[time-nuts] How to properly characterize 32kHz oscillators manually and with a microcontroller?

[Mark Sims]

Back when that local high school kid was arrested for bringing his "home made" alarm clock to school,  I bought a "bomb clock game" off of Ebay.  It looks like 7 sticks of dynamite with a circuit board with LEDs strapped to it.  The game part is you press a button and it starts counting down 10 seconds.  You have to disconnect one of 4 wires or it "goes off".   When not running the game, it is a standard alarm clock.  It has a 18650 lipo cell with charger circuit plus runs off the (5V) input.

It uses what looks like an ATMEGA328 series chip with a standard 10 cent 16 MHz microprocessor crystal.  The strange thing is it is VERY accurate.  It seems to hold +/- two seconds over several months.   I'm guessing that either I was very lucky (not likely, those crystals are rather iffy) or the maker did some sort of measurement of the processor clock and programmed an adjustment factor into EEPROM to compensate for the true frequency.  I've done the same thing with several AVR projects that had software clocks in them and the compensated 16 MHz clocks beat any of the RTC chips.