Raspberry Pi Zero 2 W (BCM2837B0) — Bootloader & Boot Process: Community Documentation

Version 1.0 | Community-Maintained Technical Report

1. Overview of the BCM2837B0 Boot Sequence

The Raspberry Pi Zero 2 W uses the BCM2837B0 SoC, which is a variant of the BCM2837 (originally introduced in the Raspberry Pi 3B). This processor contains a boot ROM embedded in the silicon — this is the first code that executes when power is applied, and it cannot be modified.

Step-by-Step Boot Flow

1. Power-On / Reset - The VideoCore IV GPU core powers on first (this is a characteristic of all BCM283x-family SoCs) - The ARM CPU remains held in reset while the GPU executes its boot ROM

2. ROM Boot Loader (Stage 1) - The GPU boot ROM scans for a valid boot medium - Boot order is fixed in ROM: SD card (first) → USB (second) - The ROM looks for a FAT filesystem on the SD card (or USB mass storage device) - If no bootable device is found, the boot ROM enters USB device mode (waiting for rpiboot)

3. Bootloader Firmware Loading (Stage 2) - Once a FAT partition is found, the ROM loads bootcode.bin into L2 cache - bootcode.bin is the secondary bootloader — it initializes SDRAM and loads the main firmware - On the Zero 2 W, bootcode.bin is required on the SD card (unlike Pi 4 which has internal EEPROM)

4. VideoCore Firmware (Stage 3) - start.elf (or start4.elf for newer firmware) is loaded — this is the VideoCore firmware - This firmware reads config.txt, cmdline.txt, and any device tree overlays - The firmware also handles GPU-side initialization (clock speeds, memory split)

5. Kernel Handoff - For Linux: the ARM CPU is released from reset; the kernel (kernel.img or kernel8.img for 64-bit) is loaded to the specified memory address - The kernel command line from cmdline.txt is passed to the kernel - Device tree blobs (.dtb) or device tree overlays (.dtbo) are passed for hardware description

Note: The Zero 2 W does not have a bootloader EEPROM (unlike the Pi 4, Pi 400, Compute Module 4/5). All firmware files must reside on the bootable SD card or USB drive.

2. Boot Firmware & Storage

Firmware Files Required on Boot Media

For the Zero 2 W to boot, the following files must be present on a FAT-formatted partition (typically the first partition on the SD card):

Source: Raspberry Pi Documentation — boot folder contents

EEPROM Status

The Zero 2 W does not have a bootloader EEPROM. This distinguishes it from the Raspberry Pi 4 series and Compute Module 4/5, which store the bootloader in SPI flash. All boot firmware must be provided externally via SD card or USB.

Firmware Source

The official firmware is maintained in the Raspberry Pi GitHub repository:

• Repository: raspberrypi/firmware (boot folder)
• URL: https://github.com/raspberrypi/firmware/tree/master/boot

Community members typically copy these files from a Raspberry Pi OS image or the official firmware repository.

3. Boot Modes Supported

The BCM2837B0 supports multiple boot modes. The primary mode is determined by the contents of the boot media and the OTP (One-Time Programmable) memory configuration.

3.1 SD Card Boot (Default)

• Primary boot device — the ROM always attempts SD card boot first
• Requires a FAT-formatted partition with the firmware files listed above
• Supports both SDIO and SPI-mode SD card interfaces (the Zero 2 W uses the standard SD interface)

3.2 USB Boot

The Zero 2 W supports booting from USB mass storage devices (flash drives, hard drives, SSDs). This requires:

1. USB boot mode must be enabled — the OTP bit for USB boot must be set
2. On a new Zero 2 W, USB boot is not enabled by default — you must enable it via program_usb_boot_mode=1 in config.txt and reboot once

Note: The official documentation states that on Raspberry Pi 3-series devices, you must enable USB boot mode via the OTP. The Zero 2 W (which uses the same SoC family) likely inherits this behavior, though community testing confirms USB boot works on most units.

Procedure to enable USB boot:

# Add to config.txt on an SD card, boot once, then remove the line
program_usb_boot_mode=1

After enabling, the boot order becomes:

1. SD card (if bootable)
2. USB device (if no SD card boot)

3.3 USB Device Boot (rpiboot)

The Zero 2 W supports being booted via USB device mode using the rpiboot tool. This is the method used for flashing eMMC on Compute Modules, but it also works on the Zero 2 W.

• The device enumerates as a USB mass storage device (or in newer firmware, as a composite device with serial console)
• The host computer runs rpiboot which serves a boot image over USB
• This allows provisioning without an SD card

Source: GitHub — raspberrypi/usbboot lists Zero 2 W as compatible with the mass-storage-gadget firmware, which runs a Linux initramfs enabling USB device boot

3.4 Network / PXE Boot

Network boot is not natively supported on the Zero 2 W in the same way it is on Pi 3B+ and Pi 4. The BCM2837B0 ROM does not include a built-in PXE client. However:

• Network boot can be achieved by booting via USB and then loading network drivers from the initramfs
• Some community projects have demonstrated network boot by using the USB gadget interface with DHCP/TFTP
• The official network boot documentation focuses on Pi 3B+ and newer — the Zero 2 W is not listed

Community Note: Network boot on the Zero 2 W requires additional software and is not a plug-and-play feature.

3.5 Boot Mode Summary Table

4. UART / Serial Console

The Zero 2 W provides access to the primary UART (UART0) on the 40-pin GPIO header. This is the default console for boot messages and Linux serial console.

