Finding u-boot code in SD card using a hex viewer

Hi,

I am using NITROGEN6X board from Boundary Devices (here
<https://boundarydevices.com/product/nitrogen6x-board-imx6-arm-cortex-a9-sbc
/> ).

The linux kernel uses U-boot as the boot loader.

I open the SD Card content in a hex viewer and I can see/decode the MBR in sector 0.

I am now trying to find the stage 1 (secondary program loader) and stage 2 code in the hex viewer.

Is there any documents describing this?

- Mani