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.
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