4.27. STMicroelectronics STM32MP

The STM32MP is a line of 32-bit ARM SoCs. They reuse peripherals of the STM32 line of microcontrollers and can have a STM32 MCU embedded as co-processor as well.

The boot process of the STM32MP SoC is a two step process. The first stage boot loader (FSBL) is loaded by the ROM code into the built-in SYSRAM and executed. The FSBL sets up the SDRAM, install a secure monitor and then the second stage boot loader (SSBL) is loaded into DRAM.

When building barebox, the resulting barebox-${board}.img file has the STM32 header preprended, so it can be loaded directly as SSBL by the ARM TF-A (https://github.com/ARM-software/arm-trusted-firmware). Each entry point has a header-less image ending in *.pblb as well.

Use of barebox as FSBL is not supported.

4.27.1. Building barebox

With multi-image and device trees, it’s expected to have stm32mp_defconfig as sole defconfig for all STM32MP boards:

make ARCH=arm stm32mp_defconfig

The resulting images will be placed under images/:

barebox-stm32mp157c-dk2.img

4.27.2. Flashing barebox

An appropriate image for the boot media can be generated with following genimage(1) config:

image @STM32MP_BOARD@.img {
    hdimage {
        align = 1M
        gpt = "true"
    }
    partition fsbl1 {
        image = "tf-a-@STM32MP_BOARD@.stm32"
        size = 256K
    }
    partition fsbl2 {
        image = "tf-a-@STM32MP_BOARD@.stm32"
        size = 256K
    }
    partition ssbl {
        image = "barebox-@STM32MP_BOARD@.img"
        size = 1M
    }
    partition barebox-environment {
        image = "/dev/null"
        size = 1M
    }
}

Image can then be flashed on e.g. a SD-Card.

4.27.3. Boot source selection

The STM32MP BootROM samples three boot pins at reset. Usually BOOT1 is pulled down and BOOT0 and BOOT2 are connected to a 2P DIP switch:

      +-------+
BOOT2 | O O-- |
BOOT0 | N O-- |  <---- SDMMC
      +-------+
      +-------+
BOOT2 | O O-- |
BOOT0 | N --O |  <---- QSPI-NOR Flash
      +-------+
      +-------+
BOOT2 | O --O |
BOOT0 | N --O |  <---- DFU on UART and USB OTG
      +-------+

4.27.4. Boot status indicator

The ROM code on the first Cortex-A7 core pulses the PA13 pad. An error LED on this pad can be used to indicate boot status:

  • Boot Failure: LED lights bright
  • UART/USB Boot: LED blinks fast
  • Debug access: LED lights weak