Questions about serial DFU

[Liu Tengfei]

Dear Support Team,

I’m using nrf52840 with PCA10056 and SDK 14.1.  I want to make a DFU feature using SPI as its transport layer. Could you please help me about the following questions?

1. I could find some Nordic bootloader examples which could be easily used as DFU target, but I did not find any DFU controller examples by C++ code. I noticed that nrfutil(python) and ios/andriod apps that could be able to perform DFU, but I wonder, do you have any c++ source code that could be reused as DFU controller?Or if I want to use DFU service, I have to implement DFU controller logic in C++ myself ?

2. I started with the examples\dfu\bootloader_secure_serial, I think this serial bootloader doesn't need to enable sofdevice at all, but why it need to program the SoftDevice before run this example? (Regarding to the instruction in Nordic docs)

3. Also for the examples\dfu\bootloader_secure_serial, I followed Nordic instructions and everything was right (program softdevice/serial bootloader, make zip package and perform DFU, but I don't merge bootloader setting), I successfully updated an application firmware of examples\peripheral\uart to my PDK through DFU over UART. Since I used debug version bootloader, RTT log showed that nrf_bootloader_app_start with address: 0x23000 after DFU done. However, the uart application couldn't work as expected. I did some test myself, if I directly programmed uart example to my PDK, it worked! Also if I change it's flash location from FLASH (rx) : ORIGIN = 0x0000, LENGTH = 0x100000 to FLASH (rx) : ORIGIN = 0x23000, LENGTH = 0x10000 in the .ld file, and then programmed it to PDK, it also worked! So why it cannot work as expected after successfully DFU and jump to the flash address of 0x23000?

4. Since my device doesn't have any buttons and leds, I have to make an DFU API in DFU controller to trigger DFU while DFU target was running application.

(1) When the host call DFU API, a specified command will be sent to DFU target(I planned to do like this), does Nordic have such command in DFU service? or I need to defined a custom command myself?

(2) When the DFU target receive the DFU command, it will do sd_power_gpregret_set(0, BOOTLOADER_DFU_START),  disconnect all the connections and then reset, go into DFU mode after reboot, right?

But how could host know about whether the DFU target was ready in DFU mode or not? Do I need to reponse a specified command to tell host that it was ready to DFU after it reboot? Do I need to clear the SD_POWER_GPREGRET_SET after DFU done?

Best regards,

Tengfei.

[gfmoore]

This has to be one of the best answers and explanations of what is going on that I have seen on this site. 

It should be expanded, updated  and put as one of the first documents that beginners should read, not stuck here in an answer. In my opinion there is to much that is assumed in knowledge as to how these things work, especially for newcomers. Well done tesc for your efforts here to an excellent set of questions by Tengfei, even of 3 years ago.