STM32746G RTC problem

[nikhilgopale]

Posted on August 10, 2017 at 08:56

Hi Everyone,

I am working on RTC with stm32f746g controller. I tried number of option but i am not getting accurate time.

After some time rtc is getting lag of 4 to 5 seconds and it keep on lagging in time.

I am using 32.768khz exteranl crystal with 24pf capacitors.

I tried by changing this capacitors value 22pf,15pf,10pf also but still my problem is not solved.

I did internal rtc calibration also to minimize this error by changing prescaler values but still i am getting time lag.

Can anyone suggest me any other option which will work out and i will get accurate time?

[STOne-32]

Posted on August 13, 2017 at 11:24

Dear

Gopale.Nikhil

‌,

I would recommend you to monitor PA8 using an universal counter and see the LSE accuracy at 32,768KHz to find the right CL1 and CL1, the right value will have the closest frequency to Nominal.

I strongly recommend to remove the 1MOhm parallel resistance and use its footprint to rework the watch crystal ( swapping its ends) as shown. It will increase the immunity of the osc-in versus the PC13 ( SD card ?).

It may happen also due to PC13 and other high speed signals crossing some pulses are skipped and so the the huge drift. .. you can see iton PA8 also with an oscilloscope.

I would recommend to start with CL1/ CL2 at 15 pF.

Cheers,

STOne -32