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'F4 - is 1MHz RTC clock when derived from HSE mandatory or max?



While I could not find a  maximum RTC input clock limit in the datasheet, I believe the underlined phrases should say "max.1MHz". As it is phrased now, it appears to call for exactly 1MHz which I doubt is mandatory.


ST Employee

Hello @waclawek.jan

1 MHz is not a max value, but if you generate 1 MHz using the RCC prescaler it will be possible to configure the RTC prescalers in order to generate 1s calendar clock.

The RTC prescalers depth allows up to 4 MHz, just need to take care to be able to generate a 1s clock with RTC prescalers, if a calendar is needed.

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Hi @Sarra.S ,

In that case, I would recommend to remove reference to "1MHz" from the manual altogether. Non-mandatory recommendations should not be in the normative material (RM/DS/ES); they should be in related ANs.

OTOH, who's to say that the RTC must clock at 1s? I can imagine quite a couple of applications where RTC ticks at higher frequency, ignoring the calendar, using it just for wakeup. There should be a maximum input frequency for RTC given in the datasheet.