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How to Build a Smartwatch: Picking a Chip
- mrheosuper 1y agoInteresting they did not go with 2-chip design(1 for main application, 1 for BLE stuff). Which is sometime makes sense because high power mcu usually does not have RF
- mschuster91 1y agothe more chips you have, the more complex the project becomes. BOM is one thing, every chip needs support passives and oscillators, but now you also need to coordinate communication between the chips, you need to devise a way to update firmwares and access both chips for debugging purposes... that might be worth to trade off for less battery life.
- mrheosuper 1y agoin 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
- Max-q 1y agoFrom a PCB layout and supply chain perspective, it’s a big difference.
- mschuster91 1y ago> in my experience they are not that much difference between 2 design. Depends! If the two chips use UART or SPI for intercommunication, okay, you need two lines between the CPU and two GPIO lines for wakeup, and JTAG can be shared anyway. But if you use stuff like shared memory, or want to do stuff like updating the display not just from the high-power chip but also from the low-power one, suddenly design becomes much more complex.
- bArray 1y agoThe "high-end" modern MCUs are pretty great, you have the NRF offerings, but also the likes of the ESP32 where you can get Bluetooth and WiFi in a single package. Personally these days I would lean towards the ESP32, they continue to iterate on it nicely and it has great community support. I'm personally developing a smart watch platform based on micropython.
- jsheard 1y agoAren't ESP32s way more power hungry than typical BT-only parts though?
- bArray 1y agoNot insanely for a smart watch. Your smart watch battery will be something like 200mAh, so for 20 hours you need to average 10mAh. With zero optimisation, screen refresh rate at 30+fps, I have smart watch chewing 30mAh. Getting down to 10mAh is not so bad. If you're not actively driving the display, you can under-clock significantly [1], if you're not using WiFi you can turn the modem off [2]. [1] https://docs.espressif.com/projects/esp-idf/en/stable/esp32/api-guides/low-power-mode/low-power-mode-soc.html https://docs.espressif.com/projects/esp-idf/en/stable/esp32/... [2] https://docs.espressif.com/projects/esp-idf/en/stable/esp32/api-guides/low-power-mode/low-power-mode-wifi.html https://docs.espressif.com/projects/esp-idf/en/stable/esp32/...
- mrheosuper 1y agono, esp32(the original one) is insanely power hungry, especially its radio. Also 20 hours of runtime is horrible.
- bArray 1y agoI'm getting 10 hours run time with the screen continuously running and drawing graphics every refresh in micropython - extending this time out is definitely possible. There are many ESP32 variants, depending on what you pick some may be more compelling for your use-case.
- numpad0 1y agoIt's just a watch. You don't need a full UNIX computer to tell time, or to record heart rates or pinging AWS for those matters.
- znpy 1y agoInteresting comment, because according to https://en.wikipedia.org/wiki/PDP-7 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.
- internetter 1y agoThat's a complete non sequitur. Just because UNIX can run on lower end hardware doesn't mean "a full UNIX computer" is the best tool for the job.
- numpad0 1y agoAnd frustratingly all "lacks MMU" and V6 UNIX binaries don't just run. Imagine the world where random IoT gadgets all run backported GPLv2 GNU/Unix systems and crashing left and right. Instead there's a massive gap between Linux-capable systems and RTOS-focused systems despite, like you said, the latter now being bigger and better than real shared UNIX systems.
- ItsHarper 1y agoThe Cortex-M33 core in the SiFli chip will be much faster than the M4 cores that the fastest released Pebble watches used, so a faster MCU than this is not something that's needed. However, more battery life is very welcome, and the fact that they're using MCUs with integrated Bluetooth this time seems to be a huge part of the upgrade from about a week of battery life to about a month of battery life.
