DFU via UART using a custom desktop application

[jPolo777]

Hi. I would like to know how to do a DFU over UART using a custom desktop application (without using nrfutil command nor nRF Connect app). What would be the UART protocol for sending a new image via UART and what would I have to do to tell the NRF to enter in DFU mode (without using an external button)?
Also, I was wondering if it would be possible to do a DFU from an external flash memory that will communicate via SPI with the nRF (with the desktop application, I would send the image via UART to the nRF, the nRF would put the image in the external memory and then it would enter in DFU mode to update its firmware from the external memory).

I'm using the DK nRF52840, SDK v.17.0.2. Any help would be appreciated!

[jPolo777]

Amanda Hsieh said:

You can enable NRF_BL_DFU_ENTER_METHOD_GPREGRET in the sdk_config.h, and write the special value in the retention register GPREGRET register a flag (BOOTLOADER_DFU_START = 0xB1)  in the application and then do a soft reset (NVIC_SystemReset) to switch. 

So, If I understand well, when I want the nRF to go into DFU mode, I have to set the GPREGRET register in the nRF application (NRF_POWER->GPREGRET = 0x01) and then call the function NVIC_SystemReset?

[Amanda Hsieh]

Set the register to 0xB1. 

-Amanda