vbat pin s32k

[bharadwaajra]

I was going through the S32K pin mapping excell sheet I could not find any V-Bat pin for RTC is provided. 

[danielmartynek]

Hi,

The S32K1xx RTC doesn't have an independent power supply pin.

Regards,

Daniel

[ueliton_rodrigu]

Hi there danielmartynek,

So should I connect all power pins to Vbat?

Regards,

Ueliton

[danielmartynek]

Hi Ueliton,

I quite don't understand, could you please elaborate?

Thanks,

BR, Daniel 

[ueliton_rodrigu]

Hi Daniel Martynek,

I'm designing a PCB using S32K146 and need use RTC in my application. S32K has internal RTC module, so I don't have to use any external I2C RTC module, just place the external oscillator.(that is my understanding).

I understood (correct me if I wrong) that I have to connect a 32.768kHz oscillator at S32K RTC_CLKIN pin.

Like Bharadwaaj RA mentioned above, I didn't find the Vbat pin in S32K to place a backup battery. Should I connect all S32K power pins to a CR2032 3V-battery (for example) to preserve RTC timer in case of external power failure? Like bellow...

Ext_power is from the onboard 12V/5V DC-DC converter.

CR2032 is 3V.

In the scenario where there is no Ext_power, the RTC timer will be preserved by the CR2032 battery.

Thank you for your help and feel free for the proper corrections.

Regards.

Ueliton

[danielmartynek]

Hi Ueliton,

Yes, the external clock 32.768kHz have to be connected to the RTC_CLKIN pin.

You would need to power the whole MCU.

Please refer to AN5426 Hardware Design Guidelines for S32K1xx Microcontrollers, Section 3 3 Power supplies.

We don't have any battery-power-design recommendation for this MCU, this is application-dependent.

However, the S32K1xx require VDD => 2.7V. (S32K1xx Data Sheet, rev9, Table 2).

Can the battery provide at least 2.7V via the diode?

Regards,

Daniel

[ueliton_rodrigu]

Thank you danielmartynek‌.

About RTC clock and power it is clear to me now where to connect clock and the need of backup power the whole MCU.

About the Power Supply pins, I had already looked the Guideline (AN5426), but decided to followed the S32K144EVB_Q100 schematic downloaded from NXP website: https://www.nxp.com/downloads/en/schematics/S32K144EVB-SCH-29248-RB.pdf 

About the 2.7V thru diode you are right, we will not be able to do that with CR2032. Even using low forward drop diodes (0.3V) it will be at the limit. 

Our application aims to log the state of the system, so we need keep the timer (RTC).

I will consider the use of another battery or even the use of a supercapacitor. 

If you have any other suggestion, please... feel free.

Regards,

Ueliton