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STM32 ROM Bootloader

The STM32 ROM Bootloader is a jump-to type bootloader, with some extra setup used to integrate with it.

By default, when integrating this bootloader, a "double tap reset to enter the bootloader" feature will be enabled, to help with designs that do not easily expose a BOOT pin.

Kconfig Symbol Enablement​

Three Kconfig symbols need to be enabled for this feature to work, namely RETAINED_MEM, RETENTION, and RETENTION_BOOT_MODE. Typically, this is done by implying the symbols for the board symbol in the Kconfig.<board>, file, e.g.:

config BOARD_TOFU65
    select SOC_RP2040
    imply RETAINED_MEM
    imply RETENTION
    imply RETENTION_BOOT_MODE

By using imply at the board level, users of the board can choose to override the setting and disable the feature if they so choose.

Adjust Existing SRAM Node​

First, we'll adjust the existing SRAM node to shrink it by one byte so Zephyr will not interfere with the retained mem:

/* Reduce SRAM0 usage by 1 byte to account for non-init area */
&sram0 {
    reg = <0x20000000 0x3FFF>;
};

Note:

  • The 0x20000000 address is the address of the RAM for the target. This is nearly always 0x20000000
  • The exact value of 0x3FFF will depend on the total RAM on the target. The value should be the total RAM, minus 1-bytes, in hex

New Memory Region & Nested Retained Mem​

/ {
    sram@20003FFF {
        compatible = "zephyr,memory-region", "mmio-sram";
        reg = <0x20003FFF 0x1>;
        zephyr,memory-region = "RetainedMem";
        status = "okay";

        retainedmem {
            compatible = "zephyr,retained-ram";
            status = "okay";
            #address-cells = <1>;
            #size-cells = <1>;

            retention0: retention@0 {
                compatible = "zephyr,retention";
                status = "okay";
                reg = <0x0 0x1>;
            };
        };
    };
};

Note:

  • The node sram@20003FFF and the corresponding reg property values are obtained by adding the base RAM address (.e.g. 0x20000000) to the shrunk RAM size (e.g. 0x3FFF) to get the new start address for the area of reserved RAM.

Chosen Boot Mode Node​

Finally, we'll set a chosen property to select the created retention node:

/ {
    chosen {
        zephyr,boot-mode = &retention0;
    };
};