Entering the Bootloader via Software

A common requirement of many applications is the ability to jump to the programmed bootloader of a chip on demand, via the code's firmware (i.e. not as a result of any physical user interaction with the hardware). This might be required because the device does not have any physical user input, or simply just to streamline the device upgrade process on the host PC.

The following C code snippets may be used to enter the bootloader upon request by the user application. By using the watchdog to physically reset the controller, it is ensured that all system hardware is completely reset to their defaults before the bootloader is run. This is important; since bootloaders are written to occupy a very limited space, they usually make assumptions about the register states based on the default values after a hard-reset of the chip.

AVR8 Architecture

The following software bootloader jump code is written for the AVR8 architecture.

  #include <avr/wdt.h>
  #include <avr/io.h>
  #include <util/delay.h>

  #include <LUFA/Common/Common.h>
  #include <LUFA/Drivers/USB/USB.h>

  uint32_t Boot_Key ATTR_NO_INIT;

  #define MAGIC_BOOT_KEY            0xDC42ACCA
  #define BOOTLOADER_START_ADDRESS  (FLASH_SIZE_BYTES - BOOTLOADER_SEC_SIZE_BYTES)

  void Bootloader_Jump_Check(void) ATTR_INIT_SECTION(3);
  void Bootloader_Jump_Check(void)
  {
      // If the reset source was the bootloader and the key is correct, clear it and jump to the bootloader
      if ((MCUSR & (1 << WDRF)) && (Boot_Key == MAGIC_BOOT_KEY))
      {
          Boot_Key = 0;
          ((void (*)(void))BOOTLOADER_START_ADDRESS)();
      }
  }

  void Jump_To_Bootloader(void)
  {
      // If USB is used, detach from the bus and reset it
      USB_Disable();

      // Disable all interrupts
      cli();

      // Wait two seconds for the USB detachment to register on the host
      Delay_MS(2000);

      // Set the bootloader key to the magic value and force a reset
      Boot_Key = MAGIC_BOOT_KEY;
      wdt_enable(WDTO_250MS);
      for (;;);
  }

Note that the bootloader magic key can be any arbitrary value. The FLASH_SIZE_BYTES and BOOTLOADER_SEC_SIZE_BYTES tokens should be replaced with the total flash size of the AVR in bytes, and the allocated size of the bootloader section for the target AVR.