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It is important to note that the STM32H750 device has only 128KB of internal (on-chip) Flash. This implies that the initial firmware binary must be small enough to fit into this amount of Flash. U-Boot supports this, by implementing the concept of the "U-Boot SPL". When the U-Boot SPL build-time option (CONFIG_SPL_BUILD) is enabled in the U-Boot configuration file (include/configs/stm32h7-som.h), the U-Boot build procedure will build two separately loadable firmware images:
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The UART based serial console is a standard feature of U-Boot. On the STM32H750 System-on-Module, USART1 is configured as the U-Boot serial console (refer to include/configs/stm32h7-som.h).
The serial console is available both in the U-Boot SPL and main U-Boot. It is important to note that in order to make sure that the USART1 related definitions are not truncated from the U-Boot SPL run-time configuration file (arch/arm/dts/stm32h7-som.dts), special constructs must be included in the .dts file, for the USART1 node and associated STM32 pins, as follows:
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The U-Boot reads the value of the PE2 GPIO input and configures the SDRAM interface appropriately, setting it up for one of the two SDRAM assembly options above. Namely, the sdram init function from board/emcraft/stm32h7-som/stm32h7-som.c calls the detect_sdram_size function from this module to get the value of the PE2 GPIO input. In case of PE2 GPIO input active, the dts_fixup function residing in this module is also called. This function patches the u-boot device tree to provide the correct values for the U-Boot memory controller driver.
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tty - is a serial device name as seen on the Linux host, e.g.
/dev/ttyUSB0, where the debug console of the STM32H750 is connected to (mandatory);uboot_img - is the path to the main U-Boot image, i.e.
u-boot.img(mandatory);rtos_img - is the path to the RTOS image, i.e.
freertos_stm32h750.img(optional);splash_img - is the path to the splash screen image (optional).
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