diff --git a/src/osal/osal.h b/src/osal/osal.h
index ea655d4f1..e30f9159a 100644
--- a/src/osal/osal.h
+++ b/src/osal/osal.h
@@ -1,4 +1,4 @@
-/* 
+/*
  * The MIT License (MIT)
  *
  * Copyright (c) 2019 Ha Thach (tinyusb.org)
@@ -53,6 +53,8 @@ typedef void (*osal_task_func_t)( void * );
   #include "osal_freertos.h"
 #elif CFG_TUSB_OS == OPT_OS_MYNEWT
   #include "osal_mynewt.h"
+#elif CFG_TUSB_OS == OPT_OS_EMBOS
+  #include "osal_embos.h"
 #else
   #error OS is not supported yet
 #endif
@@ -62,6 +64,10 @@ typedef void (*osal_task_func_t)( void * );
 //--------------------------------------------------------------------+
 static inline void osal_task_delay(uint32_t msec);
 
+//------------- Interrupt -------------//
+static inline void osal_enter_interrupt(void);
+static inline void osal_leave_interrupt(void);
+
 //------------- Semaphore -------------//
 static inline osal_semaphore_t osal_semaphore_create(osal_semaphore_def_t* semdef);
 static inline bool osal_semaphore_post(osal_semaphore_t sem_hdl, bool in_isr);
diff --git a/src/osal/osal_embos.h b/src/osal/osal_embos.h
new file mode 100644
index 000000000..fabc75264
--- /dev/null
+++ b/src/osal/osal_embos.h
@@ -0,0 +1,171 @@
+/* 
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2019 Ha Thach (tinyusb.org)
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ *
+ * This file is part of the TinyUSB stack.
+ */
+
+// Segger embOS porting layer
+// 2019-10-08 Matt Page @ Scannex.com
+
+#ifndef _TUSB_OSAL_EMBOS_H_
+#define _TUSB_OSAL_EMBOS_H_
+
+#include "RTOS.h"
+
+#ifdef __cplusplus
+ extern "C" {
+#endif
+
+//--------------------------------------------------------------------+
+// TASK API
+//--------------------------------------------------------------------+
+static inline void osal_task_delay(uint32_t msec)
+{
+  // TODO: Convert milliseconds to RTOS ticks (most BSP are millisecond based, but not all!)
+  OS_Delay(msec);
+}
+
+//--------------------------------------------------------------------+
+// Interrupt API
+//--------------------------------------------------------------------+
+static inline void osal_enter_interrupt(void)
+{
+  OS_EnterInterrupt();
+}
+
+static inline void osal_leave_interrupt(void)
+{
+  OS_LeaveInterrupt();
+}
+
+//--------------------------------------------------------------------+
+// Binary Semaphore API
+//--------------------------------------------------------------------+
+typedef OS_CSEMA              osal_semaphore_def_t;
+typedef osal_semaphore_def_t* osal_semaphore_t;
+
+static inline osal_semaphore_t osal_semaphore_create(osal_semaphore_def_t* semdef)
+{
+  OS_CreateCSema(semdef, 0);
+  return semdef;
+}
+
+static inline bool osal_semaphore_post(osal_semaphore_t sem_hdl, bool in_isr)
+{
+  (void) in_isr;
+  OS_SignalCSema(sem_hdl);
+  return true;
+}
+
+static inline bool osal_semaphore_wait (osal_semaphore_t sem_hdl, uint32_t msec)
+{
+  (void) msec;
+
+  if (msec == OSAL_TIMEOUT_WAIT_FOREVER) {
+    OS_WaitCSema(sem_hdl);
+    return true;
+  } else {
+    return OS_WaitCSemaTimed(sem_hdl, msec);
+  }
+}
+
+static inline void osal_semaphore_reset(osal_semaphore_t sem_hdl)
+{
+  OS_SetCSemaValue(sem_hdl, 0);
+}
+
+//--------------------------------------------------------------------+
+// MUTEX API (priority inheritance)
+// Within tinyusb, mutex is never used in ISR context
+//--------------------------------------------------------------------+
+typedef OS_RSEMA osal_mutex_def_t;
+typedef OS_RSEMA* osal_mutex_t;
+
+static inline osal_mutex_t osal_mutex_create(osal_mutex_def_t* mdef)
+{
+  OS_CreateRSema(mdef);
+  return mdef;
+}
+
+static inline bool osal_mutex_lock (osal_mutex_t mutex_hdl, uint32_t msec)
+{
+  if (msec == OSAL_TIMEOUT_WAIT_FOREVER) {
+    OS_Use(mutex_hdl);
+    return true;
+  }
+  else {
+    return OS_UseTimed(mutex_hdl, msec);
+  }
+}
+
+static inline bool osal_mutex_unlock(osal_mutex_t mutex_hdl)
+{
+  OS_Unuse(mutex_hdl);
+  return true;
+}
+
+//--------------------------------------------------------------------+
+// QUEUE API
+//--------------------------------------------------------------------+
+
+// role device/host is used by OS NONE for mutex (disable usb isr) only
+#define OSAL_QUEUE_DEF(_role, _name, _depth, _type) \
+  static _type _name##_##buf[_depth];\
+  osal_queue_def_t _name = { .depth = _depth, .item_sz = sizeof(_type), .buf = _name##_##buf };
+
+typedef struct
+{
+  uint16_t depth;
+  uint16_t item_sz;
+  void*    buf;
+
+  OS_MAILBOX mb;
+}osal_queue_def_t;
+
+typedef OS_MAILBOX* osal_queue_t;
+
+static inline osal_queue_t osal_queue_create(osal_queue_def_t* qdef)
+{
+  OS_MAILBOX *mbp = &qdef->mb;
+  OS_CreateMB(mbp, qdef->item_sz, qdef->depth, qdef->buf);
+  return mbp;
+}
+
+static inline bool osal_queue_receive(osal_queue_t const queue_hdl, void* data)
+{
+  OS_GetMail(queue_hdl, data);
+  return true;
+}
+
+static inline bool osal_queue_send(osal_queue_t const queue_hdl, void const * data, bool in_isr)
+{
+  if (in_isr) return (0 == OS_PutMailCond(queue_hdl, data));
+  OS_PutMail(queue_hdl, data);
+  return true;
+}
+
+#ifdef __cplusplus
+ }
+#endif
+
+#endif /* _TUSB_OSAL_NONE_H_ */
diff --git a/src/osal/osal_none.h b/src/osal/osal_none.h
index fa581215d..5dc174871 100644
--- a/src/osal/osal_none.h
+++ b/src/osal/osal_none.h
@@ -43,6 +43,21 @@ static inline void osal_task_delay(uint32_t msec)
 //  while ( ( tusb_hal_millis() - start ) < msec ) {}
 }
 
