How accurate is the internal RTC of STM32F407VG?

[Bravo Keyl]

Posted on July 27, 2017 at 11:48

I got to know that STM32F407VGT6 has an internal RTC support with either internal oscillator or an external one. Since the internal 32kHz oscillator is not that accurate, I would like to use an external 32kHZ oscillator. But I'm confused about the accuracy of the internal RTC with an external 32kHz crystal. Can I use internal RTC for date time keeping for logger applications? How many seconds do we lose in a year? Do I have to use the external chip (such as PCA8523, DS3231) for this?

#rtc #stm32f407vgt6

[waclawek.jan]

Posted on July 27, 2017 at 12:01

The RTC itself has no impact on precision, it's just a counter. All what you need to ensure is the precision - and stability - of the crystal oscillator, and that's given not only by the crystal itself, but also the surrounding circuitry, including the loading capacitors, but also ground quality, electromagnetic noise in the close vicinity of crystal, mechanical impacts (vibration), etc.. Youi would need to ensure the same with external RTC chips, so you don't need them. Read AN2867.

The RTC has facility to 'fine-tune' the output, i.e. you can individually calibrate both for initial precision and compensate temperature effects. Without any such tuning I'd count a few tens of ppm of error at 'room temperature'; to get below some 10-20ppm you'd need some form of compensation.

1ppm translates into some 30 seconds per year.

JW