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Comment by Rinzler89 | Hacker News Reader

Can I ask why you went with a MSP430?

I'm not judging, they were all the rage in low-power embedded applications 10-15 years ago, just feels weird to see it in a modern design today, when the market now is flooded with low-power ARM chips that are way more capable than that, and probably easier to program.

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Not the OP, but my understanding (admittedly a few years outdated) is that an MSP430 will, in idle, with sleep states properly set up and low power modes in use, drain a coin cell battery (e.g. CR2032) over a period of years - with the current drawn around 1.5 uA in standby and 0.1uA in RAM retention idle.

That is likely even lower than the effective self discharge rate of a lithium coin cell battery. I don't believe the Arm equivalents like the Cortex M0 can deliver such low absolute current - maybe microamps, but more than an MSP430 from what I can see.

Ref - https://metebalci.com/blog/measuring-the-power-consumption-o...

> That is likely even lower than the effective self discharge rate of a lithium coin cell battery.

That's what I've found too. Based on my measurements and calculations, Photon uses 5µA (MSP430 + motion sensor) while sleeping, while the battery's self-discharge is more like 80µA.

That's probably a good argument for using a more powerful chip though, if the battery self-discharge is that high, hah.

Maybe tiny solar panel + capacitor, like 1980/90s calculators had, then it could run "forever" - digital Millennium Camera.

This is a cool idea!

Or a different battery chemistry with a lower self discharge rate. No need to be rechargable in most of the use cases, I'd imagine.

Panasonic BR2032 has 30µA discharge rate https://www.digikey.com/en/products/detail/panasonic-bsg/BR-...

If that's the self discharge rate then the battery would be fully discharged after less than a year on the shelf (given its 190mAh capacity). Coin cells typically have a much longer shelf life than that. I wonder what that figure is, exactly.

This sounds like a job for a lithium thionyl chloride battery.

The guys with the purple LiSoCl2 batteries make ones with up to 40 years of usable life

Some of the SiLabs EFM32 parts have comparable sleep currents.

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.

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

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

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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