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K60 RTC VBAT Current Draw

Question asked by DAN WASER on Apr 15, 2016
Latest reply on May 6, 2016 by Melissa A Hunter

I am designing with a MK66FN2M0VMD18 microcontroller and using the RTC circuitry with VBAT power supplied through an external Supercap. I have an external 32.768Khz crystal oscillator for the RTC  which uses the internal tunable capacitors configurable by software.


The RTC is configured to use the RTC Alarm output (RTC_WAKEUP_B) to turn on the power supply (required function if power supply is present) and the RTC_CLKOUT pin is also enabled to output the 32.768Khz clock (optional used to monitor/test).


VBAT is powered to approximately 3.0V through diode from 3.3VCC when power is active.

Design intent to is have RTC maintain time when power is OFF for up to 72 hours over operating temp of -40C to +85C.


I looked at the Data sheet and User Guide to try to understand the current draw specifications and would like the current draw clarified.

In Section 2.2.4 Power Mode Transition Operating Behaviors Table 6, it states that IRTC (typ) current draw varies from 357-810 nA -40C to 105C) in VLLS1 mode.

In Section 2.2.5 Power Consumption Operating Behaviors Table 7, it states IDD_VBAT (avg) when CPU is not accessing RTC registers is 6.33uA (at 105C) to 0.8uA (from -40C to 25C) for VBAT = 3.0V.


From description it is not clear to me what the VBAT current draw is when the micro is otherwise unpowered in order to size the appropriate backup power source (supercap) for 72 hour backup.


My questions are:

1. What is worst case IDD_VBAT when VCC is 0V (-40C to +85C)?

2. Does use of RTC_WAKEUP or RTC_CLKOUT affect the IDD_VBAT current draw when VCC = 0V?

3. Do any RTC resister settings affect the current draw of module when VCC = 0?


Thank you.