DFU via nrfutil is taking longer to start on our custom board than it does on the nRF52840-Preview-DK board

[DBT]

I tried out DFU functionality initially using nRF52840-PreviewDK and UART transport

When downloading an app via DFU, It takes about 6 seconds from the time I start nrfutil until the progress bar shows.

Now I've ported DFU code over to our custom board and it takes about 35 seconds from the time I start nrfutil until the progress bar shows.

Why is it taking so much longer to start on our custom board?

To be clear, the DFU download works on our board and I can confirm that the app is running after DFU download.

I just don't understand why nrfutil is taking longer to start.  I confirmed with a scope that nrfutil doesn't start sending data over the UART until after about 35 seconds, and our custom board responds to the initial message within about 50 microseconds, so it's not like nrfutil is pinging our board and its taking our board longer to start responding.  It seems like its something happening in nrfutil rather than in the bootloader on our board.

Our board does not use flow control, so I use the "-fc 0" command line option: nrfutil dfu serial -pkg app_dfu_package.zip -p COM13 -b 115200 -fc 0

The nRF52840-PreviewDK board apparently does use flow control, but I tried using "-fc 0" when downloading to the nRF52840-PreviewDK board and it didn't appear to affect the start time.

Do you have any insights on this issue?

[DBT]

Update:  In addition, downloading a bootloader - app - softdevice combo doesn't work on our new board.

Based on my observation of LEDs during the process, I think 2 of the 3 entities are being downloaded, then there is a reboot but the bootloader times out before nrfutil attempts to download the third entity.

Here is what shows in my console at the end:

C:\umg\nordic\umg-dfu\Output\Release\Exe>C:\Users\TURNBODB\Downloads\Nordic\nrfutil dfu serial -pkg bl_sd_app_dfu_package.zip -p COM13 -b 115200 -fc 0
[###############################-----] 88% 00:00:08
Traceback (most recent call last):
File "nordicsemi\__main__.py", line 1464, in <module>
File "click\core.py", line 764, in __call__
File "click\core.py", line 717, in main
File "click\core.py", line 1137, in invoke
File "click\core.py", line 1137, in invoke
File "click\core.py", line 956, in invoke
File "click\core.py", line 555, in invoke
File "nordicsemi\__main__.py", line 1042, in serial
File "nordicsemi\__main__.py", line 956, in do_serial
File "nordicsemi\dfu\dfu.py", line 129, in dfu_send_images
File "nordicsemi\dfu\dfu.py", line 90, in _dfu_send_image
File "nordicsemi\dfu\dfu_transport_serial.py", line 214, in open
pc_ble_driver_py.exceptions.NordicSemiException: No ping response after opening COM port
[61948] Failed to execute script __main__

C:\umg\nordic\umg-dfu\Output\Release\Exe>pause
Press any key to continue . . .