Standard \'Watch\' or Real-Time-Clock crystal. 32.768kHz means it is precisely half of a 16-bit counter. Start counting at 0x8000 (or 32768). When the counter rolls over from 65535 to 0then you know that exactly one second has passed. Reset the interruptreset the counterand start counting again! Rated at 12.5pF load capacitance35K ohm max (18K ohm typical) series resistanceand +/- 20ppm stability. Small cylinder package.