Unbricking

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Revision as of 00:31, 2 November 2018 by MrChromebox (talk | contribs) (Prepping to flash)

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Unbricking

If you've found your way here, it's likely because you updated your firmware and, despite best efforts to minimize the possibility,something went wrong. Thankfully, most Chromebooks can be easily unbricked using cheap, readily available hardware from Amazon/eBay/Alibaba (and many other sources).

Requirements

  • A ChromeOS device with a SOIC-8 type SPI flash chip. Most Chromebooks use this type of chip, but there are a few notable exceptions:
    - Google Chromebook Pixel 2013
    - Google Chromebook Pixel 2015
    - HP Chromebook 13 G1
    - Google Pixelbook?
These devices all use a WSON-8 flash chip, which does not expose the pins of the chip, so they cannot easily be "clipped" like a SOIC-8 chip. While it is usually possible to modify a SOIC-8 chip clip to attach to a WSON-8 chip, it's less than ideal. Both Chromebook Pixels feature a Google debug header, which can connect to a debug servo with a special cable and be flashed that way, but not an option for most users (I however do have access to one, and can unbrick Chromebook Pixels for any users upon request).
  • A device running Linux from which to run flashrom. For this guide, I will use a Ubuntu 18.04 live USB.
  • A CH341a USB flash programmer (Amazon link)
  • A 1.8v adapter (Amazon link) (needed for devices which use 1.8v flash chips. Baytrail, Braswell, Skylake and most newer devices use a 1.8v flash chip)
  • An SOIC-8 chip clip (Amazon Link)

These 3 components are often bundled together at a lower cost (eBay link), and if you're unsure if your device uses a 1.8v flash chip or a 3.3v one, it makes sense to have the adapter on hand if needed.

Hardware Disassembly

While this is somewhat device-specific, the main points are the same:

  • Disconnect all external power
  • Remove bottom cover (screws are often located under rubber feet or strips)
  • Disconnect the internal battery (for Chromeboxes, disconnect the small CMOS battery)
  • Locate the SPI flash chip
Most ChromeOS devices use a Winbond flash chip, though some use a compatible chip from another manufacturer, eg Gigadevices. It will be either an 8MB or 16MB chip, with the identifier W25Q64[xx] (8MB) or W25Q128[xx] (16MB) where [xx] is usually FV or DV. We do not want to touch the EC firmware chip, which is identified by W25X40[xx].
Unfortunately, many devices have the flash chip located on the top side of the main board, and require fully removing the main board in order to flash. This is true for most Baytrail and Braswell models.

Pin 1 of the flash chip will be notated by a dot/depression on the chip; be sure to align this with pin 1 on the chip clip wiring (a red strip on the example linked above).

Googling should locate a disassembly guide for most models. If you can't find one for your exact model, try to find one for another model of the same manufacturer as the bottom cover removal tends to be very similar.

Prepping to flash

Once you have your device disassembled and flash chip located, time to boot up the flashing environment. Most any Linux setup should do as long as either flashrom is available from the distro's software repositories, or it's 64-bit x86 (in which case you can download a statically compiled build of flashrom from my site). This guide will use a Ubuntu 18.04 live session booted from USB.

So let's get to it:

  • Boot your Linux environment (Ubuntu live USB or otherwise)
  • Open a terminal/shell window
  • Download flashrom (from my website) and extract:
wget https://mrchromebox.tech/files/util/flashrom.0602.tar.xz && tar -zxf flashrom.0602.tar.xz
NOTE: while some distros include flashrom already, most (as of the writing of this document) are using an older version (pre-1.0) which lacks some of the features used here, so I've provided a statically compiled updated version (1.0) which should run on any x86_64-based system.
  • Assemble CH341a programmer, 1.8v adapter (if needed), and chip clip/wiring. Ensure that pin 1 is correct and consistent.
Ch341a annotated.png
  • Connect the chip clip to the SPI flash chip, then connect the CH341a to the Linux host machine
  • Test connectivity and ensure the flash chip is properly identified:
sudo ./flashrom -p ch341a_spi
Flashrom will produce output identifying the flash chip. If it doesn't, double check your connections to the programmer and the chip clip and retry.
Flashrom chip detect.png
  • Determine file to be flashed
Depending on your desired use for the device, you have 3 options for flashing:
- The backup file of the stock firmware created by my Firmware Utility Script
If using this, simply copy the file from USB into the home directory of the live USB user
- The custom UEFI firmware for the device
If you were flashing the UEFI firmware when things went sideways, then that's the easiest way to proceed. You can download the UEFI firmware for your device by examining the sources.sh file from the Firmware Utility Script github repo. Simply concatenate the device-specific filename to the Full ROM base path:
wget <Full ROM base path><device specific filename>
eg for the Acer Chromebook 14 CB3-431 (EDGAR)
wget https://mrchromebox.tech/files/firmware/full_rom/coreboot_tiano-edgar-mrchromebox_20180827.rom
Don't forget to get the sha1 file for verification:
wget https://mrchromebox.tech/files/firmware/full_rom/coreboot_tiano-edgar-mrchromebox_20180827.rom.sha1
Then verify the download:
sha1sum -c coreboot_tiano-edgar-mrchromebox_20180827.rom.sha1
- The shellball firmware for the device
As with the UEFI firmware above, the shellball rom can be downloaded by concatenating the shellball base path with the device-specific filename:
wget <shellball base path>/shellball.<device name>.bin
eg for the Acer Chromebook 14 CB3-431 (EDGAR):
wget https://mrchromebox.tech/files/firmware/shellball/shellball.edgar.bin

Special Note for Chromeboxes

Devices which have an Ethernet port usually store the MAC address in the VPD (vital product data) section of the stock firmware. When flashing via the Firmware Utility Script, the script will automatically extract and repackage this in a way the UEFI firmware can use, so the MAC address of the devices is maintained. Without this, the device will use a default MAC address set by coreboot. While not ideal, this is only really an issue if two or more of the same device are on the same LAN segment. But for completeness, if flashing the UEFI firmware or shellball ROM, we'll extract the VPD (either from the board itself or a backup made by the script) and inject it into the firmware to be flashed.

TODO: add steps for persisting MAC address

Flashing Your Device

Now that everything is prepped, time to flash the device. To be thorough, we'll perform a 2nd verification after flashing to ensure the integrity of the flashed firmware.

  • Flash the firmware:
If flashing your own backup created by the Firmware Utility Script (or any backup made from a live system), use
sudo ./flashrom -p ch341a_spi --ifd -i bios -w <filename>
otherwise use
sudo ./flashrom -p ch341a_spi -w <filename>
Where <filename> is the name of your backup file, UEFI firmware file, or shellball firmware file. This will usually take 30s-90s to complete; flashrom will first read the flash chip, determine which sectors differ, erase those sectors, write the new data, then verify the data written.
  • Verify the firmware
Even though flashrom does this as part of the write process, verifying the entire flash chip is quick and an easy way to ensure everything went as it should:
As before, if flashing your own backup created by the Firmware Utility Script (or any backup made from a live system), use
sudo ./flashrom -p ch341a_spi --ifd -i bios -v <filename>
otherwise use
sudo ./flashrom -p ch341a_spi -v <filename>
using the same filename as before. If the verification passes, then disconnect the CH341a from the host machine, and then remove the chip clip.

Clean Up

Reassembly is the reverse of disassembly. Reconnect the internal battery and replace the bottom cover. Flip over the device, connect external power, press the power button, and cross your fingers :)