This folder contains a simple bootloader called "riotboot". A header with metadata of length RIOTBOOT_HDR_LEN precedes the RIOT firmware. The header contains "RIOT" as a magic number to recognise a RIOT firmware image, a checksum, and the version of the RIOT firmware APP_VER. This bootloader verifies the checksum of the header which is located at an offset (ROM_OFFSET) with respect to the ROM_START_ADDR defined by the CPU, just after the space allocated for riotboot.

In case of multiple firmware slots, the bootloader iterates through valid headers and boots the newest image.

riotboot consists of:

• This application which serves as minimal bootloader,
• the module "riotboot_hdr" used to recognise RIOT firmware which riotboot can boot,
• the module "riotboot_slot" used to manage the partitions (slots) with a RIOT header attached to them,
• a tool in dist/tools/riotboot_gen_hdr for header generation,
• several make targets to glue everything together.

riotboot expects the flash to be formatted in slots: at the CPU_FLASH_BASE address resides the bootloader, which is followed by a slot 0 containing a RIOT firmware image. If present, a second firmware image (in slot 1) starts just afterwards.

The bootloader and a RIOT firmware in slot 0 are depicted below:

|------------------------------- FLASH -------------------------------------|
|----- RIOTBOOT_LEN ----|----------- RIOTBOOT_SLOT_SIZE (slot 0) -----------|
                        |----- RIOTBOOT_HDR_LEN ------|
 ---------------------------------------------------------------------------
|        riotboot       | riotboot_hdr_t + filler (0) |   RIOT firmware     |
 ---------------------------------------------------------------------------

Please note that RIOTBOOT_HDR_LEN depends on the architecture of the MCU, since it needs to be aligned to 256B. This is fixed regardless of sizeof(riotboot_hdr_t)

Also note that, if no slot is available with a valid checksum, no image will be booted and the bootloader will enter while(1); endless loop.

A board capable to use riotboot must meet the following requirements:

• Embed a Cortex-M0+/3/4/7 processor
• Provide the variables ROM_START_ADDR and ROM_LEN
• Use cpu/cortexm_common/ldscripts/cortexm.ld ld script
• Pass the cortexm_common_ldscript test in tests/
• Being able to execute startup code at least twice (board_init())
• Declare FEATURES_PROVIDED += riotboot to pull the rigth dependencies
• Being able to flash binary files, if integration with the build system is required for flashing

The above requirements are usually met if the board succeeds to execute the riotboot test in tests/.

Just compile your application using the target riotboot. The header is generated automatically according to your APP_VER, which can be optionally set (0 by default) in your makefile.

The image can be flashed using riotboot/flash which also flashes the bootloader. Below a concrete example:

BOARD=samr21-xpro FEATURES_REQUIRED+=riotboot APP_VER=$(date +%s) make -C examples/hello-world flash-combined-slot0

The above compiles a hello world binary and a bootloader, then flashes the combined binary comprising of: bootloader + slot 0 header + slot 0 image. If booted, the device will execute the Hello-World image.

A comprehensive test is available at tests/riotboot (also see below).

When several slots are available, the bootloader iterates through valid headers and boots the newest image (which has the greater VERSION)

Dedicated make targets are available to build and flash several slots:

• riotboot/slot1: Builds a firmware in ELF and binary format with an offset at the end of slot 0;
• riotboot/flash-slot1: builds and flash a firmware for slot 1;
• riotboot/flash-extended-slot0 builds + flashes slot 0 and erases (zeroes) the metadata of slot 1 (invalidating it);
• riotboot builds both slot 0 and 1.

In particular, if one wants to be sure to boot a particular image, using the target riotboot/flash-extended-slot0 is the way to go (resulting in only slot 0 being valid, thus being booted).

The following sequence of commands tests building, flashing and booting slot 0, then slot 1. tests/riotboot prints out the current booted slot in the shell.

To test building, flashing and booting the first slot:

BOARD=samr21-xpro APP_VER=$(date +%s) make -C tests/riotboot/ riotboot/flash-combined-slot0 test

For the second slot:

BOARD=samr21-xpro APP_VER=$(date +%s) make -C tests/riotboot/ riotboot/flash-slot1 test