DFU with UART through LTE Modem

Question

I'm developing FOTA through LTE Modem which is made from QUECTEL(EC21KL) 
 
 On my custom board, nRF52832 is connected with EC21 LTE Modem via UART bus(TX : P0.16, RX : P0.17). 
 
 I successfully downloaded new firmware from our server to EC21KL LTE modem, but I cannot execeute DFU through UART. 
 
 I don't know how can I set the pins for UART DFU. 
 
 How can I do this? 
 
 I attached some part of circuit design.

Summary. 
 
 1. How can I assign the pins for UART DFU? 
 2. How can I transfer new firmware through UART? 
 
 Thank you.

Hung Bui · Accepted Answer

Hi, 
 Yes it's one way of doing it. With this approach you would need to implement the AT Command in the bootloader. I'm not very familar with that so I can't comment here. Another option is to integrate the transferring (step 3) into your application. So basically: 
 1. Application tells the Modem to download the image 
 2. The image has been downloaded to the Modem 
 3. The application transfer the image from the Modem to the internal flash space 
 4. The application prepare the bootloader setting then reset to the bootloader 
 5. The bootloader read the bootloader setting and swap the image that the application downloaded to main bank 
 6. The bootloader start the new application. 
 
 Both approaches would work. In your approach you don't have to modify the application much and only need to modify the bootloader. The second approach called background DFU meaning your application still run normally when receiving the image to the NRF52 and only switch to bootloader to swap. But it require more modification in the application and is not supported out of the box (you need to follow the background DFU example ) 
 So all in all, I think your approach is more suitable if the implementation of the AT command to get the image is not too complex that it can be implemented in the bootloader, then it's fine.

DFU with UART through LTE Modem

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