How to Build a Smartwatch: Picking a Chip

I’m confused, is bangle js manufacturing new hardware (and is that open with schematics, bom, gerbers) or just shucking an existing cheap Chinese smart watch (SMA Q3)?

I'd be willing to sell you mine as I bought it a while back and never got around to playing with it. It's basically mint condition unused.

Zephyr provides an open source implementation of all layers of the BLE stack. The radios of some devices are documented extensively, Nordic nRF5 devices are probably the best example.

The Bluetooth SIG requires that you qualify your device if you advertise that you use Bluetooth IP, similar to what is required for the cellular space. Do you have to do this if you’re just “Bluetooth compatible?” Maybe not. Whatever the case you have to conduct FCC part 15C testing (intentional radiator).

I understood the criticism to be about describing it as open source when it isn't, i.e. that

"We say it's open source because we expect the reader to know that we're not telling the truth"

should be replaced by

"It's open source except for the BLE firmware blob, which can't be open source due to regulatory reasons."

To be fair, the article just repeated the claims made on the GitHub page for the SDK.

I have yet to see a real case of "binary blob for regulatory reasons". What law prevents open source RF firmware?

The device needs to operate in spec and if someone reflashes it with out of spec firmware, that's no different than someone soldering a different resistor onto the PCB or feeding the output of the chip into an amplifier and large antenna. It's a modification by the user and so the user is liable for operating out of spec. And all of this really unrelated to source code, just to reflashablitiy. The code could easily be open, but the productions device could be made unflashable if the law really required it. Yet this is not what we see here.

It's IP reasons, and that's fine, but let's not make up additional excuses for them.

I know very little about bluetooth firmware beyond using it for apps and such, but what kind of regulatory reasons are there that prevent publishing source code for bluetooth radios?

Not in Zephyr. There's a full BLE Controller in source code form there.

Same, I love Pebble and Pebble-like devices precisely because they have long battery life. I don't want or need payments on my watch. I'm also not a fitness junkie, so I don't use GPS tracking either. I mainly enjoy the well-designed UX Pebble provides and its simple functionality like alarms, stopwatch, and notifications.

With watches that need charging every one or two days, I get this Sisyphean feeling that I am owning a constantly dying device with a battery indicator that exists solely to be charged. The amount of functionality a watch can provide me (versus a smartphone) just doesn't justify the amount of charging I need to do.

Maybe I'm not a smartwatch power user like OP. But the Pebble and its ilk are a great fit for the niche audience I belong to.

Don't get me wrong, I backed the first Pebble on kickstarter. Still have it. It was a great watch for the time. I loved it.

But these days I just have bigger requirements. The whole heartrate thing I don't even care about. Notifications are the main thing, but route tracking, integration with my alarm system and above all payments are just things I can't do without anymore. I had an Amazfit for a while which also had a 3-week battery life (though no apps) but it missed the payment option.

The charging is not a big deal for me because I only wear my watch outside anyway. And it charges in 30 minutes. It makes it easy for me to keep it charged also because it becomes a routine. With the amazfit it was often empty when I needed it.

But yeah it is great that there are devices for everyone.

While the ESP32 is great for many applications, it’s not for battery operated stuff. When an nRF draws 1-2 mA when using BLE, ESP32 will draw 40 mA. And the chip they selected is even more efficient.

The low power chips can also run in low power mode without BLE running using micro amps, something the ESP can’t match.

I really like ESP32 and I hope they have a low power chip on their roadmap.

Aren't ESP32s way more power hungry than typical BT-only parts though?

I would not consider ESP32 high-end MCU, it still lacks many peripheral(DSP, GPU), its core clock is not high(only 240mhz iirc).

Recently they release ESP32P4, with very strong performance, but like you guess, without Radio

in my experience they are not that much difference between 2 design. The BLE FW is a binary blob that you will download at boot with 2 chip-design, or load it to correct address with single chip-design.

From the CPU perspective, they are the same

Interesting comment, because according to https://en.wikipedia.org/wiki/PDP-7 the PDP7 (the first computer "UNIX" ran on) had ~9.2KB of memory (supporting up to 144KB).

Most contemporary SoCs will have more memory (and compute power) than that.

Small correction, pebbles use a TFT "e-paper" display, not e-ink. This is how they have reasonable refresh rates for animations and no after image problems. It's the best compromise imo

> I kept getting distracted by my phone ... so I completely disabled all notifications

Couldn't you just disable notifications for the unimportant apps?

I had a Seiko watch for decades that needed a new battery every 5 years, and I still miss it. It never left my wrist except for battery replacements. The Casio mentioned above goes one better by using solar, so it would never have to leave my wrist - and it has a BLE connection to the phone. But it doesn't have enough sensors, like an accelerometer.

Withings smartwatches come do closest, and that's what I have now. Their standout feature is their 30 day recharge, but they still have a ways to go.

They are expensive, but worse they are fragile and no one will repair them in most countries, and the rechargeable battery means they will only live a few years. On top of that I'd like to see them add NFC, payment card and passkey integration, with better security so if they leave you wrist you have to re-authenticate or reset them. None of these things require much (if any) power. Also while the analogue time is great, interacting with the digital interface is clunky. A single button is not enough. The digital interface could be bigger, and could display more - like displaying the last few message.

So my take is while smartwatches are improving, they still have a ways to go. And that way is not the direction Apple appears to be taking, which seems to be to put a power hungry iPhone with a miniature screen on your wrist. I'm hoping the old watch manufacturers will come back with their robust very low power designs, with a few added sensors.

The Scanwatch Light seems to be very close to what I need. While they're priced reasonably for what they offer, they're crazy expensive for what I would need. I'm really not interested in fitness or sleep tracking these days anymore.

I'm also a bit scared of the many "charging issues" some people seem to have with them after a few months, but I guess every batch has a few bad devices so I could hopefully return it.

Thanks for the recommendation! I might try one soon.

Whoop.com, your wish has been granted! Just pony up $19.95/mo (or whatever) as well...

BLE is fairly straightforward to implement. It's the compliance testing which is costly. Are these fully compliant, i.e. FCC approved?

There are solution for question 2. You can either add a compatibility layer to the software stack(something like Java VM), or people can build their app against your codebase, and your watch will dynamically load it and run(like how linux run their app). In Zephyr they have LLEXT for that.

I mean, these applications are by necessity very tiny. They must fit in a very small memory. Hence, I seriously question that it would take a lot of work to rewrite the software for a different architecture. You can reuse the entire graphical design (which is probably a large chunk of the work because designing for small screens is not easy).