STM32 RTC accuracy

Posted on May 17, 2011 at 13:40

Hi yuvraj,

Maybe you could explain why you cannot use a 32Khz crystal? Even in high volume application the cost of calibrating the LSI will be more than a cheap crystal.

Regards

Trevor

Posted on May 17, 2011 at 13:40

Hi,

I'm not sure it's recommended to use internal LSI for timing measuremenet.

But anyway  an application note AN2604 from ST describes the calibration

process of the internal RTC clock in order to keep an accurate timing considering the temperature / voltage variation of the application.

Cheers.

Posted on May 17, 2011 at 13:40

AN2945 describes the clock accuracy, basically HSI (8 MHz +/- 1%) and LSI (40 KHz +/- 8%)

While the STM32 is running it is relatively easy to trim the RTC divider based on the HSI and SYSTICK. I'm not convinced that either HSI or LSI are particularly stable having watched them wander against a GPS reference. If you have an external TCXO driving HSE that would be a better reference than HSI.

If accuracy and stability is important, I'd have to agree with Trevor and say just use an external 32.768 KHz watch crystal, which would be 25 cents or less in small quantities.

-Clive