Recently I killed on of SigFox Td1208 device by misprogramming it, destroying the bootloader. In a such situation serial port is not anymore accessible for programming. The use of JTAG port is a necessity. On the TD1208, jtag has been repaced by SWD which is a 2 wires port doing the same thing.
To use this port, you generally need a specific interface. These interfaces like Jlink are expensive (> 300€), some are less like BusPirate (30€). By the way, if you do not have it, you will have to order and wait for it. That’s why I’ve taken the choice of using my BeagleBone black device GPIO to pilot these signal. This code can easily be port to RaspberryPI…