Flashing OpenWrt onto a Cisco Meraki MR33 with the later locked U-Boot is not a normal serial-console install. The documented workaround removes the TSOP48 NAND chip, backs it up, replaces the device-specific U-Boot partition with a working older image, then boots OpenWrt from a TFTP-served initramfs before installing it to NAND. This is invasive hardware rework, not a recovery method for every Cisco access point.
Why the MR33 needs an unusual recovery
The MR33’s later firmware can leave it with a locked U-Boot. The project journal identifies the affected version as U-Boot 2017.07-RELEASE-g78ed34f31579 (Sep 29 2017 - 07:43:44 -0700); the usual xyzzy serial escape no longer works. Hackaday reports that entering this bootloader through the serial port can permanently brick the CPU. That makes trying the familiar serial-shell route especially risky.
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The project author, writing on June 2, 2021, contrasted the locked bootloader with an older working one and noted the MR33’s appeal: “However, this device has three Wi-Fi radios and a BLE radio.” The following procedure is specific to the MR33 work documented by Sagacious Suricata. It does not establish that the same bootloader behavior or NAND offsets apply to another AP model or hardware revision.
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What the hard-way repair involves
The key change is made off the board: remove the NAND, alter only the U-Boot partition using a known-working image, and reinstall the chip. The project compared NAND contents from locked and working units and found that the u-boot partition differed; u-boot-backup was unused. The older U-Boot image was written at NAND offset 0x700000, after erasing a 0x200000 region. Those are MR33 project-specific values, not instructions to apply to another device.
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Equipment and skills
- A console UART setup, including a suitable USB-to-TTL serial adapter and the MR33-specific pinout.
- A TFTP server for transferring the temporary OpenWrt image.
- For the bootloader repair: hot-air rework capability, a TSOP48 socket or ZIF setup, and a development board or programmer able to read and write raw NAND.
- Experience handling NAND data, desoldering and resoldering a TSOP48 chip, and validating a full-flash backup before modifying anything.
The author used three USB serial adapters: one for the main console and two to monitor transmit and receive lines. The project’s UART instructions explicitly say: “Do not connect the VCC pin to your serial adapter.” Confirm the board pinout and the adapter’s logic level before wiring it; do not treat the connector’s VCC pin as a serial signal.
Recovery sequence documented for the MR33
- Identify the hardware and bootloader. Confirm the device is an MR33 and determine whether it has the locked U-Boot. Do not assume a recovery recipe or NAND address applies to a different model.
- Back up the NAND off-board. Remove the TSOP48 NAND and read the full flash before making changes. The project used a development board with raw NAND access and saved the backup in 32 MB chunks because larger transfers caused a memory abort on that board. Keep an untouched copy so the original data can be restored if the repair fails.
- Replace only the U-Boot partition. Compare the locked and working NAND layouts, then write the known-working U-Boot image to the
u-bootpartition. The reported offset and erase length—0x700000and0x200000—belong to this MR33 procedure. Preserve other partitions and verify the data before resoldering the NAND. - Reconnect the console and restore the chip. Resolder the NAND, wire the UART using the MR33 pinout, and leave adapter VCC disconnected. Cisco’s general AP boot guidance describes a 115200-bps terminal connection, trying 9600 bps if there is no output, and pressing ESC during startup to stop autoboot. That is general AP guidance, not a guarantee that every MR33 revision or repaired bootloader will behave identically.
- Load an intermediate U-Boot and boot the initramfs. In the documented run, the author used a modified
ubootwrite.pywith Python 2.7 and pyserial to load an intermediate U-Boot over serial. A TFTP server then suppliedopenwrt-19.07.6-ipq40xx-generic-meraki_mr33-initramfs-fit-uImage.itb. This is the historical image name used in that run, not a recommendation to install an old release today; image availability and supported releases can change. - Install OpenWrt to NAND. After booting the initramfs, the documented process removed obsolete factory UBI volumes while preserving the
ARTcalibration volume, recreated a failsafe volume using the initramfs image, and ransysupgradewith the MR33 squashfs image. The journal reports successful creation ofpart.safe,rootfs, androotfs_datavolumes. Exact shell commands are not reproduced here, so do not infer them from the volume names; use the MR33 project instructions and verify the target image and partition layout before writing.
Risks and limits to weigh first
- CPU lockout: the locked U-Boot’s serial entry path is reported to be capable of permanently bricking the CPU. Do not experiment with it as though it were a harmless shell prompt.
- NAND loss: desoldering, reading, erasing, or writing flash can destroy the bootloader or device-specific data. A complete verified backup is the practical route back to the original contents.
- Calibration data: the documented OpenWrt transition preserves
ART. Erasing or overwriting it can compromise device-specific radio calibration. - Limited repeatability: the documented offsets and partition observations are for the MR33 units in the project, not a universal Cisco layout.
- Warranty and service: the project author warns against returning modified or bricked devices to Cisco for service or replacement, and says the work is undertaken at the user’s own risk.
How Cisco’s other boot guidance fits
Cisco publishes general access-point boot guidance involving a console cable, terminal speed, and ESC to interrupt autoboot. Its newer recovery guide covers different Catalyst models; it should not be read as MR33 support. That guide explicitly excludes 2800/3800 APs and says U-Boot is not intended to be accessible on Wi-Fi 6E and Wi-Fi 7 APs. Model-specific recovery instructions matter: neither Cisco guide establishes that the MR33’s locked bootloader can be safely entered or repaired through a standard serial procedure.
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