Compute Module 1 (BCM2835) — Bootloader & Boot Process: Community Documentation

Document Version: 1.0
Target Hardware: Compute Module 1 (CM1), BCM2835 SoC
Scope: Bootloader architecture, firmware components, boot modes, and bring-up considerations

1. Overview of the BCM2835 Boot Sequence

The boot sequence on BCM2835 (used in the original Raspberry Pi 1, Pi Zero, and Compute Module 1) follows a well-documented but minimally officially documented path. Unlike newer SoCs, BCM2835 lacks an internal EEPROM bootloader; the entire boot firmware resides on external storage.

Step-by-Step Boot Flow

1. Power-On / Reset - The SoC exits reset and begins executing the first-stage bootloader embedded in the mask ROM (BootROM) on the silicon. - This BootROM is hardcoded and cannot be modified.

2. First-Stage: BootROM (SD Card Detection) - The BootROM scans for a valid SD card (MMC/SD interface). - It looks for a properly formatted FAT filesystem (typically FAT16 or FAT32 on the boot partition). - Loads bootcode.bin from the SD card into L2 cache/ SRAM.

3. Second-Stage: bootcode.bin - Enables the SDRAM (RAM initialisation). - Loads start.elf into SDRAM. - At this point, GPU firmware takes over execution. The ARM CPU remains halted until the GPU prepares the environment.

4. Third-Stage: start.elf (GPU Firmware) - Parses config.txt and cmdline.txt. - Loads the Device Tree Blob (.dtb) or Device Tree Blob for B Plus (.dtb). - Loads kernel.img (or custom kernel specified in config.txt) into memory. - Configures ARM core clock, VPU, and hardware according to config.txt settings. - Releases the ARM CPU from reset — handoff to the operating system kernel.

5. Kernel Handoff - The ARM CPU begins executing at the entry point of kernel.img. - Boot arguments are passed via Device Tree or ATAGS (legacy).

Note: This sequence is documented in community sources and inferred from the firmware binary behavior. The official Raspberry Pi documentation (Raspberry Pi Boot Modes - Official Docs) describes boot modes generically, but the specific internal BootROM behavior is not publicly disclosed by Broadcom.

2. Boot Firmware & Storage

Firmware Components (SD Card-Based)

Unlike newer Compute Modules (CM3, CM4, CM5), the CM1 has no on-board EEPROM. All firmware resides on the SD card boot partition.

These files are hosted in the official GitHub firmware repository (raspberrypi/firmware).

Storage Media

• Primary: MicroSD card (full-size SD adapter on CM1 IO board)
• Boot Partition: FAT16 or FAT32 (required for BootROM to read firmware)
• Root Partition: Can be ext4, F2FS, or other Linux filesystems

3. Boot Modes Supported

BCM2835 supports a limited set of boot modes compared to newer SoCs.

Source: Raspberry Pi Boot Modes - Official Docs confirms SD card is the primary boot mode for older hardware. Community knowledge indicates USB boot is not natively supported on BCM2835 without SD card intervention.

Boot Order

BCM2835 BootROM default order: 1. SD card (MMC0) 2. (No internal fallback to USB or network)

4. UART / Serial Console

The Compute Module 1 exposes UART pins on the IO board, but there are specific considerations for serial console access.

Default UART Configuration

• Primary UART (UART0): Exposed on GPIO pins 14 (TXD) and 15 (RXD) on the 22-pin IO header.
• Baud Rate: 115200 bps by default (unless overridden in cmdline.txt or config.txt).
• No automatic console enable: Unlike newer Pis with enable_uart=1 in config.txt, CM1 may require explicit configuration.

Serial Console Configuration

To enable serial console on CM1:

In config.txt:

enable_uart=1

In cmdline.txt:

console=serial0,115200 console=tty1 root=/dev/mmcblk0p2 rootfs=ext4 elevator=deadline fsck.repair=yes rootwait

Quirks and Community Notes

• The CM1 IO board provides a TTL-level serial header (3.3V). Do not connect RS-232 directly — use a USB-to-TTL serial adapter.
• Mini-UART vs. PL011: On BCM2835, the primary UART is the PL011 (full-featured UART). The mini-UART is available on the VideoCore subsystem but is not typically used for ARM console.
• Bootloader serial output: During early boot (before start.elf loads), minimal or no serial output is produced. The GPU firmware (start.elf) outputs to serial if configured.
• Dual serial ports: The Compute Module 1 has two UARTs available on GPIO, but only one is typically routed to the IO board.

