diff --git a/boards/samd21g18a.json b/boards/samd21g18a.json
index b1f09c8..01541d2 100644
--- a/boards/samd21g18a.json
+++ b/boards/samd21g18a.json
@@ -2,7 +2,7 @@
   "build": {
     "core": "samd",
     "cpu": "cortex-m0plus",
-    "extra_flags": "-DARDUINO_SAMD_MKR1000 -DARDUINO_ARCH_SAMD -D__SAMD21G18A__",
+    "extra_flags": "-DARDUINO_ARCH_SAMD -D__SAMD21G18A__",
     "f_cpu": "48000000L",
     "hwids": [
       [
diff --git a/builder/frameworks/arduino.py b/builder/frameworks/arduino.py
index e56eb2d..415a95c 100644
--- a/builder/frameworks/arduino.py
+++ b/builder/frameworks/arduino.py
@@ -94,14 +94,18 @@
         "-Wl,--warn-section-align"
     ],
 
-    LIBPATH=[
-        join(FRAMEWORK_DIR, "variants",
-             board.get("build.variant"), "linker_scripts", "gcc")
-    ],
-
     LIBS=["m"]
 )
 
+variants_dir = join(
+    "$PROJECT_DIR", board.get("build.variants_dir")) if board.get(
+        "build.variants_dir", "") else join(FRAMEWORK_DIR, "variants")
+
+env.Append(
+    LIBPATH=[
+        join(variants_dir, board.get("build.variant"), "linker_scripts", "gcc")
+    ])
+
 if "BOARD" in env:
     env.Append(
         CCFLAGS=[
@@ -129,8 +133,7 @@
 
         LIBPATH=[
             join(SYSTEM_DIR, "CMSIS", "CMSIS", "Lib", "GCC"),
-            join(FRAMEWORK_DIR, "variants",
-                 board.get("build.variant"))
+            join(variants_dir, board.get("build.variant"))
         ]
     )
 
@@ -153,7 +156,6 @@
 
     if BUILD_CORE == "adafruit":
         env.Append(
-            CPPDEFINES=["USE_TINYUSB"],
             CPPPATH=[
                 join(FRAMEWORK_DIR, "cores", BUILD_CORE,
                      "Adafruit_TinyUSB_Core"),
@@ -171,7 +173,7 @@
         ],
 
         LIBPATH=[
-            join(FRAMEWORK_DIR, "variants", board.get("build.variant"))
+            join(variants_dir, board.get("build.variant"))
         ],
 
         LINKFLAGS=[
@@ -208,11 +210,11 @@
 
 if "build.variant" in env.BoardConfig():
     env.Append(
-        CPPPATH=[join(FRAMEWORK_DIR, "variants", board.get("build.variant"))]
+        CPPPATH=[join(variants_dir, board.get("build.variant"))]
     )
     libs.append(env.BuildLibrary(
         join("$BUILD_DIR", "FrameworkArduinoVariant"),
-        join(FRAMEWORK_DIR, "variants", board.get("build.variant"))
+        join(variants_dir, board.get("build.variant"))
     ))
 
 libs.append(env.BuildLibrary(
diff --git a/builder/main.py b/builder/main.py
index 45aff7b..d42b3bf 100644
--- a/builder/main.py
+++ b/builder/main.py
@@ -207,10 +207,13 @@ def _jlink_cmd_script(env, source):
         ],
         UPLOADCMD="$UPLOADER $UPLOADERFLAGS $SOURCES"
     )
-    if board.get("build.core") == "adafruit":
+    if board.get("build.core") == "adafruit" and board.get(
+            "build.mcu").startswith("samd51"):
+        # special flags for the latest bossac tool
         env.Append(
-            UPLOADERFLAGS=["-U", "--offset",
-                           board.get("upload.offset_address")])
+            UPLOADERFLAGS=[
+            "-U", "--offset", board.get("upload.offset_address")])
+
     else:
         env.Append(UPLOADERFLAGS=[
             "--erase",
diff --git a/platform.json b/platform.json
index dccdcdf..f084f08 100644
--- a/platform.json
+++ b/platform.json
@@ -12,7 +12,7 @@
     "type": "git",
     "url": "https://github.com/platformio/platform-atmelsam.git"
   },
-  "version": "3.8.0",
+  "version": "3.8.1",
   "packageRepositories": [
     "https://dl.bintray.com/platformio/dl-packages/manifest.json",
     "http://dl.platformio.org/packages/manifest.json",
diff --git a/platform.py b/platform.py
index 70fdfe3..8f21b88 100644
--- a/platform.py
+++ b/platform.py
@@ -49,7 +49,8 @@ def configure_default_packages(self, variables, targets):
             self.packages["toolchain-gccarmnoneeabi"][
                 'version'] = ">=1.40803.0,<1.40805.0"
         if (board.get("build.core", "") == "adafruit"
-                and "tool-bossac" in self.packages):
+                and "tool-bossac" in self.packages
+                and board.get("build.mcu", "").startswith("samd51")):
             self.packages["tool-bossac"]['version'] = "~1.10900.0"
 
         for name in disabled_pkgs: