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MattMills
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RB750Gv2 repair manual or debug docs?

Sun Jan 16, 2022 10:35 am

Hello,

I have 2 RB750Gv2 boards that have both started to fail to boot. The power light turns on, the User light turns on, but then nothing happens. The reset button is non responsive during boot or after. Both boards give a very "meager" beep noise while booting, that is more like a clicking. I found some other repair guides here on the forum, but I couldn't find one that matches this board. Is this available somewhere?

Alternatively, is there anyway the onboard pads marked UART or E-JTAG are documented? I'm setup and familiar with doing JTAG or working on an embedded processor with a debugger, so I was trying to access the UART on one board, and I believe I've at least figured out the pinout (TX, RX, GND, 3.3V), but haven't had any success in getting an actual signal out, aside from a 200 mV peak to peak signal that has a 2.3-2.4V DC offset.

Thanks,
Matt.
 
MattMills
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Re: RB750Gv2 repair manual or debug docs?

Wed Jan 19, 2022 1:44 pm

Just a slight update, haven't gotten either device working, however I was able to dump the SPI flash on both devices. I've been able to identify that both identifies have the primary and secondary bootflash as routerboot 3.27, which sounds fairly old, although looking through the release notes for routerboot and routerboard after the merged release notes I don't see anything that would prevent booting with an old bootflash...

I'm contemplating trying to manually overwrite the boot sector within the flash with the latest routerboot firmware, but unsure if I can do that successfully since the format on the flash doesn't seem obvious.

That said, looking at SPI onboard as the device tries to boot, sends 305 address reads (some of which seem to loop between a few addresses), then it looks like it spends about 400 ms looping between reading 0x0107a0 and 0x010780 in SPI flash, reads a few more addresses and then drops into a weird behavior of trying to repeatedly send SPI write enable commands... it appears to try to erase sector 0, send a write disable here or there, and then just loops on write enable forever. I suppose all of that could just be reading through looping code and executing it, not sure how the SPI flash works on this particular chip since there is no public datasheets.

Not sure if this is the real behavior or just my logic analyzer (PulseView or the hardware) being weird because it's trying to capture 24 Mhz of data on 8 channels for a while. I was thinking it might be the SPI flash being write fatigued, but I don't think I've ever seen that happen before.
 
hecatae
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Re: RB750Gv2 repair manual or debug docs?

Wed Jan 19, 2022 2:33 pm

Do the boards look physically okay, capacitors not bulging etc?
 
MattMills
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Re: RB750Gv2 repair manual or debug docs?

Wed Jan 19, 2022 3:05 pm

Yeah they both look physically fine under microscope, and one of them was running fine at the time it stopped booting.
 
mada3k
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Re: RB750Gv2 repair manual or debug docs?

Wed Jan 19, 2022 3:53 pm

Many users doesn't understand that the SPI Flash has a limited number of writes, and places logs and graphs on it, hence wearing it out.
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MattMills
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Re: RB750Gv2 repair manual or debug docs?

Wed Jan 19, 2022 5:06 pm

Well I'm going to replace the existing SPI flash since I have a handful of winbond 16 MB SPI flash on hand, although they aren't spares, I'll take one off one of my existing projects until I get some spares stocked, so I should be able to see if it's a write fatigue issue... Surprising that there isn't some kind of code to handle that error case if that is the case, and it just bricks in a weird fashion. Wouldn't be hard to boot in a ram-only state where it boots the last config and logs errors continuously about the flash failure so you can pull the config off it and put it on something non-broken. In my case I had a bunch of VPN keys that I had been working on that weren't backed up when it failed, but I realized I had an alternate means by which I could replace them.

I didn't even realize logs would be writing to flash as that is astronomically silly to try to do on a 16 MB flash. Graphs obviously would need to, but I would think it'd be cached long enough and wear leveled so it's less of an issue, but maybe that's not the case. I do believe this machine had some log spam going, but I was pretty sure that log spam was to ram only, now I'm less sure.

Either way, IMO, there should be big red warnings and confirmation dialogs any time you're going to set something up to write to flash, doubly so if there isn't a wear leveling file system involved, as I'm pretty sure it's fairly easy to write fatigue a single sector when that's the only place you're writing, compared to writing KBs across 16 MB of locations.
 
mada3k
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Re: RB750Gv2 repair manual or debug docs?

Wed Jan 19, 2022 7:56 pm

I agree, there should be warnings. Just add a logging action=disk then your done in a couple of months.
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MattMills
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Re: RB750Gv2 repair manual or debug docs?

Fri Jan 21, 2022 2:05 pm

So after completely replacing the SPI flash on one board, I had no success, but after manipulating the board I was getting some ethernet LEDs, oddly.

I realized if I bent the PCB in a certain way, the device would boot, but also sometimes short the LEDs and things like that, so I'm guessing there is an internal defect in the PCB, or the switch chip or CPU are not well soldered to the board.

I did get it to boot once with the new SPI flash, even though now I'm guessing the SPI flash was not an issue, but it was complaining about licensing (so I guess license key is based on SPI flash serial #), and it only lasted ~30 seconds before I couldn't hold the "right" position and it died.

I still have a second one to test out, so I'll check that one too, and I might end up seeing if I can reflow the switch chip, but it seems pretty large.
 
MattMills
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Re: RB750Gv2 repair manual or debug docs?

Sat Jan 22, 2022 1:43 pm

So after moving my flashed SPI chip to the second board, it was clear the 1.8v supply had an issue, as when the board was flexed the green power LED would fade. I pulled the 1.8v power supply off the board, but the LED was still powered so I guess there was an independent LDO within the CPU chip, and I was able to reflow some of the CPU pins with hot air to get them properly attached which eliminated that LED issue, however the board doesn't boot for whatever reason... It's also using more current than the previous board (220 mA vs 150 mA).

Would love a diagram or schematic or list of expected voltages if that exists anywhere.

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