RTC XTAL frequency mismatch on RT1062

[nickolas_at_whisper]

Hello,

We recently discovered that the RTC input frequency on a custom board that we have built has the wrong frequency for the RTC XTAL frequency.

The user manual says: (Page 1135)

RTC oscillators are either implemented with 32 kHz or 32.768 kHz crystals. Please consult the Security Reference Manual for appropriate frequency selection and configuration.

The RTC oscillator input that we have selected is actually 32.875kHz.

I have a couple questions:

1. How can I figure out what the specified frequency should be for the RTC_XTALi? (I downloaded the security reference manual but cannot find any details on what oscillator frequency should have been selected).

2. What would be the negative effects of using the wrong frequency for the RTC XTAL? (32.875KHz). I'm assuming that the RTC would just run slightly fast, but want to make sure there are no failure modes I'm not thinking about in this case.

Thanks!

[jingpan]

Hi @nickolas_at_whisper ,

1. The frequency is mainly determined by the crystal itself. RT1062 or external circuit can't shift it's frequency too much.

2. There isn't negative effect with 32.875K except you want to use it as a accurate clock source. 

But I think you should take care other issue. Please see the AN3208. An1783 tells users how to set external circuit.

Regards,

Jing