RE: RTC tick is out by 24hz when set to 1000hz

paul — Thu, 20 Jun 2019 21:49:12 GMT

Thanks Einar,

No just reading the documentation:

"Macro for converting expected frequency to prescaler setting."

and expecting that if I put 1000 in I will get 1000 out ;-)=

Many thanks

Paul

Einar Thorsrud — Thu, 20 Jun 2019 07:01:41 GMT

Hi,

This is expected. It seems you have forgotten to consider that you are doing integer arithmetic.

Details:

#define RTC_FREQ_TO_PRESCALER(FREQ) (uint16_t)(((RTC_INPUT_FREQ) / (FREQ)) - 1)

So with the values from your code, the output result from RTC_FREQ_TO_PRESCALER() is floor((32768/1000)-1) = floor(31.768) = 31.

Than you find the RTC frequency by this calculation:

fRTC [Hz] = 32768 / (PRESCALER + 1 ) = 32768 / (31 + 1 ) = 1024.