This is a tiny bootloader (under 4KB) for STM32F103 (although it probably works for similar devices). It enables user to flash devices over USB with any arbitrary payloads. It features some minimal payload checking to ensure use apps are valid before booting them.

• Small size, ideally under 4KB to fit on the first four pages.
• RDP protection configurable at build time.
• Reboot into DFU mode support (by writing tag to RAM + reset).
• Watchdog support for failsafe.
• Total wipe on DFU downloads (avoid partial FW updates).
• Optional upload enable (to prevent firmware/data reads).
• Firmware checksum checking.

One can reboot into bootloader (in DFU mode) by simply writing the magic 0xDEADBEEFCC00FFEE value to the last 8 bytes of RAM and triggering a full system reset. This will make the bootloader start DFU mode instead of loading the (valid) payload present in flash.

Bootloader might enable RDP (Readout protection) that will prevent debugger over SWIO from reading data. This protection can be removed but will cause all user flash (except the DFU bootloader) to be deleted, that's cause the first 4KB are always write protected. It can also disable SWIO GPIOs to prevent any debuggers from attaching to the device once booted. The booloader also features some DFU proectections. It is possible to disable firmware read by disabling UPLOAD commands. In order to prevent data read it is possible to prevent partial writes, since what could allow a small firmware being uploaded to extract data from flash. With this protection enabled the bootloader will wipe all the blocks as soon as an erase/write command is issued.

The bootloader can be configured to detect a GPIO condition on boot and abort boot to go into DFU mode. The pin will be configured as an internal pulldown and the user will need to pull it up to force DFU mode, which will be read right after reset (there's some small delay to ensure the pin is read correctly).

The firmware can optionally enable the Internal Watchdog on a configurable period of 1 to 26 seconds. If the user app does not reset the watchdog before the period is due it will reset the system and enter DFU mode.

The use firmware should be build and linked at an offset of 0x1000 (4KB) so it can safely boot as a payload. The bootloader will check some stuff before declaring the payload valid:

• Stack points to somewhere in the RAM range (0x20000000).
• The firmware contains its size at offset 0x20 (as a LE uint32).
• The firmware 32bit XOR checksum is zero (can use offset 0x1C for that).

If these conditions are met, provided no other triggers to boot into DFU are present, the bootloader will point VTOR to the user app and boot it.

• ENABLE_DFU_UPLOAD: Enables DFU upload commands, this is, enables reading flash memory (only within the user app boundaries) via DFU.
• ENABLE_SAFEWRITE: Ensures the user flash is completely erased before any DFU write/erase command is executed, to ensure no payloads are written that could lead to user data exfiltration.
• ENABLE_CHECKSUM: Forces the user app image to have a valid checksum to boot it, on failure it will fallback to DFU mode.
• ENABLE_WRITEPROT: Protects the first 4KB of flash against writes. Essentially prevents any user app from overwriting the bootloader area.
• ENABLE_PROTECTIONS: Disables JTAG at startup before jumping to user code and also ensures RDP protection is enabled before booting. It will update option bytes if that is not met and force a reset (should only happen the first time, after that RDP is enabled and can only be disabled via JTAG). This also protects the bootloader (first 4KB) like ENABLE_WRITEPROT does, making these two options incompatible.
• ENABLE_GPIO_DFU_BOOT: Enables DFU mode on pulling up a certain GPIO. You need to define GPIO_DFU_BOOT_PORT and GPIO_DFU_BOOT_PIN to either GPIOA, GPIOB, .. GPIOE and 0 .. 15 to indicate which port to enable and what pin to read from.
• ENABLE_PINRST_DFU_BOOT: Enables DFU mode when a reset from the NRST pin occurs.

By default all flags are set except for DFU upload, so it's most secure.