Keep running Autonomo's RTC

I’m trying to connect coin battery to keep running RTC inside the SAMD21 like the arduino Zero with the positive of the battery connected to the 3v3 pin, but it doesn’t work.
It’s there another pin to do this?

I’m not exactly familiar with the Zero’s hardware. However, the RTC is not a seperate component on the board, but a part of the SAMD21 chip.

Once you configure the RTC to run, it will continue counting while the board is powered up.

If the board resets, the library attempts to preserve the existing clock time. This is contigent on the RTC being configured before the reset event. There is also the option to force it to reset the clock by passing an optional true to the begin() method.

Edit: I have checked the datasheet, and the RTC is reset when the board is power off and then on (Power On Reset).

@GabrielNotman - I’m bringing up a Autonomo/Rev5 and I can only find the Rev3_sch - I realise this is an old thread…
just wondering if there are any more thoughts on being able to maintain the RTC with a small coin battery?
Thats typically what happens with other systems.

Seems to me might be possible to do it with a small backup voltage on 3.3 that keeps it above the minimium 1.7V - but all depends on how much uA other chips tied to 3.3V use. Particularly what version of the XC6220 is used. Some LDOs are being specified for backfeed - that is 0V on Vin while there may be a Vout - I think that requires a variant XC6220A.
Though probably the largest quiescent user is any device plugged in on the uSD.

So anyway -thought I would fly this question. Any insights appreciated.


The RTC is a component within the SAMD21 micro-controller. In order to keep the RTC clock running, the micro-controller must remain powered (however in deep sleep you should be able to get this down to < 50uAs).

While you can run a SAMD21 on a lower voltage that 3.3V, I wouldn’t advise trying to do it with an Autonomo. There are a lot of complications including the maximum clock frequencies and the IO levels with other components on the board (or attached to the board).

In your case may want to look at an external RTC such as the DS3231 (just make sure you get an RTC that uses 3.3V for supply and IO levels).

Ok. I should have said I was a design engineer - so just wanted to check if I hadn’t missed something.
I was hoping that there would be a low cost way of maintaining the RTC with out an external chip.
I’m popularizing that the Kinetis KL28 Cortex M0 and K28 CM4 both have a builtin+5V regulator that helps with a graceful power management - which in addition also has an accurate onboard Vref and ADC16. The TeensyLC/36 have pioneered some of the arduino tools.