🛠 Flasher l'eMMC d'un CM3 par USB avec rpiboot

Objectives

Write a system image directly to the 4 GB soldered eMMC of a CM3, without an SD card. The BCM2837 has no bootloader EEPROM: at power-on, it looks for bootcode.bin on the eMMC, and if that fails, it waits for boot code over USB. By connecting EMMC_DISABLE_N to ground, you physically disconnect the eMMC (SDCMD pin) and force this USB device mode, where the module enumerates as a USB mass storage device controlled by rpiboot (official raspberrypi/usbboot repository).

Hardware

• CM3 (with eMMC; the CM3 Lite does not have one and is programmed via SD) on a CMIO3
• Micro-USB cable between the CMIO3's USB slave port and a Linux PC (or another Pi)
• EMMC_DISABLE_N jumper placed on the CMIO3 (eMMC-disable jumper)
• 32-bit Raspberry Pi OS image (armhf, Pi 3 branch)

Complete Code

# Sur le PC hôte (Linux) :
git clone https://github.com/raspberrypi/usbboot
cd usbboot && make && sudo ./rpiboot     # le CM3 apparaît comme disque USB

lsblk                                    # repérer le périphérique, p. ex. /dev/sdb
lsblk /dev/sdb                           # vérifier la taille (~4 Go pour un CM3)

sudo dd bs=4M conv=fsync status=progress \
  if=2024-xx-xx-raspios-bookworm-armhf.img of=/dev/sdb
sync

Remove the EMMC_DISABLE_N jumper, power on again: the module boots from its eMMC. The firmware reactivates the eMMC via EMMC_EN_N_1V8 during USB programming (datasheet §8).

Expected Verification

After sudo ./rpiboot, lsblk shows a new disk of approximately 4 GB; after dd and rebooting without the jumper, the Raspberry Pi OS desktop appears on HDMI and lsblk on the CM3 shows the root filesystem on /dev/mmcblk0.

Common Errors

• Writing to the wrong disk (/dev/sda = the PC's disk!): always check the size with lsblk before dd.
• Forgetting to remove the EMMC_DISABLE_N jumper after flashing: the module requests USB boot again instead of booting from eMMC.
• Expecting a BOOT_ORDER like on CM4/Pi 4: the CM3 has no EEPROM, the boot order is hardcoded in silicon — USB forcing is purely hardware-based.
• Switching to USB boot costs ~2 s of eMMC timeout after power-on: this is normal, wait a few seconds before plugging in the cable.
• Trying to flash a CM3 Lite this way: without eMMC, there is nothing to flash — write an SD card instead.