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Manjunatha S
Manjunatha S
Associate III
November 24, 2014
Question

STM32 RTC Time lags

  • November 24, 2014
  • 16 replies
  • 3585 views
Posted on November 24, 2014 at 15:13

Hi,

I'm using STM32l151rbt6 MCU. But after few days of running the program the internal RTC starts lagging. I'm not doing RTC calibration. Any other possible reasons?

Thanks.

#stm32 #rtc
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16 replies

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Tesla DeLorean
Tesla DeLorean
Guru
December 10, 2014
Posted on December 10, 2014 at 17:13

I measured LSE via MCO pin. It was 32.75 to 32.89KHz. I think it's fine. What do you say?

I think it should be solid/stable. Your crystal appears to be out of specification, the 9-12pF crystals are not suitable for STM32 designs, you need the 6-7pF ones.

http://www.mouser.com/ProductDetail/Citizen-Finetech-Miyota/CM130-32768KDZF-UT/?qs=6siQ5y5nVCzKpPfDh9phvQ%3D%3D

ST uses these MC-306 32.7680K-E3:ROHS

http://www.mouser.com/ProductDetail/Epson/MC-306-327680K-E3ROHS/?qs=/ha2pyFaduh9AAJ6s9/wjsMnN39w6VheRZymAkVEKGpCmuk2ykMFoA%3d%3d

Is it possible to provide any means of alert when the LSE fails?

Fails how. There's no real mechanism to checking if it's not oscillating properly, other than it stops.

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Manjunatha S
Manjunatha SAuthor
Associate III
December 11, 2014
Posted on December 11, 2014 at 12:59

Hi Clive,

Thanks for the suggestion. I have forwarded this to hardware team. I just want to know in what terms this crystal affects the stability of the RTC?

Tesla DeLorean
Tesla DeLorean
Guru
December 11, 2014
Posted on December 11, 2014 at 15:16

I would expect it to be off frequency, and stop/start randomly over voltage and temperature, fail to start, or take a long time to start.

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Manjunatha S
Manjunatha SAuthor
Associate III
December 11, 2014
Posted on December 11, 2014 at 15:24

Okay. Thanks for the info.

Manjunatha S
Manjunatha SAuthor
Associate III
December 12, 2014
Posted on December 12, 2014 at 09:05

Hi Clive,

Please let me know the recommended capacitance values of the Loading capacitors. 

Tesla DeLorean
Tesla DeLorean
Guru
December 12, 2014
Posted on December 12, 2014 at 14:36

For the EPSON part I mentioned ST uses 6.8pF

http://www.st.com/web/en/resource/technical/document/application_note/CD00221665.pdf

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