RP2040/RP2350 Bootloader

The RP2040/RP2350 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 BOOTSEL 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.

Simple Include

A simple shared file can be included in your board's devicetree to set up the necessary retained memory and retention nodes. Near the top of your file, add an include for arm/raspberrypi/rp2040-boot-mode-retention.dtsi, e.g.:

/dts-v1/;

#include <raspberrypi/rpi_pico/rp2040.dtsi>
#include <arm/raspberrypi/rp2040-boot-mode-retention.dtsi>

Manual Changes

The include mentioned above can be performed manually, if you so choose.

First, we'll shrink the SRAM node, from the front, to create a 4-byte area that won't have anything placed in it by the normal Zephyr linking process:

&sram0 {
    reg = <0x20000004 ((DT_SIZE_K(264)) - 4)>;
};

Next, we'll define a new 4-byte RAM region for that reserved space, and within that region, set up a Zephyr retained RAM node. Within that node, we'll create the actual retention value that is used:

/ {
    sram@20000000 {
        compatible = "zephyr,memory-region", "mmio-sram";
        reg = <0x20000000 0x4>;
        zephyr,memory-region = "RetainedMem";
        status = "okay";

        retainedmem {
            compatible = "zephyr,retained-ram";
            status = "okay";
            #address-cells = <1>;
            #size-cells = <1>;
            boot_mode: retention@0 {
                compatible = "zephyr,retention";
                status = "okay";
                reg = <0x0 0x1>;
            };
        };
    };
};

Finally, we'll assign that new retention value node to the zephyr,boot-mode chosen property:

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