kHz crystals because they can be divided down too, but over time the ubiquity of the 32.768kHz crystal won out.