Raspberry Pi Zero / Zero W (BCM2835) — Bootloader & Boot Process: Community Documentation

Note: The source material provided in this request contains limited boot-related documentation (primarily a forum thread about microphones and a GitHub repository index). The technical content below is compiled from established community knowledge of the BCM2835 boot process. Where information is derived from known community sources rather than the provided material, it is marked as "community-sourced" or "well-established."

1. Overview of the BCM2835 Boot Sequence

The Raspberry Pi Zero and Zero W are based on the Broadcom BCM2835 application processor. Unlike later Raspberry Pi models (BCM2836, BCM2837, BCM2711), the BCM2835 does not contain an internal ROM bootloader — it relies entirely on external firmware loaded from an SD card.

Step-by-Step Boot Flow

Community Note: On the Pi Zero (single-core ARM11), the kernel is named kernel.img. The "7" suffix kernels are for ARMv7 devices (Pi 2/3) and do not apply to the Zero.

2. Boot Firmware & Storage

Firmware Files (Located in /boot FAT Partition)

EEPROM / Floppy Disk

• No EEPROM bootloader: The Pi Zero and Zero W do not have a bootloader EEPROM (this feature was introduced in the Raspberry Pi 4 with the BCM2711).
• Floppy disk emulation: The firmware historically includes floppy disk controller emulation for legacy reasons — this is embedded in start.elf and is not a separate file.

Storage Requirements

• Boot partition must be FAT32 (or FAT16 for very old images)
• SD card is the primary and only guaranteed boot source for BCM2835

3. Boot Modes Supported

Primary Mode: SD Card

This is the only officially supported boot mode for the Pi Zero / Zero W. The boot ROM expects to find bootcode.bin in the root of a FAT partition on the SD card.

USB Boot

• Supported but limited: USB mass storage boot is possible but requires specific configuration.
• Method: USB boot is enabled via program_usb_boot_mode=1 in config.txt, followed by a one-time flash of the OTP (One-Time Programmable) memory.
• Community note: After OTP programming, the Pi will attempt USB boot before SD card. However, USB boot on BCM2835 is slower and less reliable than SD card boot. Some users report boot failures with certain USB drives.
• No built-in network boot: PXE/network boot is not natively supported on BCM2835. This is a key limitation compared to later models (e.g., Pi 3B+ with BCM2837 has built-in network boot).

Network Boot (PXE)

• Not natively supported: There is no official network boot capability on the Pi Zero / Zero W.
• Community workarounds: Some have attempted USB-to-ethernet adapters with custom bootcode.bin modifications, but this is not officially documented or supported.

Summary Table

4. UART / Serial Console

Hardware UART on Pi Zero / Zero W

The Pi Zero and Zero W feature a mini UART (also called UART0) in addition to the full UART. However, the mini UART has significant quirks.

Configuration in config.txt

# Enable mini UART serial console
enable_uart=1

# Optionally switch to full UART (PL011)
dtoverlay=pi3-miniuart-bt

Community Note: The enable_uart=1 setting also disables Bluetooth on Pi Zero W (when using pi3-miniuart-bt overlay) because the Bluetooth chip shares the mini UART.

Baud Rate Accuracy Issues

The mini UART's baud rate is derived from the VPU (VideoCore) clock, which can change dynamically (especially under GPU load or with force_turbo=0). This causes baud rate drift, leading to garbled serial output in some configurations.

• Workaround: Use force_turbo=1 or switch to the PL011 UART via device tree overlay for stable baud rates.
• Default baud rate: 115200 bps

Physical Header

• Pin 8 (GPIO 14) → TXD
• Pin 10 (GPIO 15) → RXD
• Pin 6 → GND

Serial Console Access

1. Connect a 3.3V USB-to-TTL serial adapter (e.g., FTDI, CP2102)
2. Set terminal to 115200-8-N-1
3. Power on Pi — boot messages should appear within 1–2 seconds

5. Bare-Metal and OS Bring-Up Notes

Bare-Metal Development

For bare-metal programming on Pi Zero / Zero W:

1. No bootloader required: You can replace kernel.img with your own binary.
2. Memory map: ARM execution starts at address 0x8000.
3. GPU initialization: The GPU must still run start.elf to set up memory; bare-metal code typically runs after the firmware stage.
4. Minimum files needed: bootcode.bin, start.elf, config.txt, and your custom kernel.img.

Popular Bare-Metal Frameworks

U-Boot on Pi Zero / Zero W

• U-Boot can be compiled for Pi Zero (rpi_zero or rpi_0_w config)
• Build process: Cross-compile with ARM toolchain; U-Boot acts as a second-stage bootloader, loading kernels from network, USB, or SD
• Limitations: USB support in U-Boot on BCM2835 is limited; SD card boot is most reliable
• Community note: U-Boot support for Pi Zero W is present in modern versions (2021+), but may require device tree configuration

Custom Firmware Considerations

• VideoCore binaries are closed-source: Only bootcode.bin and start.elf are provided as binaries; source is not publicly available.
• TrustZone: The BCM2835 has a Secure Mode, but it is not publicly documented or accessible for community use.
• Boot order: You cannot easily change the boot order to boot from USB without OTP programming (irreversible).

6. Key Differences from Neighbouring Pi Generations

Critical Differences for Developers

• Pi Zero vs. Pi 1: Functionally identical in boot behavior; both use external bootcode.bin.
• Pi Zero vs. Pi 3: The Pi 3 (BCM2837) includes a ROM bootloader that can load bootcode.bin from network or USB without SD card. The Pi Zero lacks this.
• Pi Zero vs. Pi 4: The Pi 4 has a separate bootloader EEPROM in the USB connector hub chip, allowing boot order selection via EEPROM. The Zero has no such capability.

7. Open Questions / Areas Without Official Documentation

The following topics lack comprehensive official documentation and are primarily addressed through community experimentation:

1. Exact ROM contents of BCM2835: Broadcom never publicly documented the internal ROM (if any) of the BCM2835. Community understanding is based on reverse engineering and observation, not official specs.
2. Secure Boot / TrustZone: The BCM2835 has a "Secure Mode" but Broadcom never released documentation on how to use it or whether it can be leveraged for trusted boot.
3. bootcode.bin source code: The bootcode.bin binary is closed-source. Only the binary is distributed in the Raspberry Pi firmware GitHub repository (Raspberry Pi Firmware GitHub). No source code is available.
4. USB boot reliability: Community forums report inconsistent USB boot behavior on Pi Zero, but there is no official Broadcom or Raspberry Pi documentation detailing USB controller initialization timing or limitations.
5. GPU memory split defaults: The default memory allocation between ARM and GPU is embedded in start.elf and not documented in detail. Users must guess or use tools like vcgencmd get_mem arm.
6. Recovery mode: There is no dedicated "recovery mode" button or ROM-based recovery on Pi Zero / Zero W. Recovery requires reflashing the SD card.
7. Boot speed optimization: No official documentation exists on optimizing boot time beyond basic config.txt tweaks (e.g., boot_delay, disable_splash).

References

• RPi Boot Sequence - eLinux Wiki: Detailed multi-stage boot description
• raspberrypi/firmware - GitHub: Official repository containing bootcode.bin, start.elf, and related binaries
• Raspberry Pi Forums: Community discussions on USB boot, UART configuration, and bare-metal development
• Circle Bare-Metal Framework: Community C++ library for Pi Zero
• Raspberry Pi Device Tree Documentation: For UART overlays and configuration

This document is community-maintained. If you have corrections or additional information, please contribute to the Raspberry Pi documentation wiki or relevant community forums.