Voltage on VBAt pin in K60

[amleng]

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

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?

[amleng]

Hi Hui Ma,

Actually this is my point. If the wake up pin is an open drain pin it shouldn't be changed by external load. to me it's a bug in the silicon!

[Paul_Tian]

Hi, Amleng

As you know, open-drain pin using as an input pin will cost very low current than normal IO. But open-drain has no drive ability, its input voltage level depends on external circuit. So I think the external load can effect wakeup pin input voltage. I suggest you have a check on your wake-up pin circuit. Thanks.

Best Regards

Paul

[Hui_Ma]

I do a test with TWR-K60D100M RTC_WAKEUP pin, I use 100K ohm pull up resistor connects with RTC_WAKEUP pin, the voltage is 3.22V (VDD is 3.3V).

I also test to use 500K ohm pull up resistor connects with RTC_WAKEUP pin, the voltage is 2.87V (VDD is 3.3V).

If you could remove the RTC_WAKEUP pin peripheral circuit and measure that pin related voltage.

Wish it helps.

[Hui_Ma]

If you check TWR-K60D100M tower board, you could find below schematics about VBAT circuit, which provides battery power and board power selection with VBAT pin.

There is no need to set any register before using the VBAT pin.

About RTC_WAKEUP open drain phenomenon, I need to check with Kinetis product engineer.

I will let you know when I could get any feedback.

Thank you for the patience.