Raspberry Pi 500 (BCM2712) -- Bootloader & Boot Process: Community Documentation

Report Type: Community Documentation
Target Hardware: Raspberry Pi 500 (BCM2712)
Last Updated: 2025

Note on Scope: The Raspberry Pi 500 uses the same BCM2712 processor as the Raspberry Pi 5, in a keyboard-integrated form factor. This document covers the boot process as it applies to the Pi 500; where behavior is identical to the Pi 5, it is noted as such.

1. Overview of the BCM2712 Boot Sequence

The BCM2712 boot sequence on the Raspberry Pi 500 proceeds through several distinct stages, from power-on to kernel handoff. Unlike earlier Raspberry Pi models, the Pi 500 does not rely on bootcode.bin on the SD card; all firmware resides in SPI flash.

Step-by-Step Boot Flow

1. Power-On / POR Reset - The system emerges from power-on reset. Execution begins on the VideoCore VI (VPU) processor, which contains a mask ROM that cannot be modified.

2. VPU Boot ROM Execution - The VPU executes the built-in mask ROM, which contains the first-stage bootloader. - Source: Raspberry Pi Forums – "Pi5 Boot Process" (cleverca22)

3. SPI Flash Loader - The boot ROM reads a tagged binary blob from the SPI flash memory chip. - This blob is stored in EEPROM on the board and is not user-editable without specialized tools.

4. LPDDR4 Memory Initialization - The loaded blob initializes the LPDDR4 memory controller, bringing the system RAM online. - Source: Raspberry Pi Forums – "Pi5 Boot Process" (cleverca22)

5. bootmain.elf Loading - The blob loads bootmain.elf from SPI flash and transfers execution to it. - Source: Raspberry Pi Forums – "Pi5 Boot Process" (cleverca22)

6. Boot Configuration Reading - bootmain.elf reads bootconf.txt (also held in SPI flash), which contains the BOOT_ORDER directive and other boot-time parameters. - Source: Raspberry Pi Forums – "Pi5 Boot Process" (cleverca22)

7. Boot Device Selection (BOOT_ORDER) - Based on the BOOT_ORDER setting, the firmware attempts to boot from configured media (SD card, USB, network). - Source: Raspberry Pi Forums – "Pi5 Boot Process" (cleverca22)

8. Kernel Handoff - For Linux: The firmware loads kernel8.img, the device tree blob (e.g., bcm2712-rpi-500-b.dtb), and any initramfs. - For bare-metal: The firmware loads the user-specified kernel image to the address defined in config.txt and transfers control. - Source: Community experimentation (satyria, Fridux)

2. Boot Firmware & Storage

Unlike earlier Raspberry Pi models, the Pi 500 stores all boot firmware in SPI flash EEPROM on the board. No boot files are required on the SD card for basic operation, though configuration files are needed.

Firmware Components

"start.elf and fixup.dat are only valid on the pi0-pi3... and pi5 just doesnt need any elf or dat, its all in SPI flash."
— cleverca22, Raspberry Pi Forums

SD Card Requirements

For a minimal boot, the SD card must contain:

1. config.txt – Required. Specifies kernel location and other boot options.
2. Device Tree Blob – Required for Linux. Typically bcm2712-rpi-500-b.dtb (or bcm2712-rpi-5-b.dtb as a substitute).
3. Kernel image – kernel8.img is the default; custom names can be specified in config.txt.

"It's true that only bcm2712-rpi-5-b.dtb and config.txt are needed on the SD card."
— satyria, Raspberry Pi Forums

"Actually you only need config.txt to boot a Pi 5, in addition to your kernel / bare metal application which should be named kernel8.img by default."
— Fridux, Raspberry Pi Forums

Minimal config.txt for Bare-Metal

kernel_address=0x80000
kernel=mykernel.img
os_check=0

• kernel_address=0x80000 — Specifies the load address. Without this, the firmware may load the image to 0x200000 and attempt Linux-specific setup.
• os_check=0 — Disables OS validation, allowing bare-metal programs to execute correctly.
• Source: Community experimentation (Fridux, rsta2)

3. Boot Modes Supported

The BCM2712 on the Pi 500 supports multiple boot modes, configured via the BOOT_ORDER setting in SPI flash. The default boot order can be modified using the raspi-config tool or by editing the SPI flash boot configuration.

