RP2040: add bootloader via double tap support

Author: facchinm

variants/NANO_RP2040_CONNECT/double_tap_usb_boot.c

@@ -0,0 +1,37 @@
+#include "pico.h"
+#include "pico/time.h"
+#include "pico/bootrom.h"
+
+// Allow user override of the LED mask
+#ifndef USB_BOOT_LED_ACTIVITY_MASK
+#define USB_BOOT_LED_ACTIVITY_MASK 1
+#endif
+
+// Doesn't make any sense for a RAM only binary
+#if !PICO_NO_FLASH
+
+static const uint32_t magic_token[] = {
+        0xf01681de, 0xbd729b29, 0xd359be7a,
+};
+
+static uint32_t __uninitialized_ram(magic_location)[count_of(magic_token)];
+
+// run at initialization time
+static void __attribute__((constructor)) boot_double_tap_check() {
+    for (uint i = 0; i < count_of(magic_token); i++) {
+        if (magic_location[i] != magic_token[i]) {
+            // Arm for 100 ms then disarm and continue booting
+            for (i = 0; i < count_of(magic_token); i++) {
+                magic_location[i] = magic_token[i];
+            }
+            busy_wait_us(100000);
+            magic_location[0] = 0;
+            return;
+        }
+    }
+
+    magic_location[0] = 0;
+    reset_usb_boot(USB_BOOT_LED_ACTIVITY_MASK, 0);
+}
+
+#endif