What determines the accuracy of time by RTC ICs?

The accuracy of time by RTC ICs depends on several factors as shown below:

• [1] Dispersion of Xtal oscillators' frequency
• [2] Temperature characteristic of Xtal oscillators
• [3] Change over the years of Xtal oscillators
• [4] Frequency dispersion of RTC_ICs
• [5] Power supply voltage dependence of RTC_ICs
• [6] Parasitic capacitance dispersion of PCBs

Supposed that the gain or loss of the time set by a Xtal osccillator, RTC_IC, and a PCB is "X", then, the accuracy of time,

X = ± [1] ± [2] ± [3] ± [4] ± [5] ± [6]

[1] is defined as specification of a Xtal oscillator. Generally, ±10ppm or ±20ppm tolerance is common. Supposed that ±20ppm- Xtal oscillator is selected.

[2] depends on the operating temperature around the application, here, supposed that the temperature is room temperature, around 25 degreeC. In that case, the tolerance of this part is ±0ppm.

[3] Generally, frequency change over 1 year is commonly ±10ppm for a Xtal oscillator. But initially, this part is also ±0ppm.

[4] is mentioned as ±5ppm to ±10ppm in our datasheet, but actually, the dispersion is very small, so supposed that this tolerance is considered as ±5ppm.

[5] As shown in the typical characteristics in the datasheet, in case of Ricoh RTC_ICs, this tolerance can be almost ignored.

[6] Supposed that this tolerance is considered as ±5ppm.

Then,

X = ± [1] ± [2] ± [3] ± [4] ± [5] ± [6] = ±20ppm ±0ppm ±0ppm ±5ppm ±0ppm ±5ppm = ±30ppm

In monthly rate,

±30ppm x 60sec. x 60min. x 24hrs. x 31days = ±80sec./month

To improve the accuracy of time, fine adjustment is necessary after all components mounting on board.

By fine adjustment, the errors caused by [1] [4] [5] [6] can be cancelled.

As for our module, R2025, R2045, a Xtal oscillator is built in and fine adjustment has been done before shipping to the users, and realized the time accuracy ±5ppm (at 25°C). In monthly rate, 13.4sec./month.

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