Frequency deviation of crystal oscillator leads to inaccurate time of electronic clock/clock crystal
Electronic clocks / clock crystal used for timing the key electronic components for the quartz crystal oscillator,
the nominal frequency of 32.768KHz. With the aging of the quartz crystal oscillator,
its frequency will be with the Q quality factor down and drift phenomenon.In the actual circuit application
of the clock crystal 32.768KHz, the capacity of the external capacitor can be fine-tuned or rely on the
CMOS integrated circuit in the logic frequency adjustment method to actively
compensate for the purpose of adjusting the time day difference.
External capacitor adjustment method
If the electrical capacity increases, the time slows down; if the electrical capacity decreases, the time speeds up.
Electronic clock / quartz clock crystal selected by the quartz crystal oscillator frequency deviation
(adjustment frequency deviation) should be ≤ 10PPM. If the crystal oscillator frequency deviation
itself exceeds the specification requirements. In other words, the use of the above approach is still
unable to meet the time day difference specification, it is recommended to use qualified
quartz crystal oscillator to replace the existing crystal oscillator, fundamentally eliminate the problem of inaccurate time.
Clock Crystal oscillator 32.768KHz frequency accuracy and the amount of time error variation
±10PPM:
The frequency deviation : 32.736KHz~32.800KHz
The time error :0.864×1=0.864 seconds per day
The total time deviation : 5.256 minutes per year
±20PPM:
The frequency deviation : 32.703KHz~32.833KHz
The time error : 0.864×2=1.728 seconds per day
The total time deviation: 10.512 minutes per year.
±30PPM:
The frequency deviations: 32.670KHz~32.866KHz
The time error: 0.864×3=2.592 seconds per day
The total time deviation : 15.768 minutes per year
If the clock crystal oscillator 32.768KHz accuracy is ±10PPM, the time of day error calculation formula is as follows：
10 (PPM) x 24 (1 day = 24 hours) x 60 (1 hour = 60 minutes) x 60 (1 minute = 60 seconds) = 864,000 * 1/100,000 = 0.864 seconds
The time error of this RTC crystal is no more than 0.864 seconds per day. If it is +10PPM,
it is at most 0.864 seconds faster per day; if it is -10PPM, it is at most 0.864 seconds slower per day.