I use the STM32F429 for my new products, and the Crystal for the RTC stops running when power goes away. I have a battery on it to run RTC, so I can I get the 32.768kHz crystal on OSC32 to run using the battery on Vbat??

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[waclawek.jan]

> Crystal for the RTC stops running when power goes away

How do you know?

Read out and check/post content of relevant registers (RCC.BDCR, RCC).

Check schematics of reference boards (e.g. https://www.st.com/en/evaluation-tools/32f429idiscovery.html ) and compare to yours. Post as much information as possible so that we don't need to guess.

JW

JW,

I do see the external crystal stops running when power goes away.

What can I do to keep the crystal running? I try but can’t get it running off the battery.

Please help me know how to get the Crystal running!!!

This new technology I invented with this CPU is awesome. Please help my business!!

I need to make RTC run OK.

[waclawek.jan]

LSE is an ultralow power circuit, if you observe crystal directly using normal oscilloscope probe, you are quite likely to influence it so that it stops. You should observe it only indirectly - directing it to MCO1 pin (I'm not sure it works when VDD is off), or observing RTC's 1Hz/512Hz output, or simply observing whether time in RTC changes properly.

What is your VDD? And how exactly the battery is connected? Check also for bad solder joints.

You may want to experiment with "known good" boards such as Disco or Nucleo (they may have the LSE crystal not populated, read carefully their respective manual).

JW

My VDD is 3.3V. My battery is always connected 100% to the Vbat pins. It is connected correct to the CPU.

I used and oscilloscope to see the LSE crystal oscillating. Every time the VDD powers off, the crystal does not oscillate. Even if I don't touch the crystal the RTC does not move up in time when power is off. I really need a list of the registers and bits in the register that MUST be setup to make sure this runs correct.

Can anyone please give me the register list and bits to setup to make sure this works??

[GLASS]

Maybe consider application note AN​2867.

Stm32f429 are gen1 stm32f4. No possibility to change LSE drive level​.

the external 32768Hz quartz must be carefully choosen (some part are stated stm32f4 compatible) and routed, as like two condensator​s..

Also you MUST verify all power pins and gnd..and good decoupling...

Generally startup code produced by cube mx and hal do the job (if you take care of not initing Rtc at all startup but only when cold startup after V backup lost).

But ref manual​ can help... (smile for jw...)

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