Further adventures in Chromebook-land

28.01.2013 21:33

As you might remember from my previous blog post, I have a disassembled ARM-based Samsung Chromebook lying around, occupying various horizontal surfaces that might otherwise be put into better use. After an initial success with replacing the original, feature-challenged OS with armhf port of Debian Wheezy I hit on a couple of snags. First, I found out that running the computer with a non-Google signed OS means having to look at an annoying warning message at each boot and having to press Ctrl-D (and being careful not to turn off developer mode by touching any other keys by mistake) or wait for a minute or so. And second, by carelessly playing around with alsamixer, I managed to get the left built-in speaker to melt through the bottom casing of the laptop.

Naturally, the smart decision would at that point be to return the laptop to the shop and demand my money back. Of course, I chose the other way.

Chromebook left speaker close-up

As far as the speaker is concerned, it's quite beyond repair. While the body (which I guess is kind of a resonance chamber?) and the magnet are quite unharmed, the membrane and the coil (made with a piece of flexible PCB as far as I could see) ended up in a puddle of molten plastic. I suppose the other speaker is still working correctly, but I didn't test it as I have it disconnected from the motherboard for now.

Chromebook motherboard, top side

Once you remove the human interface, the business part of the laptop is a surprisingly small motherboard, containing little more than the Exynos system-on-chip surrounded by a bunch of memory chips and power supply circuits.

Chromebook motherboard, bottom side

The problem with the annoying bootloader turned out to be harder to solve than I thought. As I understand the boot process, the CPU first runs pre-boot code (apparently some proprietary initialization code from Samsung). This then loads secondary program loader which in turn loads an U-Boot. This one then annoys you and goes on to load anything you want from the SSD, provided you are in developer mode, of course. I'm not sure about the first two parts, but U-boot is stored on an Winbond 25Q32DW series serial flash chip with an SPI interface.

Serial flash IC on Chromebook motherboard

This chip has a active-low write-protect pin. The pin is pulled low by default somehow, which prevents the main Exynos CPU from writing to flash. It doesn't seem to be tied to ground though, so I'm guess it might be controlled from the embedded system controller (or a GPIO pin from Exynos, but that would be kind of stupid). If you browse Google's documentation there are some mentions of a mysterious servo2 debug board that apparently allows you to overwrite the flash and even boot the computer if the flash is corrupted. I haven't been able to find any kind of details about it, not even where are you supposed to connect it. There are no special debug connectors on the laptop's motherboard as far as I can see, so it's either plugged into one of the externally accessible connectors (USB, HDMI, SD card, audio) and does some magic through there (possibly with the help of ESC), or servo2 refers to a special version of the motherboard that has some additional debug capabilities.

In any case, without debug board's magic, replacing the bootloader doesn't look simple. I can rewire the write-protect pin, but that will give me exactly one try at programming the flash. If I botch it, Exynos will crash on boot and I won't get another chance. I'm not sure I'm capable of desoldering the flash chip without destroying it, reprogram it externally and solder it back without messing up any of tiny SMD components around it. Although there seems to already be an Arduino-based programmer available for these chips, so at least I would be spared the task to code that myself.

I've built the Chromium OS development environment which in turn can also be used to build the flash images. While everything seems to build without problems, I'm kind of confused as to whether the images built in this way still include the annoying warning or not. The build process itself turned out to be quite convoluted, involving surprising amounts of complex Python code (what's wrong with Makefiles?) hidden behind Gentoo's Portage scripts and has so far resisted my attempts to find out how the flash image is actually constructed.

Unfortunately, while poking inside the laptop I managed to add a third problem on top of the previous two. While re-attaching the copper cooling plate my screwdriver slipped and shattered one of the tiny bare-die flip-chip packages around the STM32F10086 controller.

Shattered flip-chip component

These seem to be bare silicon soldered directly to the PCB without any kind of packaging and are surprisingly brittle. From what's left of it and by looking at similar components on the board, I'm guessing the laser-etched back-side marking originally said 2822HN. I'm not sure what its function was and I can't find any references on the web for these components (the other type of a similar component used on the motherboard is 28DCV7). Perhaps a discreet logic gate? In any case, it's quite beyond my capabilities to replace, even if I would manage to get a replacement part.

Surprisingly, with this component in the broken state as it is, the laptop still boots. So far I haven't yet tested if any peripheral isn't working. One effect seems to be that the power button is now kind of unreliable, taking several presses before the computer turns on. But that might also not be related - with the casing open, everything is kind of wobbly and I wouldn't be surprised if keyboard isn't properly supported in this setup.

In any case, this Chromebook seems to be a failure as far as replacing my EeePC goes. In this broken form it certainly won't become a computer I can rely on when traveling, even if I manage to replace the bootloader. Might eventually turn out useful for some other project though.

Posted by Tomaž | Categories: Life

Add a new comment

(No HTML tags allowed. Separate paragraphs with a blank line.)