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Perfect timing. I just started planning to build a DIY smartwatch and was looking into the S3. Having native zigbee support could be nice.

The S3 (and other Espressif chips) are pretty power-hungry. May want to keep looking for a smartwatch application.

S3 was just for me to do an initial prototype. Longer term was going to try with the C3, though I'm open to suggestions :)

You could consider similar devices and what they have used. Another angle is to look at ULPMark results: https://www.eembc.org/ulpmark/