How to Build a Smartwatch: Picking a Chip
23 comments
·May 14, 2025rgoulter
pjc50
BLE radio device firmware is always going to be provided as binary blobs for a combination of IP and regulatory reasons.
stateoff
More information on the chip here: https://www.cnx-software.com/2025/05/14/sifli-sf32lb52j-big-...
mrheosuper
Interesting 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
bArray
The "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.
Max-q
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.
mrheosuper
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
jsheard
Aren't ESP32s way more power hungry than typical BT-only parts?
bArray
Not 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/...
[2] https://docs.espressif.com/projects/esp-idf/en/stable/esp32/...
numpad0
It'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.
mschuster91
the 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
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
Max-q
From a PCB layout and supply chain perspective, it’s a big difference.
amelius
> The most interesting and difficult constraint is actually software compatibility.
But it is probably the easiest constraint to get around.
alin23
Glad 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_f...
[1] https://www.reddit.com/r/smartwatch/comments/174hq9x/need_a_...
[2] https://tildes.net/~tech/18nf/smartwatch_primarily_for_notif...
0xEF
I 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.
null
MrAlex94
Would Withings[1] watches not fit the bill? I have a ScanWatch and the battery life lasts almost a month.
alin23
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.
MrAlex94
For what it’s worth, after I bought one, around five other people I know got them, varying models from the Steel HR to Nova, and haven’t been made aware of any issues. Of course, personal experience doesn’t apply to the mass of products, and these are all UK SKUs, so who knows if issues vary between countries as well?
Overall everyone has been happy though and I haven’t seen Withings come up on HN which is surprising tbh!
"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/6c82a9b15db49871...
Which isn't a problem. But, I wish if something is described as "open source", you could read the source code for it.