+//--------------------------------------------------------------------+
+// Interrupt API
+//--------------------------------------------------------------------+
+static inline void osal_enter_interrupt(void)
+{
+  //
+}
+
+static inline void osal_leave_interrupt(void)
+{
+  //
+}
+
+
+
 //--------------------------------------------------------------------+
 // Binary Semaphore API
 //--------------------------------------------------------------------+
diff --git a/src/portable/microchip/samd21/dcd_samd21.c b/src/portable/microchip/samd21/dcd_samd21.c
index 231abc7d0..9848f445c 100644
--- a/src/portable/microchip/samd21/dcd_samd21.c
+++ b/src/portable/microchip/samd21/dcd_samd21.c
@@ -30,6 +30,7 @@
 
 #include "device/dcd.h"
 #include "sam.h"
+#include "osal/osal.h"
 
 /*------------------------------------------------------------------*/
 /* MACRO TYPEDEF CONSTANT ENUM
@@ -37,6 +38,14 @@
 static TU_ATTR_ALIGNED(4) UsbDeviceDescBank sram_registers[8][2];
 static TU_ATTR_ALIGNED(4) uint8_t _setup_packet[8];
 
+
+//---------------------
+// Forward declarations
+//---------------------
+void maybe_transfer_complete(void);
+void USB_IRQHandler(void);
+//=====================
+
 // Setup the control endpoint 0.
 static void bus_reset(void)
 {
@@ -291,11 +300,13 @@ void maybe_transfer_complete(void) {
   }
 }
 
-void USB_Handler(void)
+void USB_IRQHandler(void) // Renamed
 {
-  uint32_t int_status = USB->DEVICE.INTFLAG.reg & USB->DEVICE.INTENSET.reg;
+  uint32_t int_status;
+  osal_enter_interrupt();
+  int_status = USB->DEVICE.INTFLAG.reg & USB->DEVICE.INTENSET.reg;
   USB->DEVICE.INTFLAG.reg = int_status; // clear interrupt
-
+  //printf("USB_IRQ %x\r\n", int_status);
   /*------------- Interrupt Processing -------------*/
   if ( int_status & USB_DEVICE_INTFLAG_SOF )
   {
@@ -338,6 +349,7 @@ void USB_Handler(void)
 
   // Handle complete transfer
   maybe_transfer_complete();
+  osal_leave_interrupt();
 }
 
 #endif
diff --git a/src/tusb_option.h b/src/tusb_option.h
index 6178b2cf9..d4ef1d75b 100644
--- a/src/tusb_option.h
+++ b/src/tusb_option.h
@@ -78,6 +78,7 @@
 #define OPT_OS_NONE       1 ///< No RTOS
 #define OPT_OS_FREERTOS   2 ///< FreeRTOS
 #define OPT_OS_MYNEWT     3 ///< Mynewt OS
+#define OPT_OS_EMBOS      4 ///< Segger embOS
 /** @} */