RTC why 32768 crystal?

larrwill · Joined: 18 Apr 2011 Posts: 1

Why is it that a 32768 crystal seems to be the standard choice for a real time clock even though timer 1 is 16 bit? Why not use a 65536 crystal and have timer 1 overflow every second without having to change the register value as with 32k?
Is the lower frequency more accurate, lower power, or ???

Thanks

Douglas Kennedy · Joined: 07 Sep 2003 Posts: 755 Location: Florida

You guessed it generally the lower the frequency the lower the power. Now this doesn't mean there is a race to develop a 1 hertz crystal.

temtronic · Posted: Mon Apr 18, 2011 7:17 am

Also, the 32kHz 'watch crystal' was made well before the PIC came out ! It was designed for the first 'digital' watches( analog movements 'pulsed' every second).
It is the 'optimum' frequency considering speed,size,power and easily divisable by powers of 2.And of course price...probably BILLIONS of them in the world !

Ttelmah · Joined: 11 Mar 2010 Posts: 19569

For each different crystal 'cut', there is a range of frequencies over which it works best. Then each cut has a temperature tolerances etc.. Now the particular cut used at 32768, has the advantage of small size (try looking at the sizes of some 100K SC cut crystals, and you will see why these are not used in watches!). The 'X' cut commonly used in the watches, has a temperature turnover point that is close to 20C, and the frequency is quite accurate, over a range of perhaps 10C to 30C. This makes it easy to use without an oven, in 'room temperature' applications. For this cut, the 'useable' range, that can easily be made, is about 16KHz, to perhaps 150KHz, with the highest gain and lowest drive requirement, occurring between around 30 and 50KKHz. For a one second divider, the nearest 'binary' count to this, 'best' point, was 32768, so this became the standard. Perhaps however worth commenting, that the very first crystal clock used 50KHz, and some Seiko watches made a few years ago, used a 2MHz AT cut crystal instead, but with a rather high battery consumption....
So, perfectly possible to get a 65K X cut crystal. Cost small - but not as small as the 32768Hz unit. Extra power needed, tiny.
Most clocks though use dividers other than a PIC, so are just happy to divide by 2^15.
On the PIC, for most applications, a 2 second counter is entirely adequate.
If you want though to make a 'clock', with a 1 second 'tick', then use a PIC with the RTCC module. This divides by 16384, and can handle events at intervals from 0.5 seconds up.

Best Wishes