Thanks! After reading/thinking a bit more; just building U-Boot is probably the way to go. I have a cross compiling environment for a BeagleBone anyways, seems to be the same family SoC.
Would then need to get the flash layout and a copy of the filesystem from a working unit, plus likely some things I'm not thinking of yet... Will get there eventually. -Ian-
I've been trying to do this on the SDG805, but not getting anywhere. I can't seem to get the SPL up and running. I figured it might be useful to summarise what I've done so far though:
The AM335x has a ROM boot loader, this will accept the u-boot spl over UART one (which is exposed on the PCB) using xmodem. There are a few binary builds of the u-boot spl around, from the TI AM335x SDK and elsewhere. However none of them work for me. I suspect that the DDR memory isn't configured correctly.
If you look in the TI SDK the DDR init function read an EEPROM on the TI evaluation board, and beagle bone and set the DDR configuration and power management settings accordingly. I doubt this would work on the Siglent instruments. While they seem to have copied a lot of things from the TI SDK, I kind of expect the EEPROM and DDR config to be different. The sdg800 u-boot does refer to the beagle bone in places though, so it's possible many things are similar.
The DDR memory used on the SDG805 is a k4b1g164gg this is largely similar to a chip that's already in the ddr_config.h (IIRC). TI have a guide to setup new DDR chips in u-boot and I followed that.
I also removed some of the beagle board checks to allow the board to init the TPS65910 power management IC.
There's another thing in there that worries me, which is that on the beaglebone there's a GPIO which enables the DDR memory. I'm guessing there might be something similar on the SDG800s but without knowing which GPIO(s) to toggle it might be though figure it out.
Anyway, none of my current attempts have worked. The spl gets loaded over xmodem and I can see it display it's version header but then it just hangs. You should be able to load the reset of U-boot over Y-modem at this point. I'm back to trying to find a original siglent spl. This would all be a lot easier if Siglent followed the GPL and shared their u-boot changes as they are legally required to do.
Anyway, now I've been trying to extract a copy of the spl from the firmware update packages. No luck so far, but it does look like it's around. I'll post an update if I get anywhere.
Failing any of these options, I'm going to buy another SDG805 anyway. When I get it I'll extract the u-boot spl (and hopefully a complete copy of the rest of the flash) I'll let you know. Before I hosed my system I think I did get a copy of the kernel image, but the FS images I downloaded were corrupt.
I'd hope the SDG800 and SDG2000 are similar enough that binaries will work across both systems.