LPC55S16 Battery for RTC

[Matthias9265]

Hi!

I'm developing a system that needs to hold the RTC to keep time through longer sustained power down states (potentially a year+). If I understand correctly, I need to connect a battery or super cap to VBAT_PMU and that will hold up the RTC. Is there anything else to consider?

Does this now mean VBAT_DCDC needs to be connected to the 3.3V supply (and not connected to VBAT_PMU)?

Will this cause any problems with a 3V battery on VBAT_PMU as that battery drains?

Thanks!

[Matthias9265]

Thank you for the reply.

In this case, should I expect the RTC to hold through a complete removal of 3.3V to the MCU core (all other supply pins)? 

Thanks!

[xiangjun_rong]

Hi,

Yes, you can just connect the VBAT_PMU of LPC55S16 to battery or 3.3V, and float the VBAT_DCDC and the other power supply such as VDD, VDDA, Vrefp, in the case, the RTC will work and hold the time.

Hope it can help you

BR

XiangJun Rong

[Matthias9265]

At this time, the RTC is resetting to 1/1/1970 when VBAT_PMU is connected to 3V and the remaining supply inputs are cycled. It continues to count when the running, but will not hold time through a main supply interruption.

Any thoughts?

[xiangjun_rong]

Hi,

I copy the reset Fig from UM11295.pdf, as you see that the POR(Power On Reset) also Reset the RTC, that is why that the RTC can not hold time after main supply is powered.

I suppose it is a bug, the POR should not reset RTC.

Hope it can help you

BR

XiangJun Rong

[Matthias9265]

Thank you for the response.
Is there a method to disable the POR for the RTC?

[xiangjun_rong]

Hi,

Unfortunately, there is not a method to disable the POR for the RTC, I think this is a bug.

BR

XiangJun Rong

[xiangjun_rong]

Hi,

If you just want to run the RTC, in the case, it is okay that you can just connect the VBAT_PMU to a battery or super CAP, the VBAT_DCDC is not powered.

It is okay that you  connect the VBAT_PMU to a battery or super CAP, the VBAT_DCDC is powered with 3.3V power supply, but the LPC55S16 is in power down or deep power down mode, in the case, the RTC can generate event to wake-up the LPC55S16.

Hope it can help you

BR

XiangJun Rong