1Get a X9 Pro Activity Tracker
Aliexpress, Banggood, and Ebay are your best bet. Pricing will vary widely so it pays to look around.
NOTE: I have purchased many X9 Pro activity trackers. Some of them have a green PCB and some have a blue one. It doesn't make a difference for the purposes of this tutorial. The traces on the PCB are arranged a little differently but all the components are the same.
2Cut Glue Between Translucent Plastic Cap and Metal Enclosure Body
Use a hobby knife ("X-Acto" knife) to cut as much of the glue bonding the plastic cap to the metal body as possible. Getting a thin blade between the plastic and metal will gradually wedge the two apart. Take your time (I would budget at least 5 minutes) and work your knife all the way around the cap.
3Lift Plastic Cap Off the Metal Enclosure
Use a small screw driver or prying tool to carefully lift the plastic cap off the metal enclosure body. Take your time and work thy prying tool around the cap.
Separate the cap and put it aside for now.
4Unscrew Plastic Retainer
Unscrew the plastic retainer and put the screws somewhere you won't lose them.
5Remove Inner Plastic Electronics Enclosure
Carefully pry out the internal plastic enclosure.
6OPTIONAL Remove the Main PCB From the Internal Enclosure
The PCB test pads that you need to solder are on the exposed side the the PCB, but all those cool chips are on the hidden reverse side. If you are like me you will definitely want to take a look. Carefully push back the plastic teeth retaining the PCB. You will see all the ICs in their glory and also the LiPo battery. Once you are done taking a peak put the main PCB back in the plastic internal enclosure.
7Coat the Exposed SWD Programming Connecting PCB Pads With Flux
I always use flux. Flux dissolves accumulated oxidized metal from the top of the pads, making solder bonds easier and stronger.
8Solder Wires to the 4 SWD Connection PCB Pads
You may want to dab some glue on the solder point to reinforce it but that is strictly optional.
9Reassemble Device and Connect the X9 Activity Tracker Wires to an SWD Programmer
Cut a groove in the plastic cap for the wires to fit through and reassemble the device just like you took it apart. Connect the wires to a bread board. Connect the Ground and Voltage Supply wires to a 3.3v bench power supply, as well as the ground and reference voltage connections from your SWD programmer.
10Install Sandeep Mistry ArduinoCore Arduino IDE Package and Add X9 Variant From Project GitHub Repository
If you have not already installed the Arduino IDE, go ahead and do it. Then go to Sandeep Mistry's Nordic ArduinoCore GitHub repository (Arduino wrapper for the Nordic nRF5x SDK), download the ArduinoCore package and install on the Arduino IDE. There are excellent instructions in the GitHub repository README. Use the variant file for the X9 activity tracker found in the GitHub repository for this project.
Load the nrf52_X9Project_OLED sketch (attached to this project as a file) into the Arduino IDE. Make sure board is set to "Wearable nrf52 X9" (will show up once you add my X9 variant), and that the SoftDevice (this option will show up after installing Nordic ArduinoCore) to "S132". These settings can be found under the Arduino IDE "Tools" tab. Under the Arduino IDE "Sketch" tab press "Export Compiled Binary". This will create a raw compiled version of your application/sketch in the file folder for the nrf52_X9Project_OLED sketch.