- alin23 1y agoGlad to finaly see someone in the low-power chip industry going in the open source direction. Thanks for the insight! When I saw rePebble be announced, I signed up for it right away. Only later I realized I actually don't want a smartwatch, I want a dumb watch with vibration notifications. I know I'm in the minority, but it's a niche that has a few very interested people in it [0] [1] [2] After wearing the Casio F105 for the past 2 years, I can't go back to something larger, heavier or thicker than this. I could accept weekly battery charging for the benefit of having some bluetooth functionality. So nowadays I'm looking for a super small bluetooth chip that can power a small vibration motor, which can receive all notifications from my iPhone. I would like to glue that chip, motor and a small lithium battery between the two straps of my F105, because in my tests it seems I don't notice if I add a small weight there. I still remember when I first used my first Mi Band 1, a forgotten fitness band that had no display, just 3 RGB LEDs that could even get specific colors based on the app that sent the notification. I could know right away when I got a blue Messenger chat that I needed to answer now, or a yellow Google Keep reminder that I could ignore until I got back to my computer. [0] https://www.reddit.com/r/pebble/comments/9xw2j2/im_looking_for_a_dumb_smartwatch_that_just/ https://www.reddit.com/r/pebble/comments/9xw2j2/im_looking_f... [1] https://www.reddit.com/r/smartwatch/comments/174hq9x/need_a_minimalistic_yet_reliable_watch_for_email/ https://www.reddit.com/r/smartwatch/comments/174hq9x/need_a_... [2] https://tildes.net/~tech/18nf/smartwatch_primarily_for_notifications_and_battery_life https://tildes.net/~tech/18nf/smartwatch_primarily_for_notif...
- 0xEF 1y agoI still rock my OG Pebble and while I am excited they're coming back, I have no plans to upgrade. Like you, my needs are simple; vibrating alarm, notifications, but with one key factor; I need a display that I can read in broad daylight that plays nice with my far-sighted eyes. The eInk display on the OG Pebbles hits the mark. Being able to read a text without pulling out my phone is also nice. Plus they can be got on eBay for about $30 USD and a fresh battery is about $15, so they don't break the bank. The Rebble.io community's work is still functioning well enough for my use, as well.
- Knork-and-Fife 1y ago
- stateoff 1y agoMore information on the chip here: https://www.cnx-software.com/2025/05/14/sifli-sf32lb52j-big-little-arm-cortex-m33-bluetooth-mcu-powers-the-core-time-2-smartwatch/ https://www.cnx-software.com/2025/05/14/sifli-sf32lb52j-big-...
- rgoulter 1y ago"The SDK is open source" Oh, a BLE MCU with open source code SDK? AFAICT, the BLE code is provided as a binary blob. https://github.com/OpenSiFli/SiFli-SDK/tree/6c82a9b15db49871cf49570b3e3658e214d50610/middleware/bluetooth/lib https://github.com/OpenSiFli/SiFli-SDK/tree/6c82a9b15db49871... Which isn't a problem. But, I wish if something is described as "open source", you could read the source code for it.
- pjc50 1y agoBLE radio device firmware is always going to be provided as binary blobs for a combination of IP and regulatory reasons.
- WJW 1y agoI 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?
- tcbawo 1y agoTo avoid intentionally or unintentionally interfering with frequency spectrum it should not be.
- archi42 1y agoThe BT blob (or WiFi blob, or general radio blob) often controls low level aspects of the actual RF device. So you can often alter frequencies, power levels and other parameters. This results in the device acting out of spec. Now one might say that's the fault of the person doing the modifications/manipulations, but regulations in various countries require the device to prevent these manipulations. (N.B. I'm not in the exact business, but that's my take away from looking into the topic some time ago)
- HPsquared 1y agoSurely it's possible to maintain control over that while still publishing the source code? Seems like security by obscurity.
- amelius 1y ago> The most interesting and difficult constraint is actually software compatibility. But it is probably the easiest constraint to get around. I would put this one more towards the end of the list.
- ItsHarper 1y agoThere's two big factors that make software compatibility very important for this product specifically. 1. His team is positively tiny compared to what Pebble used to have, and the less software work that's needed, the better. 2. All of the apps and watchfaces people wrote for the original Pebbles were distributed as compiled ARM binaries, so if you picked an MCU with an entirely different instruction set, you'd lose backwards compatibility. ESP32 would fall into that category, for example (not that it would have been a good choice anyway).
