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I am inspired by one of the topic but very very old (4 years) to flash espruino on cheap fitness tracker.
1 - Hack something.
3 - Get know how about nrf software, SDK, softdevice, application etc.
2 - To learn the flashing process using DFU and SWD programmer (ST-Link v2, J-link ) or OpenOCD with raspberry pi.
3 - To have some fun
Since I already have a microbit so I think that fitness tracker is sharing the same SOC as the mBit.
I found one ebay listing for such fitness tracker using nRF51822 32 KB ram 256 KB flash. Ordered and yet to arrive.
I googled for more information available in the advertisement and found that I may be a clone on V07 SPORT BP Heart Rate Tracker. Searching on the same on FCCID found this.
So assuming this is the same as in the image, I can open the tracker with just 4 screws, so no cut & open.
I can see 5 pads in a line, center of the PBC. Last 2 are SCLK and SWD. So this would be the SWD pads where my programmer will connect.
I am still not clear about the display type and controller.. Serial (SPI/I2C/) or Parallel (8 or may be 16??)
I will post more updates soon.