One main reason I chose the MSP430 was for the FRAM. Photon stores all its state (ie the photo ring buffer indices, and user settings) in one big FRAM struct, which can be read/written just like RAM, but persists across power cycles and crashes.
Another reason was MSP430's low power consumption, but like you said it sounds like ARM has caught up.
Would love to hear about alternative designs -- are there low-power ARM chips that have something like FRAM and don't require erase/programming dances to write data?
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.
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.
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.
Another reason was MSP430's low power consumption, but like you said it sounds like ARM has caught up.
Would love to hear about alternative designs -- are there low-power ARM chips that have something like FRAM and don't require erase/programming dances to write data?