Embedded Firmware

The CC uses two different embedded firmware for the bed and the hotend boards with allegedly the same bootloader. The firmwares as of 2025-05-19 are a substitute of Klipper commit 28f60f7e with a lot of changes.

Official update method

To update both the hotend and the bed boards, the main firmware attempts to send a file through YModem protocol when the said boards are power cycled during bootup. The firmware files are not encrypted and are stored on the rootfsA or rootfsB partition (whichever is active) in the /lib/firmware folder. The hotend firmware is called upgrade-hotend.bin and the bed board's is upgrade-bed.bin.

STM32 flash structure

Start offset	End offset	Description
0x08000000	0x08007FFF	32Kb Bootloader
0x08008000	0x0800BFFF	16Kb Unknown
0x0800C000	...	Elegoo-Klipper Firmware

The firmware upgrade files include the 16Kb of unknown (mostly 0xFF with some random bytes) data so it must be flashed on address 0x08008000.

Flashing custom firmware

No going back

Flashing a new firmware will ERASE THE ELEGOO BOOTLOADER that we have no image file for, nor a substitution!

Currently the stock mainboard CANNOT boot from custom firmwares nor the stock firmware without the original bootloader. This essentially means you cannot use the stock mainboard anymore after wiping the firmware from the hotend or bed boards!

Nobody in the OpenCentauri community holds responsibility of any bricked or damaged machine. Anything you do is your own responsibility!

Readout Protection

The hotend and bed boards come with the readout protection enabled by default. As long as you don't remove it, the stock flash is safe. If you instruct the programmer to deactivate the readout protection, the entire flash memory, with the bootloader and the firmware will be wiped.

Hotend

Remove the hotend board (only the main one, the supplementary board is not needed).
On the back side, there is a 2x4 copper pad row. Short the 3.3v and the BOOT with a tweezer
Connect it to your PC via an USB-C cable (no adapter is needed) while shorting the pins
Keep it shorted for ~2 seconds, then you can let go
Open STM32CubeProgrammer software
On the top right, select the USB mode. If you shorted the pins correctly, it should find the hotend board. if not, you'll get a "No DFU detected".
Go to the second tab in the left side vertical button column
In the "Download" section, select the firmware file.
Click Start Programming

Bed

Either via an added connector or solder jumper wires directly to the right side serial pins to an USB-TTL transciever as
- Bed Board USB-TTL
  
  5v 5v
  
  RX RX
  
  TX TX
  
  GND GND
- Note: Depending on the USB-TTL board, RX and TX lines might need to be switched.
Short the BOOT pin to 3.3v
Connect the USB-TTL board to your PC
Short the RESET pins on the bed board (no need to keep it shorted, just touch it)
In STM32CubeProgrammer, select UART mode and select your USB-TTL device
Go to the second tab in the left side vertical button column
In the "Download" section, select the firmware file.
Click Start Programming

Firmware Creation

The STM32F402 is an STM32 variant specifically made for the Chinese market. For any configuration, use STM32F401RTC6 (sometimes just STM32F401).

Vanilla Klipper and/or custom firmware

Select "No bootloader" in the make menuconfig and flash it to the default address. If you plan to use the stock bootloader and/or a similar size, select 32Kb bootloader and flash to default address + 0x8000.

Communication interfaces

Board	Pins
Hotend	USB PA11 / PA12
Bed (stock RS232 transciever)	UART PA2 / PA3
Bed (right side serial)	UART1 PA9 / PA10

Boostrapping the stock firmware

You need to find a bootloader either source code or image. Then you need to merge it with the Elegoo firmware into one file, with the following format:

Start offset	End offset	Description
0x0	0x7FFF	Bootloader, padded with 0x00
0x8000	...	Stock firmware file

And flash the merged file to the default address.

Bed Board	USB-TTL
5v	5v
RX	RX
TX	TX
GND	GND