ESP32S3 External 32 kHz Oscillator clock levels for GPIO15

[EklundJL]

Guidelines seemed to have changed between v5.2 and v6.0.2(stable); can I drive with a 3.3v external clock?:

https://docs.espressif.com/projects/esp ... _time.html
External 32 kHz oscillator: Allows using a 32 kHz clock generated by an external circuit.
The external clock signal must be connected to the external oscillator pin.
The input amplitude must ensure that the digital circuit can correctly distinguish between
logic high and low levels. Test results show that 1.7 V is the threshold between high and low levels.
Therefore, make sure the input waveform has a peak value greater than 1.7 V and a minimum value lower
than 1.7 V. For details on pin connections, please refer to the datasheet.


https://docs.espressif.com/projects/esp ... _time.html
External 32 kHz oscillator at XTAL_32K_P pin: Allows using 32 kHz clock generated by an
external circuit. The external clock signal must be connected to the XTAL_32K_P pin.
The amplitude should be less than 1.2 V for sine wave signal and less than 1 V for square wave signal.
Common mode voltage should be in the range of 0.1 < Vcm < 0.5xVamp, where Vamp stands for signal
amplitude. In this case, the XTAL_32K_P pin cannot be used as a GPIO pin.*/

[eriksl]

That's weird (indeed). To me it looks like a typo. Seems to me they actually mean the amplitude needs to be at least 1.7V. I'd file a bug report. In my experience the bug reports are read by quite a lot more people from Espressif than the forum.

[MicroController]

The confusing part here is "Vcm < 0.5xVamp" which limits the peak voltage to Vamp, which in turn is much lower than the 1.7V "threshold". Both docs might be correct but in different conditions (passive crystal vs. active signal). Some clarification would indeed be needed.

[lichurbagan]

Those limits are probably target-specific, rather than a general change between ESP-IDF versions. The allowable XTAL_32K_P signal depends on the exact ESP32 variant and its RTC oscillator input circuitry.

If the documentation for your chip specifies less than 1 V for a square wave, do not apply a 3.3 V clock directly, it may exceed the pin rating even if the logic threshold is around 1.7 V on another device. Use attenuation or a suitable level shifter and meet the stated amplitude and common-mode requirements. Which exact ESP32 model and datasheet revision are you using?

[EklundJL]

ESP32-S3 Series Datasheet v2.2

Thank you for your response. Level shifting and amplitude attenuation are easy. The inputs are for an external 32kHz oscillator, not an external 32 kHz crystal. The point here is that the original text stated "The amplitude should be ... less than 1 V for square wave signal" but the current text states "...make sure the input waveform has a peak value greater than 1.7 V", contradictory.

[MicroController]

"The amplitude should be ... less than 1 V for square wave signal" but the current text states "...make sure the input waveform has a peak value greater than 1.7 V", contradictory.

No contradiction here. For example, 1.65V+/-0.3V (i.e. a signal between 1.35V and 1.95V, amplitude of 0.3V) would fit, and likely be close to what an actual crystal would be driven at.