Added "double-tap" feature to bootloader

Bootloader is activated by quickly double tapping to the reset button

bootloaders/zero/main.c

@@ -97,6 +97,32 @@ static void check_start_application(void)
         led_port->DIRSET.reg = (1<<30);
         led_port->OUTCLR.reg = (1<<30);
   
+#if defined(BOOT_DOUBLE_TAP_ADDRESS)
+	if (PM->RCAUSE.bit.POR)
+	{
+		/* On power-on initialize double-tap */
+		BOOT_DOUBLE_TAP_DATA = 0;
+	}
+	else
+	{
+		if (BOOT_DOUBLE_TAP_DATA == 0) {
+			/* First tap */
+			BOOT_DOUBLE_TAP_DATA = 1;
+
+			for (uint32_t i=0; i<50000; i++) /* 200ms */
+				/* force compiler to not optimize this... */
+				__asm__ __volatile__("");
+
+			/* Timeout happened, continue boot... */
+			BOOT_DOUBLE_TAP_DATA = 0;
+		} else {
+			/* Second tap, stay in bootloader */
+			BOOT_DOUBLE_TAP_DATA = 0;
+			return;
+		}
+	}
+#endif
+
 	uint32_t app_start_address;
 
 	/* Load the Reset Handler address of the application */

bootloaders/zero/main.h

@@ -34,6 +34,16 @@
 #define CPU_FREQUENCY                     8000000
 
 #define APP_START_ADDRESS                 0x00002000
+
+/*
+ * If BOOT_DOUBLE_TAP_ADDRESS is defined the bootloader is started by
+ * quickly tapping two times on the reset button.
+ * BOOT_DOUBLE_TAP_ADDRESS must point to a free SRAM cell that must not
+ * be touched from the loaded application.
+ */
+#define BOOT_DOUBLE_TAP_ADDRESS           0x20007FFC
+#define BOOT_DOUBLE_TAP_DATA              (*((volatile uint32_t *) BOOT_DOUBLE_TAP_ADDRESS))
+
 #define BOOT_LOAD_PIN                     PIN_PA21 //Pin 7
 //#define BOOT_LOAD_PIN                     PIN_PA15 //Pin 5
 #define BOOT_PIN_MASK                     (1U << (BOOT_LOAD_PIN & 0x1f))

bootloaders/zero/samd21j18a_flash.ld

@@ -50,7 +50,7 @@ SEARCH_DIR(.)
 MEMORY
 {
   rom (rx)  : ORIGIN = 0x00000000, LENGTH = 0x00040000
-  ram (rwx) : ORIGIN = 0x20000000, LENGTH = 0x00008000
+  ram (rwx) : ORIGIN = 0x20000000, LENGTH = 0x00008000-0x0004 /* 4 bytes used by bootloader to keep data between resets */
 }
 
 /* The stack size used by the application. NOTE: you need to adjust according to your application. */

variants/arduino_zero/linker_scripts/gcc/flash_with_bootloader.ld

@@ -25,8 +25,8 @@
  */
 MEMORY
 {
-  FLASH (rx) : ORIGIN = 0x00000000+0x2000, LENGTH = 0x00040000-0x2000
-  RAM (rwx) : ORIGIN = 0x20000000, LENGTH = 0x00008000
+  FLASH (rx) : ORIGIN = 0x00000000+0x2000, LENGTH = 0x00040000-0x2000 /* First 8KB used by bootloader */
+  RAM (rwx) : ORIGIN = 0x20000000, LENGTH = 0x00008000-0x0004         /* Last 4B used by bootloader   */
 }
 
 /* Linker script to place sections and symbol values. Should be used together