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I'm thinking about building a simple electronic watch in my spare time.

Not a lot of functions, no LCD, just keeping time and displaying the time using LEDs.

However, I have never created a time-keeping circuit so I'm wondering if using a µC's internal clock is enough to keep track of time or do I need something else?

I have seen some people using 32.xxkHz for time-keeping. Why do people choose this particular value (maybe it's just coincidence I came across a few projects using that value).

Links to projects are always welcome.

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32768Hz divide by 2 = 16384; 16384 / 2 = 8192; 8192 / 2 = 4096; 4096 / 2 = 2048; 2048 / 2 = 1024; 1024 / 2 = 512; 512 / 2 = 256; 256 / 2 = 128; 128 / 2 = 64; 64 / 2 = 32; 32 / 2 = 16; 16 / 2 = 8; 8 / 2 = 4; 4 / 2 = 2; 2 / 2 = 1. And dividing by two is very easy in an electronic circuit. Basically what can be done is divide 32768 by 2^15 to get 1Hz. –  jippie Dec 14 '12 at 22:48
    
I should have noticed. Thank you! –  Delusion Dec 14 '12 at 22:54
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There are 365 days * 24 hours * 60 minutes = 525600 minutes in a year. If you want at most 1 minute deviation per year then you want at max 100% / ( 365 * 24 * 60 ) = 0.000190% frequency fault. An internal RC oscillator in a microcontroller doesn't even come close. More commonly this is represented as (approx) 2 ppm (parts per million). –  jippie Dec 14 '12 at 22:56
    
Thank you. You don't happen to know a website where I can read up on this stuff? –  Delusion Dec 14 '12 at 23:00
    
@Delusion Here's Atmel's real time clock app note atmel.com/Images/doc1259.pdf it may help you understand what's happening. –  Garrett Fogerlie Dec 15 '12 at 2:19
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3 Answers 3

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32.768 kHz is used in watches for two reasons:

  1. It's a power of two, so it can be divided down to 1.000 Hz with a simple, binary divider.

  2. It's a relatively low frequency that can still be put in a tiny package. The low frequency keeps the power consumption in the IC low.

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Thank you! One step closer. –  Delusion Dec 14 '12 at 22:57
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32.768 kHz divided by 2^15 is 1 second. And since that is a standard frequency and a very high volume part, they are dirt cheep.

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I would interface the micro with an RTC (Real time clock)

https://www.sparkfun.com/products/99

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