I am designing a system with a PCF8523 RTC. The datasheet states that it has a 32.768 kHz oscillator built into the IC, but it outputs the OSCI & OSCO pins and the datasheet application chapter sketches an external crystal.

I did not find an answer in the datasheet whether the oscillator has an internal crystal or not. Do I need an external crystal for the oscillator to work?


1 Answer 1


There is a difference in the terms crystal, oscillator, and crystal oscillator.

The RTC has an internal oscillator circuitry for crystal.

But it does not have an internal crystal.

So it needs an external crystal to work.

Note any possible requirement for external crystal loading capacitors too, and any other components, such as external series or parallel resistors.

  • \$\begingroup\$ This is confusing to me too: The PCF8523 contains: An on-chip 32.768 kHz oscillator with two integrated load capacitors. \$\endgroup\$
    – Rodo
    Nov 3, 2022 at 17:05
  • \$\begingroup\$ @Rodo It contains a suitable oscillator and loading caps for an external crystal. It never says it contains the crystal for building a crystal oscillator. \$\endgroup\$
    – Justme
    Nov 3, 2022 at 17:32
  • \$\begingroup\$ The crystal itself is a physically-vibrating sliver of quartz or more recently, a "MEMS" device. As such, a quartz crystal is incompatible with the process (and size!) of the integrated circuit, so are not "included" inside an IC. MEMS Oscillators are gaining popularity. \$\endgroup\$
    – rdtsc
    Nov 3, 2022 at 19:03
  • \$\begingroup\$ @Justme I never thought it had an internal crystal. An "oscillator" implies that it can generate the wave. An amplifier/buffer needs a crystal (with caps) to generate a wave. I sell an older version in eBay (PCF8583) and its datasheet explicitly say: "A 32.768 kHz quartz crystal has to be connected to OSCI (pin 1) and OSCO (pin 2)." There is nothing in the datasheet for the PCF8523 that clearly states that. \$\endgroup\$
    – Rodo
    Nov 3, 2022 at 22:01
  • 1
    \$\begingroup\$ figure 38 perhaps. \$\endgroup\$ Nov 4, 2022 at 5:34

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