RTC with 32.768 kHz crystal

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Joined: Wed Aug 23, 2017 11:33 am

Re: RTC with 32.768 kHz crystal

Postby Kazuya91 » Thu May 10, 2018 10:23 pm

Any news? Can i wake the ESP32 with an external RTC like DS3231 with Arduino?

I found this:
https://github.com/espressif/arduino-es ... WakeUp.ino

Posts: 15
Joined: Sat Apr 11, 2020 8:11 am

Re: RTC with 32.768 kHz crystal

Postby fededim » Mon May 04, 2020 7:39 am


I am connecting to this thread...I would like to add a 32.768 Khz crystal in order to have less RTC clock drift during deep sleep (I have measured almost 35 seconds in 2 hours time with standard internal RC 150 Khz source!). The hardware design guidelines at page 11 show the typical circuit below:

3 questions:
1) Can anybody tell me typical values for capacitors C22 and C23 ? Or at least how can I determine them ?
2) As it regards the R4 resistance it is stated to be between 5 MΩ < R4 ⩽ 10 MΩ, what is its typical value ?
3) There is also a note about equivalent ESR less than 70kΩ, shall I need to buy low ESR capacitors ? Or at least can anybody tell the equivalent impedance formula of the circuit ?

If the answer to 1) and 2) and 3) depends on selected crystal can you suggest the values for a 32.768Khz SMD crystal available on AliExpress such as NX3215SA , FC-145 , FC-255 ?

Last but not the least is it possible under Arduino Framework to change RTC clock source ? On ESP-IDF there seems to be the function select_rtc_slow_clk but I can't find any equivalent on Arduino.

Posts: 15
Joined: Sat Apr 11, 2020 8:11 am

Re: RTC with 32.768 kHz crystal

Postby fededim » Wed May 06, 2020 1:31 pm

For those interested in the ESP-WROVER-KIT-V3 schematics (https://dl.espressif.com/dl/schematics/ ... _SCH-3.pdf, on the right of the picture search for C12) - made by the same EspressIf - there are two capacitors of 12pf and R4 isn't even present. I'll try with these values, even though the capacitors should be based on the load capacitance of the crystal in datasheet; unluckily the electronics store where I bought the crystal did not have the datasheet.

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Joined: Sat Mar 24, 2018 4:35 pm

Re: RTC with 32.768 kHz crystal

Postby jumpjack » Fri May 22, 2020 3:49 pm

any news?
Do I have an accurate RTC on my M5stickC (ESP32 based) ?

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Joined: Fri Sep 01, 2017 3:50 pm

Re: RTC with 32.768 kHz crystal

Postby SomeCallMeTim » Fri May 22, 2020 5:14 pm

I'll chime in that for us we weren't able to make the crystal stable and ended up going with a MEMS oscillator instead.

After we did our design with the MEMS oscillator, there is now an ECO indicating a fix for "Optimized 32.768 kHz crystal oscillator stability". See https://www.espressif.com/sites/default ... de__EN.pdf.

- Tim

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