The library bootloaders will now correctly start the user application after a watchdog-based application start, even if the /HWB line is held low externally during the reset phase.

Bootloaders/CDC/BootloaderCDC.c

@@ -56,6 +56,27 @@ static uint32_t CurrAddress;
  */
 static bool RunBootloader = true;
 
+/** Magic lock for forced application start. If the HWBE fuse is programmed and BOOTRST is unprogrammed, the bootloader
+ *  will start if the /HWB line of the AVR is held low and the system is reset. However, if the /HWB line is still held
+ *  low when the application attempts to start via a watchdog reset, the bootloader will re-start. If set to the value
+ *  \ref MAGIC_BOOT_KEY the special init function \ref Application_Jump_Check() will force the application to start.
+ */
+uint32_t MagicBootKey ATTR_NO_INIT;
+
+
+/** Special startup routine to check if the bootloader was started via a watchdog reset, and if the magic application
+ *  start key has been loaded into \ref MagicBootKey. If the bootloader started via the watchdog and the key is valid,
+ *  this will force the user application to start via a software jump.
+ */
+void Application_Jump_Check(void)
+{
+	// If the reset source was the bootloader and the key is correct, clear it and jump to the application
+	if ((MCUSR & (1 << WDRF)) && (MagicBootKey == MAGIC_BOOT_KEY))
+	{
+		MagicBootKey = 0;
+		((void (*)(void))0x0000)();
+	}
+}
 
 /** Main program entry point. This routine configures the hardware required by the bootloader, then continuously
  *  runs the bootloader processing routine until instructed to soft-exit, or hard-reset via the watchdog to start
@@ -80,6 +101,9 @@ int main(void)
 
 	/* Disconnect from the host - USB interface will be reset later along with the AVR */
 	USB_Detach();
+	
+	/* Unlock the forced application start mode of the bootloader if it is restarted */
+	MagicBootKey = MAGIC_BOOT_KEY;
 
 	/* Enable the watchdog and force a timeout to reset the AVR */
 	wdt_enable(WDTO_250MS);

Bootloaders/CDC/BootloaderCDC.h

@@ -67,6 +67,9 @@
 		/** Eight character bootloader firmware identifier reported to the host when requested */
 		#define SOFTWARE_IDENTIFIER          "LUFACDC"
 
+		/** Magic bootloader key to unlock forced application start mode. */
+		#define MAGIC_BOOT_KEY            0xDC42CACA
+		
 	/* Type Defines: */
 		/** Type define for a non-returning pointer to the start of the loaded application in flash memory. */
 		typedef void (*AppPtr_t)(void) ATTR_NO_RETURN;
@@ -75,6 +78,8 @@
 		static void CDC_Task(void);
 		static void SetupHardware(void);
 
+		void Application_Jump_Check(void) ATTR_INIT_SECTION(3);
+
 		void EVENT_USB_Device_ConfigurationChanged(void);
 
 		#if defined(INCLUDE_FROM_BOOTLOADERCDC_C) || defined(__DOXYGEN__)

Bootloaders/DFU/BootloaderDFU.c

@@ -92,6 +92,27 @@ static uint16_t StartAddr = 0x0000;
  */
 static uint16_t EndAddr = 0x0000;
 
+/** Magic lock for forced application start. If the HWBE fuse is programmed and BOOTRST is unprogrammed, the bootloader
+ *  will start if the /HWB line of the AVR is held low and the system is reset. However, if the /HWB line is still held
+ *  low when the application attempts to start via a watchdog reset, the bootloader will re-start. If set to the value
+ *  \ref MAGIC_BOOT_KEY the special init function \ref Application_Jump_Check() will force the application to start.
+ */
+uint32_t MagicBootKey ATTR_NO_INIT;
+
+
+/** Special startup routine to check if the bootloader was started via a watchdog reset, and if the magic application
+ *  start key has been loaded into \ref MagicBootKey. If the bootloader started via the watchdog and the key is valid,
+ *  this will force the user application to start via a software jump.
+ */
+void Application_Jump_Check(void)
+{
+	// If the reset source was the bootloader and the key is correct, clear it and jump to the application
+	if ((MCUSR & (1 << WDRF)) && (MagicBootKey == MAGIC_BOOT_KEY))
+	{
+		MagicBootKey = 0;
+		AppStartPtr();
+	}
+}
 
 /** Main program entry point. This routine configures the hardware required by the bootloader, then continuously
  *  runs the bootloader processing routine until instructed to soft-exit, or hard-reset via the watchdog to start
@@ -695,6 +716,9 @@ static void ProcessWriteCommand(void)
 		{
 			if (SentCommand.Data[1] == 0x00)                                   // Start via watchdog
 			{
+				/* Unlock the forced application start mode of the bootloader if it is restarted */
+				MagicBootKey = MAGIC_BOOT_KEY;
+
 				/* Start the watchdog to reset the AVR once the communications are finalized */
 				wdt_enable(WDTO_250MS);
 			}

