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If external F-RAM is an option, one can use F-RAM from, say Cypress (now Infineon, [1]) together with ARM chips from Ambiq, e.g. Apollo 3 Blue Thin [2], which likely have even better power consumption than an MSP430.

[1] https://www.infineon.com/cms/en/product/memories/f-ram-ferro...

[2] https://ambiq.com/apollo3-blue-thin/

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Dang very cool. If there's ever a Photon 2 I'll definitely look at the low-power ARM world.

(FWIW Photon's USB stack is handled by a STM32F730 and I've been perfectly happy with it. Expensive though.)

IMO that's an odd choice for a micro. It initially looked like it could translate the custom SD card writing to FAT in SW, but with only 64K of flash that's not great. Some STM HAL irqs are almost that size. My recommendation is to find 1 micro that can do low power and USB, and maybe put external FRAM/QSPI for more code storage is needed.

Are there any RISC-V chips that do the same?

ESP32 has a variety of sleep modes down to around 12-20uA and up, depending what you want to leave awake.

With the 3rd CPU, the ULP, running continuously it is around 150uA and then you can be reading from i2c devices, or more. You can get that down to under 50uA with a periodic timed wake up, if you are keen.

Also it will appear as a USB device if needed when fully powered.

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