RTC clock and backup battery configuration for KL25

[dhananjayankutt]

Hello friends,

I am working on a system design based on MKL25Z128VLK4. The system will be powered by an external supply with requirements to maintain time. I need some ideas about RTC clock source with  a coin cell battery backup.

Is there any option to provide battery backup only for RTC not for whole MCU? We understand that there is no separate power domain for RTC logic. Does this implies that  we have to go for external RTC chip with battery backup interfaced through I2C? Or can we use a ORING Battery + External power input and detect external power failure to force the processor to low power mode?

Is the 32Khz Clock to run the RTC can be given only through external clock source or it can be internally sourced through internal oscillator ("OSC logic") with ERCLK32K clock? Again will it work across power modes?

Please anyone put your ideas forward. Thanks in advance.

[Paul_Tian · Accepted solution]

1. First, sorry, KL series does not like K series, it owns BATT pin. So I think KL cannot only powred to RTC. But I do not think you have to use external RTC chip for your application. KL series has very low leakage stop power mode(about 300nA), the core consume is very low.

2. For your external battery voltage detection, KL can do this function.

3. For your RTC clock source question, there are three clock source for RTC module. They are external clock(32.768KHz), internal 1KHz LPO and RTC_CLKIN (external reference clock in). Reminder is only RTC_CLKIN, this clock source can work in VLLS0 mode(the lowest power mode).

[dhananjayankutt]

Sorry for late reply.

Thank you so much for your ideas Mr. Zhe Tian.