Amlogic SoC Boot Flow
Amlogic SoCs follow a pre-defined boot sequence stored in SoC ROM code. The possible boot sequences of the different SoC families are:
GX* & AXG Family
1 |
2 |
3 |
4 |
5 |
|
---|---|---|---|---|---|
S905 S905D S905L S905W S905X S905Y S912 |
POC=0: SPI NOR |
eMMC |
NAND |
SD |
USB |
S805X A113D A113X |
POC=0: SPI NOR |
eMMC |
NAND |
USB |
POC pin: NAND_CLE
Some boards provide a button to force USB boot by disabling the eMMC clock signal and allowing the eMMC step to be bypassed. Others have removable eMMC modules; removing an eMMC module and SD card will allow boot from USB.
An exception is the Libre Computer AML-S805X-XX (LaFrite) board which has no SD card slot and boots from SPI. Booting a LaFrite board from USB requires either:
Erasing the first sectors of SPI NOR flash
Inserting an HDMI boot plug forcing boot over USB
The VIM1 and initial VIM2 boards provide a test point on the eMMC signals to block the storage from answering, allowing boot to continue with the next boot step.
USB boot uses the first USB interface. On some boards this port is only available on a USB-A type connector and requires a special Type-A to Type-A cable to communicate with the BootROM.
G12* & SM1 Family
POC0 |
POC1 |
POC2 |
1 |
2 |
3 |
4 |
|
---|---|---|---|---|---|---|---|
0 |
0 |
0 |
USB |
SPI-NOR |
NAND/eMMC | SD |
||
0 |
0 |
1 |
USB |
NAND/eMMC |
SD | - |
||
0 |
1 |
0 |
SPI-NOR |
NAND/eMMC |
SD |
USB |
|
0 |
1 |
1 |
SPI-NAND |
NAND/eMMC |
USB |
||
1 |
0 |
0 |
USB |
SPI-NOR |
NAND/eMMC |
SD |
|
1 |
0 |
1 |
USB |
NAND/eMMC |
SD |
||
1 |
1 |
0 |
SPI-NOR |
NAND/eMMC |
SD |
USB |
|
1 |
1 |
1 |
NAND/eMMC |
SD |
USB |
The last option (1/1/1) is the normal default seen on production devices:
POC0 pin: BOOT_4 (0 and all other 1 means SPI NAND boot first)
POC1 pin: BOOT_5 (0 and all other 1 means USB Device boot first
POC2 pin: BOOT_6 (0 and all other 1 means SPI NOR boot first)
Most boards provide a button to force USB BOOT which lowers BOOT_5 to 0. Some boards provide a test point on eMMC or SPI NOR clock signals to block storage from answering and allowing boot to continue from the next boot step.
The Khadas VIM3/3L boards embed a microcontroller which sets POC signals according to its configuration or a specific key press sequence to either boot from SPI NOR or eMMC then SD card, or boot as a USB device.
The Odroid N2/N2+ has a hardware switch to select between SPI NOR or eMMC boot. The Odroid HC4 has a button to disable SPI-NOR allowing boot from SD card.
Boot Modes
SD
The BootROM fetches the first SD card sectors in one sequence then checks the content of the data. It expects to find the FIP binary in sector 1, 512 bytes offset from the start.
eMMC
The BootROM fetches the first sectors of the main partition in one sequence then checks the content of the data. On GXL and newer boards it expects to find the FIP binary in sector 1, 512 bytes offset from the start. If not found it checks the boot0 partition, then the boot1 partition. On GXBB it expects to find the FIP binary at an offset that conflicts with MBR partition tables, but this has been worked around (thus avoiding the need for a partition scheme that relocates the MBR). For a more detailed explanation please see: https://github.com/LibreELEC/amlogic-boot-fip/pull/8
SPI-NOR
The BootROM fetches the first SPI NOR sectors in one sequence then checks the content of the data. It expects to find the FIP binary in sector 1, 512 bytes offset from the start.
NAND & SPI-NAND
These modes are rarely used in open platforms and no details are available.
USB
The BootROM supports a custom USB protocol and sets the USB Gadget interface to use the USB ID 1b8e:c003. The Amlogic update utility uses this protocol. It is also supported in the Amlogic vendor U-Boot sources.
The pyamlboot utility https://github.com/superna9999/pyamlboot is open-source and also implements the USB protocol. It can load U-Boot into memory to start the SoC without the storage being attached, or to recover the device from a failed/incorrect image flash.
HDMI Recovery Dongle
The BootROM also reads 8 bytes at address I2C 0x52 offset 0xf8 (248) on the HDMI DDC bus during startup. The content boot@USB forces USB boot. The content boot@SDC forces SD card boot. The content boot@SPI forces SPI-NOT boot. If an SD card or USB device does not enumerate the BootROM continues with the normal boot sequence.
HDMI boot dongles can be created by connecting a 256bytes EEPROM set to answer on address 0x52, with boot@USB or boot@SDC or boot@SPI programmed at offset 0xf8 (248).
If the SoC is booted with USB Device forced at first step, it will retain the forced boot order on warm reboot. Only cold reboot (removing power) will reset the boot order.