Products · Real-time clocks

Real-time clocks.
Clock and crystal in one package.

RTC modules with the crystal built in — no separate watch crystal, no layout risk.

Catalogue values. Final selection and current datasheets with our FAE team — free of charge, from Ladenburg near Heidelberg. Find the datasheet for a part number →

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Micro Crystal, NXP or an Epson end-of-life type in your design? The RV and PCF series are in the cross-reference →

Basics · category 03

It keeps running when the power is gone.

A real-time clock keeps date and time independently of the main system — for years from a single coin cell. Indispensable wherever events are logged, tasks are scheduled or access has to be documented.

Integrated

The crystal is already inside

No separate watch crystal, no sensitive tracks, no stray capacitance — and every module individually frequency-trimmed and tested at the factory.

Supply current

From 100 nA in backup

This decides whether the coin cell lasts three years or ten. Anyone who guarantees a service life calculates with the maximum figure — 450 nA on the RX8111CE. Both numbers are listed for every type.

Interface

I²C or SPI

Plus alarms, time-stamp inputs and reset — depending on the type. The RX6110SA handles both buses. We match that against your controller.

±1 ppm or ±3 ppm — the number alone says nothing.

Real-time clock accuracy is stated in two different ways, and the two numbers are not comparable. Comparing the ppm figure alone means comparing the wrong thing.

Stated at +25 °C

Calibration accuracy

Tells you how precisely the module was trimmed at room temperature in the factory. What happens at −20 °C or +70 °C is not part of that figure. The 32.768 kHz tuning-fork crystal has a parabolic temperature response of about −0.034 ppm/K² — at −20 °C that comes to roughly −69 ppm, some 6 seconds of deviation per day. This applies to every uncompensated module, however small the number at 25 °C.

Stated over the range

Timekeeping accuracy

The value is the upper limit across the entire temperature range, usually −40 to +85 °C. It is made possible by a digitally temperature-compensated oscillator (DTCXO): a sensor inside the module measures the die temperature and keeps correcting the divider chain. In the list above these types carry the DTCXO marker.

What that means in practice

Ask for the curve

A module rated “±1 ppm at +25 °C” can run considerably less accurately in the field than one rated “±3 ppm over −40 to +85 °C”. Check the temperature statement in the datasheet, not the ppm number. For conversion: 1 ppm ≈ 0.09 s/day ≈ 2.6 s/month ≈ 31.5 s/year.

What else an RTC module can do.

Time stamps

Logging without the processor

The module stores the time itself when an event occurs — the MCU does not have to wake up for it. Three triggers are possible: a sensor on the event input (EVIN), an internal flag — the module stamps its own supply dropout — or a command from the MCU. The RX8901CE stores up to 32 events with a resolution down to 1/1024 second.

Self-monitoring

It reports when the cell runs down

Status flags for crystal oscillation stop, main supply dropout and backup voltage dropout. That lets you build a maintenance concept without extra circuitry — the device reports for itself when the coin cell is nearing its end.

Reset and logic levels

Fewer parts on the board

The RX8130CE and, as an ordering option, the RA8000CE and RA4000CE output a reset signal with delay and thereby replace the external reset IC. The RX8111CE, RX4111CE and RX8130CE have a V_IO pin of their own: the RTC runs on 3 V yet talks to a 1.8 V MCU without a level shifter.

Discontinued types — what to design in now.

With notice 25-012E of 28 October 2025 Epson discontinued six versions. The last order date was 16 April 2026, the last delivery goes out at the end of October 2026. For each type you will find the comparison with its successor here, together with the route that changes the least.

End of life