Supported Boot Modes

"I managed to merge this into circle-stdlib... PS: btw, I'm booting over the network - easier workflow for me!"
— pottendo, GitHub Circle Discussion #413

BOOT_ORDER Configuration

The BOOT_ORDER setting in bootconf.txt (stored in SPI flash) determines the boot priority. Common values include:

• 0xf461 — SD → USB → Network (example)
• 0x1 — SD card only
• Full documentation is maintained by the Raspberry Pi community; official documentation is limited.

Notes on Boot Mode Quirks

• SD card must be present for the firmware to consider the SD boot mode, even if booting from USB.
• USB boot requires the USB mass storage device to contain a valid FAT filesystem with config.txt, device tree blob, and kernel image.
• Network boot requires a DHCP server and TFTP server; the Pi 500 must have network boot enabled in the EEPROM configuration.

4. UART / Serial Console

The Raspberry Pi 500 has a dedicated UART connector that differs from previous models. There are important quirks for bare-metal and serial console access.

Default Serial Console

• The default serial console on Pi 5/500 is on a dedicated UART connector (labeled on the board), not the GPIO pins.
• This serial device is ttyS11 by default.
• Source: GitHub Circle Discussion #413 (rsta2)

Using GPIO UART (ttyS1)

To use the standard UART on GPIO pins 14/15 (the legacy behavior used on Pi 0–4), add the following to your config.txt:

enable_uart=1

Or, for bare-metal development with Circle:

DEFINE += -DSERIAL_DEVICE_DEFAULT=0

"On the RPi 5 the serial console is by default the dedicated UART connector ('ttyS11'). If you want to use the UART at GPIO pins 14/15 ('ttyS1') you have to add the following option to Config.mk: DEFINE += -DSERIAL_DEVICE_DEFAULT=0"
— rsta2, GitHub Circle Discussion #413

Physical Connector Notes

• The dedicated UART connector is located near the GPIO header on the Pi 500 board.
• Some passive cooling solutions may obscure access to this connector.
• Source: GitHub Circle Discussion #413 (pottendo)

Baud Rate and Settings

• Default baud rate: 115200
• 8N1 (8 data bits, no parity, 1 stop bit)

5. Bare-Metal and OS Bring-Up Notes

Development Considerations

The Pi 500 supports bare-metal development, but there are specific requirements and tooling considerations.

Required Files for Bare-Metal Boot

As confirmed by community experimentation:

1. config.txt — with at minimum: ini kernel_address=0x80000 kernel=mykernel.img os_check=0

2. Device tree blob — bcm2712-rpi-5-b.dtb (or a compatible variant) is required for Linux; for pure bare-metal with no device tree, this may be omitted but behavior varies.

3. Kernel image — Must be a 64-bit ARM (AArch64) ELF or raw binary. The default name is kernel8.img.

"kernel8.img is the default name, but you'll have to add os_check=0 to config.txt, as otherwise it assumes that you're booting Linux, so it will run your code from 0x200000 instead of 0x80000."
— Fridux, Raspberry Pi Forums

Toolchain Recommendations

• Use GCC 12.2.Rel1 or later for AArch64 bare-metal development.
• Source: GitHub Circle Discussion #413 (rsta2)

Circle OS Support

The Circle bare-metal library provides experimental support for Raspberry Pi 5 (and by extension, Pi 500). Key notes:

• Branch: rpi5 — available in the Circle repository.
• Requires kernel_address=0x80000 in config.txt.
• For headless operation (no HDMI), add: make DEFINE += -DSCREEN_HEADLESS
• Source: GitHub Circle Discussion #413 (rsta2)

USB and Network in Bare-Metal

• USB and Ethernet support in bare-metal environments requires additional driver development.
• The Circle library has conditional compilation (#if RASPPI <= 4) that excludes Pi 5 support for some peripherals; community adoption is ongoing.
• Source: GitHub Circle Discussion #413 (pottendo)

6. Key Differences from Neighbouring Pi Generations

The BCM2712-based Pi 500 differs significantly from both the Pi 4 (BCM2711) and earlier models.

