For a 32.768 kHz crystal, the static inaccuracy of the RTC is bounded to ±30.5 ppm if the selected crystal has a larger static inaccuracy. For example, a crystal inaccuracy of +50 ppm in one-million ticks will generate 50 extra ticks (or off by
1-9/16 of a tick every second). By increasing the calibration value by one leaves the 9/16 of the tick. Therefore, a crystal’s static +50 ppm impacts the RTC similar to a +17.17 ppm crystal, because some of the inaccuracy is accounted for through the seconds calibration.
By enabling the fractional calibration feature, the second calculation logic can perform further calibration by delaying the clearing of the tick counter by one to 15 oscillator ticks every 16 seconds. In the earlier example, after every 16 seconds, the clock is nine ticks ahead. Therefore, by programming the value of nine into the fractional calibration field of the calibration register, the time is adjusted by nine ticks every 16 cycles, which corrects the static inaccuracy of the oscillator.
By using the fractional calibration feature with a 32.768 kHz oscillator, the static inaccuracy of the RTC is bounded to ³ 2 ppm, no matter the static inaccuracy of the oscillator. If a higher frequency crystal is used, this number is lowered. For example, by using a 62.5 kHz oscillator, the static inaccuracy is bounded to 1 ppm.