Power for RTC on LPC54101

[sturzi]

Hi,

I have a question regarding the RTC on the LPC5410x.

We are developing a new product based on the LPC54101.

On all other LPC Chips we used so far the RTC runs from a separate power pin VBAT.

But on the LPC5410x the RTC runs from the same power pins as the rest of the chip.

This makes things more complicated for us.

Our use case is the same as with the old LPC Chips:

When the product is running it is powered by a power supply.

When the product is off the power supply is removed, and the RTC should be kept running from a battery or super capacitor.

If the RTC runs from a separate VBAT pin this is straight forward to implement.

But since on the LPC5410x the RTC runs from the same power source this is not so easy anymore.

If the power supply is removed we have to get an early notification so the chip can be set to the deep power down mode.

• External interrupt from a power supply chip
• A/D converting the input voltage and triggering if we drop below a threshold
• ...
  

If the chip is not put into deep power down mode in time, it will quickly deplete the battery or super capacitor and we lose the RTC.

Additionally we have to separate the battery/super capacitor from the regular power supply somehow so that:

• the battery or super capacitor can be charged
• if the power supply is removed it can power the RTC

Once the power supply is connected again we have to wake up from deep power down

Waking up from deep power down can only be done via an RTC alarm, or by the reset pin.

Since the RTC alarm is not an option we have to somehow trigger the reset pin when the power supply is reconnected.

It looks like the RTC on this chip is designed for purely battery operated devices instead of our use case.

Implementing our old use case is much more complicated and error prone.

Does anyone maybe have an application note or suggestion how we best implement this?

Thanks

[jeremyzhou]

Hi ,

Thank you for your interest in NXP Semiconductor products and for the opportunity to serve you.
According to your introduction, it needs to utilize a complicated power-supply design to assure the power supply is uninterrupted even the regular power supply is removed, just like you pointed out before.

If the chip is not put into deep power down mode in time, it will quickly deplete the battery or super capacitor and we lose the RTC.

Additionally we have to separate the battery/super capacitor from the regular power supply somehow so that:

• the battery or super capacitor can be charged
• if the power supply is removed it can power the RTC

In addition, you said that the reset pin is used to wake up the MCU from the deep power-down mode and RTC alarm is not an option actually. When the MCU exit from the deep power-down mode, the MCU reboots, in another word, it will reset.
So in my opinion, when the power supply is reconnected, it can trigger power-on-reset (POR) and it meets your requirement.

Have a great day,
TIC

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