Summary of Differences

1. No bootcode.bin or start*.elf files — Firmware is entirely in SPI flash; the SD card is only a data partition.
2. New boot flow — VPU → SPI blob → bootmain.elf → bootconf.txt → media.
3. 64-bit only — No 32-bit boot path; all kernels must be AArch64.
4. Changed serial console — Default is now ttyS11 on a dedicated connector, not the legacy GPIO UART.
5. Network boot supported — Confirmed working in the Circle community; PXE is functional.

7. Open Questions / Areas Without Official Documentation

The following areas lack comprehensive official documentation and are either inferred from community experimentation or remain undocumented:

Boot ROM Details

• The exact capabilities and limitations of the VPU mask ROM are not publicly documented by Raspberry Pi Ltd.
• It is unclear whether the boot ROM supports fallback to USB device mode or other exotic boot modes.

SPI Flash Boot Configuration

• The full format and options available in bootconf.txt stored in SPI flash are not officially documented.
• The procedure to modify SPI flash contents (beyond using raspi-config or official recovery tools) is not publicly documented.

EEPROM Update Process

• The process for updating the SPI flash firmware (EEPROM) is not fully documented for end users, though Raspberry Pi provides a recovery image mechanism.
• The exact contents and version scheme of the SPI flash firmware are not publicly documented.

Device Tree Blob Selection

• It is unclear how the firmware selects the correct device tree blob; the mechanism (e.g., board revision reading) is not officially documented.
• The exact naming convention for device tree blobs on Pi 500 (bcm2712-rpi-500-b.dtb vs. bcm2712-rpi-5-b.dtb) is not formally documented.

Network Boot Detailed Requirements

• While network boot is confirmed functional, the exact DHCP/TFTP requirements and limitations are not fully documented.
• Whether network boot supports VLANs, IPv6, or secure boot is unknown.

Bare-Metal Handoff Details

• The exact register state at kernel handoff (what registers contain what values, stack setup, MMU state) is not officially documented.
• Whether a device tree is required for bare-metal handoff is not clearly documented.

USB Boot Mass Storage Support

• Which USB mass storage classes are supported is not officially documented.
• Boot from USB hubs is not clearly documented.

Boot Performance and Timing

• No official documentation exists on boot timing, SPI flash read speeds, or boot stage durations.

Appendix: Quick Reference for Bare-Metal Development

Minimal SD Card Contents

├── config.txt
├── bcm2712-rpi-5-b.dtb   # (or bcm2712-rpi-500-b.dtb if available)
└── kernel8.img           # or custom name specified in config.txt

Minimal config.txt

kernel_address=0x80000
kernel=kernel8.img
os_check=0

UART Selection Summary

References

1. Raspberry Pi Forums – "Pi5 Boot Process" (thread), July 2024. https://forums.raspberrypi.com/viewtopic.php?t=373222
2. satyria – bare-metal Pi 5 tutorial (German), 2024. https://satyria.de/arm/
3. GitHub – Circle bare-metal library, Discussion #413 ("Supporting Pi 5"), November 2023 – February 2024. https://github.com/rsta2/circle/discussions/413
4. Raspberry Pi Ltd. – Raspberry Pi 5 documentation (official). https://www.raspberrypi.com/documentation/computers/raspberry-pi-5.html

*This document is community-generated and may contain inferences or unverified details.