• OK, so you did firmware update, great. That means there is a bootloader for doing DFU for sure (some devices are not upgradable so may not have the bootloader at all). And the device code is 151. Did you see some picture on OLED screen while doing the upgrade? That would mean the botloader is customized with display code and is definitely worth backing up - the update zip is just application, not backup of whole flash memory so if you erase the flash completely and want to restore it later you will be missing bootloader code and UICR settings.

    I think you should first get SWD working with openocd and try to backup existing flash if possible. if the device is not protected you may even try to let it run default firmware and pause it in the middle of something and see various HW registers to see how it is configured (i2c,spi, GPIO pin configuration), see example for nrf52 here. To me it was easier than decompiling whole app. Also if the bootloader is customized with display code it is also a bit easier task as it is smaller.

    I am really really sorry for asking such a novice questions.

    Well, yes, while it is not that hard it is not novice level so you can't avoid learning a lot of things, which is hopefully the point. I'd start with Nordic documentation about nrf51
    https://infocenter.nordicsemi.com/topic/­struct_nrf51/struct/nrf51.html?cp=5 and some older SDK that still supports nrf51 like SDK11 od SDK12. As for the nrf51 reference manual it looks like it is only a pdf, for nrf52 they made it also html based which is easier to read.

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