A proper smartwatch can cost quite a bit of money. However, there are some cheap Bluetooth-connected watches that offer basic functions like show your incoming calls, dial numbers and display the state of your phone battery. Not much, but these watches often sell for under $20, so you shouldn’t expect too much.
Because they’re so cheap, [Lee] bought one of these (a U8Plus) and within an hour he had the case opened up and his camera ready. As you might expect, the biggest piece within was the rechargeable battery. A MediaTek MT6261 system on a chip provides the smart part of the watch.
[Lee] noticed some pads on the underside of the board. Two were marked as RXD and TXD, so he (correctly) assumed they would be a serial port. He also identified pads he thinks are for JTAG. Using a Bus Pirate, he was able to read the output from the serial port easily.
If you Google MT6261, you’ll see there are several ways to unlock and flash the chip that is essentially an ARM CPU with supporting I/O devices. That means for under $20 you could have a wearable development platform if you put in the work to get an adequate toolchain going. The MediaTek wiki would be a good place to start although it doesn’t list the MT6261 directly. A quick web search indicates that the MT6571 may be similar enough to get started.