NAS540 don't boot

This is one of the cases where "if you have to ask, it's not for you" can be applied.

This box has a 3 stage bootloader. The CPU has some loader code build in, which is just smart enough to read and execute code from SPI flash. This is the 2nd stage 'uloader' you see. The uloader does some initialisation, and then loads barebox (3th stage) from SPI flash in memory, and starts it. Here the box stalls, and some watchdog kicks in and reboots the box, or barebox itself decides to reboot, or some illegal code causes a reboot.

In all cases probably barebox is damaged in the SPI flash. The solution would be to desolder the SPI flash from the motherbord, put it in a SPI programmer, reflash barebox, and solder it back. Maybe the reflashing can be done while the chip is still on board, but in that case you'll have to localize the ISP pins or pads.

If you want I can provide a dump of the barebox partition, but for the rest I can't help you. A few years ago I was not even able to localize the SPI chip. https://web.archive.org/web/20180720225145/http%3A//zyxel.nas-central.org/wiki/PCB_Photos_%28NAS540%29

BTW, the function of barebox is to load and execute the Linux kernel from NAND flash.