Flashing OpenWrt on a Cisco Meraki MR33 with the later locked U-Boot is not a normal serial-console installation. The documented recovery removes the TSOP48 NAND chip, backs it up off-board, replaces the device-specific U-Boot partition with a working older image, and resolders the chip. Only then does the process continue by booting an OpenWrt initramfs over TFTP and installing the permanent image. This is high-risk board-level rework, not a general Cisco access-point procedure.
Why the MR33 needs a hardware-level recovery
The project concerns the Cisco Meraki MR33 specifically. Its author reports that a later Cisco firmware update installed a locked U-Boot that can permanently brick the CPU when entered through the serial console. The older, unlocked bootloader accepted the usual xyzzy escape; the locked version documented in the project is U-Boot 2017.07-RELEASE-g78ed34f31579 (Sep 29 2017 - 07:43:44 -0700), and that escape no longer works.
That difference changes the risk calculation: trying to interrupt autoboot or enter the locked bootloader is not a harmless first troubleshooting step. The project’s recovery avoids relying on that route by modifying the NAND off-board. Cisco’s general AP boot guidance describes connecting a console, using 115200 bps (or trying 9600 bps if there is no output), powering on, and pressing ESC to stop autoboot. It is general guidance, not an MR33-specific workaround for the locked U-Boot behavior described here.
The author’s motivation was the MR33’s hardware: it has three Wi-Fi radios and a BLE radio. That capability does not make the recovery routine or its flash layout applicable to other access points.
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What the hard-way recovery requires
The documented route combines UART access, TFTP, and off-board NAND programming. It is intended for people already comfortable with fine-pitch surface-mount rework and raw NAND handling.
| Stage | Equipment or capability | Purpose |
|---|---|---|
| Console access | UART interface and terminal software; the project used three USB serial adapters | Use the main console and separately monitor transmit and receive activity. |
| NAND removal and programming | Hot-air rework equipment, a TSOP48 socket/ZIF arrangement, and a development board capable of raw NAND access | Read, preserve, modify, and write the MR33 NAND outside the access point. |
| OpenWrt transfer | A TFTP server and a host able to run the documented serial-loading script | Load an intermediate bootloader over serial, then provide the initramfs image over TFTP. |
Check the UART voltage, pinout, and adapter wiring before connecting. The project author explicitly warns: do not connect the VCC pin to the serial adapter. The serial connection is for signal lines and ground; do not use the adapter to power the AP.
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Recover the locked bootloader from NAND
The central operation is to remove the MR33’s TSOP48 NAND, make a complete backup, and change only the bootloader partition. This is the point at which a wiring mistake, bad rework, incorrect NAND operation, or wrong image can turn a recoverable access point into a permanently damaged one.
- Establish the console wiring. Connect UART signal and ground using the MR33 pinout shown in the project journal. Keep the adapter’s VCC disconnected. The author used one adapter for the main console and two to monitor transmit and receive lines.
- Remove the NAND and read it off-board. The journal used a TSOP48 socket adapter and an embedded development board with raw NAND access. Preserve the complete original contents before making any change so the stock data can be restored if the modified image fails.
- Back up in manageable chunks. The author read the full flash in 32 MB chunks because larger transfers caused a memory abort on the development board. That is a limitation observed with that setup, not a universal NAND chunk size.
- Compare the device’s partitions before writing. In the project’s comparison, only the
u-bootpartition differed between the locked unit and a working unit;u-boot-backupwas unused. The working image was written at NAND offset0x700000after erasing a0x200000region. - Resolder and verify the hardware. Reinstall the chip and inspect the work before powering the AP. The offset and erase size above are specific to the MR33 layout used in the project; do not apply them to another model or assume they fit every hardware revision.
Do not treat an image copied from another unit as universally safe. The project’s partition comparison supports its own MR33 repair; it does not establish that the same bootloader image, NAND geometry, offsets, or behavior apply across other Cisco products.
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Boot OpenWrt temporarily, then install it
After restoring a working older U-Boot, the project used a modified ubootwrite.py script with Python 2.7 and pyserial to load an intermediate U-Boot over the serial connection. A TFTP server then provided the official MR33 initramfs image used in the documented run: openwrt-19.07.6-ipq40xx-generic-meraki_mr33-initramfs-fit-uImage.itb.
OpenWrt 19.07.6 is the version recorded in that run, not a claim that it is the best or currently supported release to install. Image names and release availability change. Before attempting the transfer, confirm that the image is explicitly for the MR33 and that the bootloader-loading method and image format match the files you have. The available project account does not provide a current release recommendation.
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Once the initramfs has booted, the documented permanent-install sequence is to remove obsolete factory UBI volumes while preserving the ART calibration volume, recreate a failsafe volume using the initramfs image, and run sysupgrade with the MR33 squashfs image. The project reports creating part.safe, rootfs, and rootfs_data volumes, with about 78.2 MB available in the overlay on that build. Those are results from the author’s documented installation, not guaranteed values for another image or device.
Be especially careful with UBI volume cleanup: the author specifically preserved ART, which contains calibration data. Erasing or overwriting it can compromise the device’s radio calibration. The project description establishes the sequence and resulting volume names, but not a universally safe command set for every firmware image; do not substitute guessed erase or volume commands.
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What this method does—and does not—establish
- It documents an MR33 path: off-board NAND work repairs the locked bootloader, after which serial loading and TFTP boot the temporary OpenWrt image.
- It is invasive: the NAND must be removed and resoldered; this is not a serial-only flash.
- A complete backup is essential: raw NAND changes are difficult to reverse without a sound original dump.
- It is not a generic Cisco recipe: the cited recovery guide for newer Catalyst models has different scope, explicitly excludes 2800/3800 APs, and says U-Boot is not intended to be accessible on Wi-Fi 6E and Wi-Fi 7 APs. Those statements do not establish MR33 compatibility.
- It is not a promise of repeatability: the documented partition findings and offsets belong to the MR33 unit and setup described by the project, not all Cisco APs or every hardware revision.
The project author warns against returning modified or bricked devices to Cisco for service or replacement and says the work is at the operator’s own risk. The practical takeaway is to attempt this only if you can safely perform TSOP48 rework, program raw NAND, verify the MR33-specific layout, and accept that a mistake may be irreversible.
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