Pinout

• Voltage: 3.3V (TTL level) — do not connect directly to RS-232 without a level shifter
• Default baud rate: 115200 baud, 8N1

Enabling Serial Console

1. In cmdline.txt: console=serial0,115200 console=tty1 root=/dev/mmcblk0p2 rootfs=ext4 fsck.repair=yes rootwait

2. In config.txt: - No special configuration needed for default UART - To disable serial console: enable_uart=0

3. Device name: On Raspberry Pi OS, the primary UART is mapped to serial0, which typically points to /dev/ttyS0 (or /dev/ttyAMA0 on older images)

Boot Messages via UART

The GPU firmware (start.elf) outputs boot debug messages to the UART before the ARM kernel starts. This includes:

• Memory initialization messages
• Loading of firmware files
• Device tree information

If no output is seen on the UART, common causes include:

• Incorrect baud rate
• Missing ground connection
• enable_uart=0 set in config.txt
• USB-to-TTL adapter issues (particularly with cheap clones)

quirks and Community Observations

• No mini-UART vs PL011: The Zero 2 W uses the PL011 UART (not the mini-UART found on Zero and Zero W). This is important because the mini-UART has a divided clock that makes baud rate generation less accurate. The PL011 on Zero 2 W is more reliable for high-speed serial communication.

• Serial console on USB: When using the mass-storage-gadget USB boot mode (via rpiboot), the device also creates a USB CDC-ACM serial interface. This provides a second serial console over USB in addition to the hardware UART. Both are available in the Linux initramfs environment.

Source: GitHub — raspberrypi/usbboot — "Because it runs Linux, it also provides a console login via both the hardware UART and the USB CDC-UART interfaces."

5. Bare-Metal and OS Bring-Up Notes

5.1 Bare-Metal Development

For bare-metal programming on the Zero 2 W (BCM2837B0), developers interact with:

• VideoCore mailbox interface — for communication between ARM and GPU
• ARM local peripherals — timer, interrupt controller
• GPU memory — firmware loads at address 0x00000000 (ARM's view of GPU memory)
• SDRAM — initialized by bootcode.bin and start.elf

Popular bare-metal frameworks and tutorials:

• Circle — C++ bare-metal framework supporting Raspberry Pi 1 through 3 (including BCM2837)
• https://github.com/rsta2/circle
• bcm2837 — community bare-metal examples for the Raspberry Pi 3 series (BCM2837)
• Raspberry Pi bare metal forum — active community discussions

Community Note: While Circle supports the Pi 3 family, specific Zero 2 W (BCM2837B0) compatibility should be verified. The B0 variant is a die shrink of the original BCM2837 with improved power management, but the peripheral map is largely identical.

5.2 U-Boot

U-Boot can be used as a first-stage bootloader on the Zero 2 W. This is useful for:

• Network boot via TFTP
• Loading kernels from ext4 filesystems
• More flexible boot scripts

To use U-Boot:

1. Compile U-Boot for rpi_3 or bcm2837 (the Zero 2 W uses the same device tree as Pi 3)
2. Rename to boot.scr.uimg or load via bootcmd
3. Place on SD card alongside firmware files

Community Note: Official U-Boot support for BCM2837 is included in mainline U-Boot. The Zero 2 W should work with the rpi_3_defconfig, but this is community-verified, not officially documented.

5.3 64-bit OS Support

The BCM2837B0 is a 64-bit capable ARMv8 processor (Cortex-A53). The Zero 2 W can boot:

• 32-bit kernels — kernel.img (ARMv7)
• 64-bit kernels — kernel8.img (ARMv8, ARM64)

To boot a 64-bit OS (e.g., 64-bit Raspberry Pi OS):

• Ensure kernel8.img is present on the boot partition
• The firmware automatically detects and loads the 64-bit kernel if present

Community Note: Raspberry Pi OS 64-bit is officially supported on Zero 2 W. Some community reports indicate that 64-bit images may have slightly different boot behavior, but firmware files are identical.

5.4 Boot Speed and Optimization

• SD card class: Use Class 10 or UHS-I cards for optimal boot times
• USB boot: USB 2.0 is the limiting factor; USB 3.0 devices work but are limited to USB 2.0 speeds
• Boot delay: The boot_delay option in config.txt adds a wait period before loading the kernel (useful for recovery)

6. Key Differences from Neighbouring Pi Generations

The Zero 2 W sits in the product line between the original Zero/W and the Raspberry Pi 4 series. Below are the most relevant distinctions:

Key Takeaways for Zero 2 W

1. Like Pi 3: Uses the same ARM architecture (BCM2837 family) and supports 64-bit; shares the PL011 UART
2. Like original Zero: Lacks EEPROM, requires SD card or USB for firmware; compact form factor
3. Unlike Pi 4: Does not support network boot natively, no GPIO boot mode, no PCIe

7. Open Questions / Areas Without Official Documentation

The following areas lack official Raspberry Pi documentation or have conflicting/incomplete community information:

7.1 USB Boot OTP Status

Question: Is the USB boot mode OTP bit pre-programmed on the Zero 2 W, or must users explicitly enable it?

• The Pi 3 series requires explicit OTP programming via program_usb_boot_mode=1
• Community reports suggest most Zero 2 W units boot USB devices without this step, but this is not officially confirmed
• Community behavior: Most Zero 2 W units appear to boot USB devices out of the box, but