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I am looking for the typical power consumption of the OSCULP32K (Ultra Low Power Internal 32kHz RC Oscillator) of ATSAMC20J18A. Found nothing in family datasheet so far, only XOSC32K. Can I expect the RC oscillator to draw even less current than crystal one?

Background: I'm migrating an AVR project to SAM C20. The former uses a dedicated Maxim Dallas DS1337 RTC (typical timekeeping current 425nA). So my plan for SAM C20 is

  • a 32.768kHz crystal on XOSC32K
  • clock the RTC from OSCULP32K when on battery
  • wake up periodically, say once a minute, turn on XOSC32K (External 32 kHz Crystal Oscillator) and adjust OSCULP32K calibration.

I just need to be reasonably confident that the timekeeping power consumption is going to be about the same or better than DS1337. No precise specification to match, luckily.

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    \$\begingroup\$ Carefully check out start-up time for the 32 kHz Xtal oscillator...waiting for it to power-up may flummox your low-current plan. RC oscillators start up very fast. \$\endgroup\$
    – glen_geek
    Oct 18, 2019 at 13:37

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Can I expect the RC oscillator to draw even less current than crystal one?

You can expect nothing. However, a guideline to reading datasheets that I rather disdained when I heard it from an old guy with bushy white eyebrows 35 years ago, and which I've come to believe solidly since, is that if the parameter isn't mentioned in the datasheet, it's because the manufacturer isn't proud of it. If the number were a good one, it'd be in there.

It could be an oversight -- I'm using a couple of processors from that family at work, and the manuals are woefully inadequate compared to what ST provides. But it could also be that the numbers just aren't stellar.

If you have an important spec to meet (and "the same or better than the DS1337" is far better formed and easy to pin down than many I've seen) then you should choose your hardware based on that spec, and don't proceed unless the datasheet shows you how, with margin. Otherwise you're just rolling the dice, and the dice are weighted against you.

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  • \$\begingroup\$ ST = ST Microelectronics? Appreciate your answer though. Let's see if I can get a reply from Microchip how much is "ultra low"... "This request may fall under Non-Standard Customer Action Request (NSCAR). NSCAR is our way of handling requests for information not available via Microchip’s website or other publications." \$\endgroup\$
    – Xpector
    Oct 18, 2019 at 16:01
  • \$\begingroup\$ The older I get the cynicaller I get (and my grammar has gone down hill, too). If you ask, be sure to ask over the temperature range you're interested in and at your desired VCC. Or, somewhere in their 600+ page data "sheet" they actually tell you, and it got lost in the other information. \$\endgroup\$
    – TimWescott
    Oct 18, 2019 at 16:27
  • \$\begingroup\$ Yes, ST Microelectronics. I love their stuff. \$\endgroup\$
    – TimWescott
    Oct 18, 2019 at 16:27

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