- amelius 1y agoI 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).
- mrheosuper 1y agoThere 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.
- rjmunro 1y agoPebbleOS isn't the only player in this space. There's also https://www.espruino.com/ https://www.espruino.com/ which powers https://banglejs.com/ https://banglejs.com/. It's a tiny implementation of Javascript for microcontrollers, so it's really easy to hack your device in real time. It also powers the Fallout Pip boy and possibly some other stuff from the wand company: https://www.thewandcompany.com/fallout-pip-boy/ https://www.thewandcompany.com/fallout-pip-boy/ See: https://github.com/orgs/espruino/discussions/7577 https://github.com/orgs/espruino/discussions/7577
- swyx 1y agoappreciate the shouting out of good work for lower profile competitors. Pebble is awesome but they deserve some love too. thx also everyone watch the Amazon Fallout show if u have the chance its p good
- sabellito 1y agoHad never heard of bangle js, love the idea of fiddling with it with a languange I already know. Would love to buy the 1st version, as I don't like square designs, but it seems they don't sell it anymore.
- djfergus 1y agoI’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)?
- dingensundso 1y agoThey sell existing hardware with their firmware. You're right, the bangle.js 2 is a SMA Q3 (https://hackaday.io/project/175577-hackable-nrf52840-smart-watch https://hackaday.io/project/175577-hackable-nrf52840-smart-w...)
- noman-land 1y agoI'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.
- outadoc 1y agoThank you for the write-up. Cool to see some degree of open-source hardware running PebbleOS. Though, can we stop having left-aligned blogs in 2025? Wide screens have been here for a while, it makes it unnecessarily hard to read :(
- wkat4242 1y agoThis smartwatch isn't really all that smart by today's standards. I would really miss the mod cons I have on my current smartwatch. Payments over NFC, dual-band GPS tracking, 4G LTE connectivity. The Pebble (and repebble) trades all that in for a multi-week battery life. But the minor hassle of once-every-two-days charging on my galaxy watch really isn't enough to forego all these powerful features for me.
- micromacrofoot 1y agoIt is for me, I don't care about any of those features. I just want a watch I can customize a bit that gets notifications and can do rudimentary heart rate monitoring. If I have to charge it more than once a week I won't wear it.
- me_online 1y agoThe battery life alone suggests otherwise.
- dwayne_dibley 1y agoI too would miss GPS, BUT I've still never found anything that really fills the pebble shaped hole in my life. My Garmin compromises the other stuff (battery, widgets, calendar, display readability etc.). Everything is a trade off.
- bigstrat2003 1y agoConversely, I don't care about any of those features, and I do care about long battery life. It's great that different devices catering to different users exist. I don't think the fact that they are aiming at different users from you makes it "not smart", it just makes it not suitable for your use.
- wkat4242 1y agoDon'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.
- Shalomboy 1y agoLove this write-up. nimBLE is a great bluetooth stack; this should open up some interesting opportunities for Core Devices moving forward.
- zoobab 1y agoI started to hack around cheap chinese Freqchip SOCs: https://github.com/zoobab/FR801xH https://github.com/zoobab/FR801xH You could get smartwatches for 3EUR on Ali with this chip.
- ENadyr 1y agoWow, some of these chips are 2.6rmb at 3k MOQ ($0.36) for an Arm Cortex m3 with BLE (and sig-mesh) support... this is wild!!! How usable is the SDK?
- bgnn 1y agoBLE is fairly straightforward to implement. It's the compliance testing which is costly. Are these fully compliant, i.e. FCC approved?
- stavros 1y agoCan you link to one of the watches? I haven't managed to find anything with two quick searches.
- ENadyr 1y ago@ericmigi these are great BTW, wish you did them during YC while we were going through all these decisions haha
- aswanson 1y agoAwesome feature set for that price.
- gitroom 1y ago[flagged]
- gazarubbleparty 1y ago[flagged]