I had been using 32.768KHz XTALs for RTC application, for a long time. And I wonder why this particular value? [8|]
My answer (based on logical guess) -->
This frequency is arrived because of legacy designs, which were based on pure hardware. If we connected the XTAL to a counter with output in binary D15 to D0, then we simply have to monitor the pin D15 and when that pin goes high, we have 1Sec complete. Refer to the following calculation: -
(1/32.768KHz) * 32768 = 1Sec
(1/32768) * 32768 = 1Sec
(1/32768) * 0x8000 = 1Sec // Hence just monitor the MSB line (D15), to know your 1Sec pulse.
If we use micro controller for RTC function, then it really does not matter if we have a 32.768KHz or any low value, like 10KHz. Thats because, we will anyway compare the entire number and not just 1 bit.
If my above logic is correct, why did NOT we ever used XTALs like 16.384KHz or 8.192KHz or 4.096KHz? Is it something to do with manufacturing constraint?