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Voltage on VBAt pin in K60

Question asked by amleng on Sep 19, 2013
Latest reply on Sep 24, 2013 by Zhe Tian

Hello everyone,

 

I use MK60DN512 MCU for my project. In my customized board I'm going to use the RTC and wakeup pin for power management purposes. I just noticed that there is a 3.3 V on VBAT pin which comes directly from CPU (no battery connected)! Based on datasheet this pin is supposed to be an isolated Input pin. So, I wonder if I should set up some registers before using this pin or something else going on here? I have 4 customized boards which all of them have the same issue (3 voltage on the VBAT).

Below you can see our schematic for this part. (Header 2 is battery which is not connected).

 

Cheers,

Sina

 

VBAT.JPG

 

Apparently, there is a relationship between VBAT and RTC wakeup pin. Based on the datasheet RTC wakeup pin is an open drain, active low output. so, we connect this pin to a 10K pullup resistor (as you can see below). If we increase this pull up to like 100k the VBAT pin voltage drops from 3V to 1.7V! So, I wonder if this wakeup pin is not a real open drain! Is there any internal pullup resistor on this pin? is there any leakage current from wakeup pin to the CPU?

 

wakeup pin.jpg

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