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OpenCentauri Firmware Notes

A collection of PD's notes on the current status of low-level OpenCentauri firmware development.

2025-08-29 Update

Recent Progress

Release v0.0.4 introduced Serial UART console and uBoot access on power-up.

Recommended Setup

  • Set uBoot boot delay:

    fw_setenv bootdelay 5

    This sets a 5-second delay before uBoot continues the start-up sequence, allowing you to interrupt via the Serial console.

  • List all parameters:

    fw_printenv

    Tip: Save a copy of the stock settings before making changes.

Serial Console Access

  • Connect a USB serial adapter (or other serial interface) to the Tx, Rx, and Ground pins on the Centauri mainboard.
  • Baud rate: 115200
  • Reboot the Carbon and observe the boot-up process.
    Press any key to interrupt boot!

Booting Without DSP

To boot from uBoot without loading DSP firmware:

run setargs_mmc
run boot_normal

This skips the boot_dsp0 argument, preventing DSP firmware from loading.

Removing DSP from Linux Device Tree (Temporary)

To remove the DSP from the Linux device tree for a single boot (changes revert on reboot):

fdt addr ${fdt_addr}
fdt print /
fdt rm dsp0
fdt rm dsp0_gpio_int
run setargs_mmc
run boot_normal

Modifying Pin Control for PE9 and PE6

To force a pull-up resistor on PE9 and PE6, use the following device tree overlay:

target-path = "/pinctrl@2000000";
__overlay__ {
    pe9_gpio: pe9-gpio {
        pins = "PE9";
        function = "gpio_in";
        bias-pull-up;
    };
    pe6_gpio: pe6-gpio {
        pins = "PE6";
        function = "gpio_in";
        bias-pull-up;
    };
};

Equivalent uBoot fdt Commands

  1. Disable DSP blocks:

    fdt set /dsp0 status disabled
fdt set /dsp0_gpio_int status disabled

  2. Configure PE9 and PE6 as GPIOs with pull-ups:

    fdt mknode /pinctrl@2000000 pe9-gpio
fdt mknode /pinctrl@2000000 pe6-gpio

# For PE9 (gpio137)
fdt set /pinctrl@2000000/pe9-gpio pins "PE9"
fdt set /pinctrl@2000000/pe9-gpio function "gpio_in"
fdt set /pinctrl@2000000/pe9-gpio bias-pull-up

# For PE6 (gpio134)
fdt set /pinctrl@2000000/pe6-gpio pins "PE6"
fdt set /pinctrl@2000000/pe6-gpio function "gpio_in"
fdt set /pinctrl@2000000/pe6-gpio bias-pull-up

Note: Configuring pinctrl for these GPIOs in the device tree may make them unavailable for general-purpose GPIO use in Linux.

Register Interface for GPIO Configuration

If device tree changes are insufficient, you can use the /sys/class/sunxi_dump/* interface to modify GPIO registers directly.

Reference: R528 User Manual

Relevant Registers

  • GPIO Base: 0x02000000
  • PE Configure Register 0: 0x00C0
  • PE Configure Register 1: 0x00C4
  • PE Data Register: 0x00D0
  • PE Multi_Driving Register 0: 0x00D4
  • PE Multi_Driving Register 1: 0x00D8
  • PE Pull Register 0: 0x00E4
Pull Register Bits
  • PE9_PULL (bits 19:18):
    • 00: Pull-up/down disable
    • 01: Pull-up
    • 10: Pull-down
    • 11: Reserved
  • PE6_PULL (bits 13:12):
    • Same as above

Example Commands

# Set PE9 and PE6 pull-up
echo "0x020000E4 0x50041050" > /sys/class/sunxi_dump/write
cat /sys/class/sunxi_dump/read

# Configure PE6 IN, PE9 OUT
echo "0x020000C0 0xF0FFFFFF" > /sys/class/sunxi_dump/write
echo "0x020000C4 0xFFFFFF1F" > /sys/class/sunxi_dump/write

Additional Resources

Next Steps

  • Continue experimenting with /sys interface to configure PE6/PE9 UARTs for Klipper MCU.
  • Investigate missing GPIOs accessible by DSP and remap via uBoot device tree hacks.
  • Consider building a replacement kernel for easier device tree control.
  • Explore device tree overlays or sideloading replacement DTs.
  • Evaluate if uBoot commands alone provide sufficient control for current needs.