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How to run RTC wihtout using an external oscillator?

Question asked by Gaurav Banyal on Mar 31, 2017
Latest reply on Apr 26, 2017 by xiangjun.rong



I have a custom board for my project using an MKL03Z32. It does not have an on-board external oscillator. So I want to use the RTC without the external oscillator.


The example project for the FRDM-KL03Z board seems to use the external oscillator. I confirmed this by removing R1 and R3 from the board. 


I am running the core clock at 8MHZ. Please let me know how to configure the RTC in this situation. 



PS: I was able to get 1 second interrupts from the RTC block by running it with LPO source clock. The problem is however, in this approach, every time you get the interrupt, you have to configure the RTC prescaler to a value (32768-1000) to get the next 1s interrupt. This is not what I desire. In my design, I just want to run the RTC so that the TSR increments every second in RUN mode and in VLLS3 mode and continues to run in the transitions between these modes. The use case is that the RTC is started while in RUN mode, MCU sleeps in VLLS3 mode, wakes up with an external interrupt source to RUN mode, reads the RTC->TSR and stores the value in the flash and sleeps again in VLLS3 mode. I am using the VLPR for other things in my application, so can't use even that.



Best regards,