How to upgrade 52833 firmware with UART interface ？

[can23]

My project consists of STM32 + 4G + nrf52833. When the 52833 firmware needs to be updated, the cloud service will
send the 52833 firmware to STM32, and then STM32 will send the firmware to 52833 through the UART interface. Now
there are some questions:

1. How to enter DFU mode

If there is a new firmware, STM32 sends a command to nrf52833 through UART interface. How does nrf52833 enter DFU，what API or setting?

2. How to send firmware to 52833

After nrf52833 enters DFU mode, how does STM32 send firmware to 52833 and package new firmware?

3. Are there any demo codes？

sdk:nRF5_SDK_17.0.2_d674dde

The following is the structure diagram of the product：

[Amanda Hsieh]

Hi, 

1. See DFU mode doc that Entering DFU mode is triggered by one of the optional sources:

• Button (NRF_BL_DFU_ENTER_METHOD_BUTTON),
• Pin reset (NRF_BL_DFU_ENTER_METHOD_PINRESET),
• Special value in GPREGRET register (NRF_BL_DFU_ENTER_METHOD_GPREGRET),
• Request from the application written to the settings page (NRF_BL_DFU_ENTER_METHOD_BUTTONLESS).

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. 

2. Have a look at this post. Another project that does more or less the same is from one of our FAEs' github: nrf-slim-serial-uart-dfu-host-c-code.

3. There is no demo exactly as your requirement, but please see the above links I provide. They are not written for an STM, but it can be a starting point. In case you need it, you can also check the bootloader's serial transport documentation on infocenter.

Regards,
Amanda

[can23]

thanks for your reply！