IMX8MP USB boot mode (uuu)

[Chantale]

Hello,
I have an Olimex IMX8MP board (schematic and KiCad repo) paired with its baseboard (schematic and KiCad repo).

I'm trying to test the usage of uuu with a blank board, i.e. using ROM USB code to burn u-boot to the emmc. No matter how I try, however, the usb device doesn't show up on my host machine (Linux). Since it's a baseboard with USB-A connectors only I'm using a USB-A-to-USB-A connector cable, but I also tried with a USB-A-to-USB-C, a cable without a VBUS and a few other options, but there's nothing in dmesg one way or another. The boot switch seems to be set to the correct 1-0-0-0 combination, other combos like setting SD boot without a card that's supposed to fall back to USB work exactly the same. The SoM itself is operational, I can boot from SD just fine (and I see eMMC so I'm assuming I can do eMMC boot too). The only missing piece is the initial blank board boot in order to get u-boot installed.

Is there any secret connection that needs to be made so that ROM code executes? I *seem* to have it pretty similarly connected to the official NXP schematic for the board's EVB, there are no hubs in between etc. Is it even supposed to be working like that or do I need to burn u-boot first by other means (e.g. booting from an SD to write the eMMC)?

Feedback greatly appreciated, thanks in advance!

[Chantale]

UPD: the base board is missing the VBUS pin, apparently the ROM code uses it to trigger the USB serial download in first place. 

[Rita_Wang]

Board need to set to serial download mode in the board. I can not see the design and the pdf of the schematic, you need to check in your side.

You said the SD can boot well on your side. For the download images to sd card and emmc use different command, download mode are the same. 

When finished downloading, the boot mode for sd card and emmc are not the same.

Here you need to check if you have connect the proper line and power, use the proper boot mode and download command.

[Rita_Wang]

I will confirm it for you then give you reply.