Bootloaders/DFU/BootloaderDFU.h

@@ -66,6 +66,9 @@
 
 		/** Minor bootloader version number. */
 		#define BOOTLOADER_VERSION_REV   0
+		
+		/** Magic bootloader key to unlock forced application start mode. */
+		#define MAGIC_BOOT_KEY            0xDC42CACA
 
 		/** Complete bootloader version number expressed as a packed byte, constructed from the
 		 *  two individual bootloader version macros.
@@ -206,6 +209,8 @@
 			static void ProcessWriteCommand(void);
 			static void ProcessReadCommand(void);
 		#endif
+		
+		void Application_Jump_Check(void) ATTR_INIT_SECTION(3);
 
 #endif
 

Bootloaders/HID/BootloaderHID.c

@@ -41,6 +41,28 @@
  */
 static bool RunBootloader = true;
 
+/** Magic lock for forced application start. If the HWBE fuse is programmed and BOOTRST is unprogrammed, the bootloader
+ *  will start if the /HWB line of the AVR is held low and the system is reset. However, if the /HWB line is still held
+ *  low when the application attempts to start via a watchdog reset, the bootloader will re-start. If set to the value
+ *  \ref MAGIC_BOOT_KEY the special init function \ref Application_Jump_Check() will force the application to start.
+ */
+uint32_t MagicBootKey ATTR_NO_INIT;
+
+
+/** Special startup routine to check if the bootloader was started via a watchdog reset, and if the magic application
+ *  start key has been loaded into \ref MagicBootKey. If the bootloader started via the watchdog and the key is valid,
+ *  this will force the user application to start via a software jump.
+ */
+void Application_Jump_Check(void)
+{
+	// If the reset source was the bootloader and the key is correct, clear it and jump to the application
+	if ((MCUSR & (1 << WDRF)) && (MagicBootKey == MAGIC_BOOT_KEY))
+	{
+		MagicBootKey = 0;
+		((void (*)(void))0x0000)();
+	}
+}
+
 /** Main program entry point. This routine configures the hardware required by the bootloader, then continuously
  *  runs the bootloader processing routine until instructed to soft-exit.
  */
@@ -58,6 +80,9 @@ int main(void)
 	/* Disconnect from the host - USB interface will be reset later along with the AVR */
 	USB_Detach();
 
+	/* Unlock the forced application start mode of the bootloader if it is restarted */
+	MagicBootKey = MAGIC_BOOT_KEY;
+
 	/* Enable the watchdog and force a timeout to reset the AVR */
 	wdt_enable(WDTO_250MS);
 

Bootloaders/HID/BootloaderHID.h

@@ -52,9 +52,14 @@
 		/** Bootloader special address to start the user application */
 		#define COMMAND_STARTAPPLICATION   0xFFFF
 
+		/** Magic bootloader key to unlock forced application start mode. */
+		#define MAGIC_BOOT_KEY            0xDC42CACA
+		
 	/* Function Prototypes: */
 		static void SetupHardware(void);
 
+		void Application_Jump_Check(void) ATTR_INIT_SECTION(3);
+		
 		void EVENT_USB_Device_ConfigurationChanged(void);
 		void EVENT_USB_Device_UnhandledControlRequest(void);
 

LUFA/DoxygenPages/ChangeLog.txt

@@ -35,11 +35,13 @@
   *   - Pipes are now configured via instances of a new struct USB_Pipe_Table_t in all host mode class drivers, rather than a list of pipe parameters
   *   - Added support for various assert and debugging macros for the UC3 devices
   *   - Changed MIDI event structure MIDI_EventPacket_t to use a single field for the combined virtual cable index and command ID, to prevent bitfield packing issues
-  *     on some architectures (thanks to Darren Gibbs).
+  *     on some architectures (thanks to Darren Gibbs)
   *  - Library Applications:
   *   - Raised the guard bits in the AVRISP-MKII clone project when in PDI and TPI to 32, to prevent communication errors on low quality connections to a target
   *   - Added additional bootloader API data to expose the bootloader start address and class to the DFU and CDC class bootloaders
   *   - Reverted AVRISP-MKII clone project watchdog based command timeout patch in favour of a hardware timer, to allow for use in devices with WDTRST fuse programmed
+  *   - The library bootloaders will now correctly start the user application after a watchdog-based application start, even if the /HWB line is held low externally
+  *     during the reset phase
   *
   *  <b>Fixed:</b>
   *  - Core: