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Programming the BT 121 can be done over UART. I considered just using a generic USB converter cable but always ran into issues with clone chips and driver problems. I decided to create a programmer from scratch with a genuine FTDI converter chip. I also added into the programmer a button to hold BOOT0 high to enable reprogramming the micro controller in the even you load some bad code (missing the DFU stuff etc.). I also added a right angle header row and straight header row so you can either plug the board directly on the programmer or plug the programmer into the breadboard.