Community Note: Some users have reported that early boot messages from bootcode.bin or the BootROM are not accessible via the standard GPIO UART. This is consistent with the closed-source nature of the GPU firmware.

5. Bare-Metal and OS Bring-Up Notes

U-Boot

• Supported: U-Boot can be used as a bootloader on BCM2835, replacing or supplementing the default kernel.img handoff.
• Build: Requires a BCM2835-specific defconfig (e.g., rpi_defconfig for original Pi, which applies to CM1).
• Usage: Place u-boot.bin or u-boot.img as kernel.img (or specify via kernel= in config.txt).
• Limitations: U-Boot must be built for the ARM1176 (ARM11) CPU core.

Circle (Bare-Metal C++ Framework)

• Supported: Circle is a C++ bare-metal framework for Raspberry Pi (including BCM2835).
• Use Case: Ideal for custom firmware, RTOS-style development, or learning bare-metal programming on CM1.
• Note: Circle handles SDRAM initialization and provides its own startup code, bypassing start.elf in some configurations.

Custom Firmware Considerations

• Custom bootcode.bin: Not feasible — binary-only and signed (community understanding).
• Custom start.elf: Not publicly documented; binaries are closed-source.
• Custom kernel: Fully supported; can replace kernel.img.
• Device Tree: BCM2835 supports both Device Tree and legacy ATAGs. Device Tree is the modern approach.

SD Card Layout Requirements

For successful boot, the SD card must contain: - A FAT partition (bootfs) marked as bootable (partition type 0x0C or 0x0E). - Files: bootcode.bin, start.elf, fixup.dat, config.txt, cmdline.txt, kernel.img, *.dtb.

6. Key Differences from Neighbouring Pi Generations

Why These Differences Matter

• No EEPROM: CM1 cannot boot without an SD card. This limits its use in embedded or headless deployments where SD card failure is a concern.
• Limited boot media: No native USB or network boot means deployment flexibility is reduced compared to CM4.
• Older GPU architecture: The VideoCore IV in BCM2835 is significantly less powerful than the VideoCore VI in BCM2711.

7. Open Questions / Areas Without Official Documentation

1. BootROM Source Code
   - The BootROM in BCM2835 is closed and not publicly documented. Its exact behavior (SD card detect timing, error handling) is inferred, not specified.

2. Bootcode.bin Internals
   - bootcode.bin is binary-only and not open source. The exact sequence of SDRAM initialization is not publicly documented.

3. Secure Boot
   - BCM2835 has no documented secure boot chain. There is no OTP-based boot verification.

4. Exact Boot Order Behavior
   - While SD card is known to be first, the precise fallback behavior (if SD fails, does it attempt USB?) is not officially documented for BCM2835.

5. GPU Firmware Configuration
   - The full range of config.txt options for BCM2835 is documented in the official docs, but the underlying GPU firmware behavior is closed-source. Some undocumented options exist in community repositories.

6. USB Boot Limitations
   - Whether bootcode.bin on BCM2835 can actually boot from USB mass storage devices (without an SD card) is unclear and likely not supported.

References

• Raspberry Pi Boot Modes - Official Docs — https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#raspberry-pi-boot-modes
• raspberrypi/firmware GitHub Repository — https://github.com/raspberrypi/firmware (boot firmware binaries)
• Raspberry Pi Forums — Community discussions on boot behavior and hardware (note: forum content varies in technical depth)
• Compute Module Hardware Documentation — Official Compute Module IO board and CM1 specifications

This document is community-maintained. Information is based on publicly available sources, community experimentation, and inference from firmware behavior. For official support, consult Raspberry Pi Ltd. directly.