diff --git a/src/class/cdc/cdc_device.c b/src/class/cdc/cdc_device.c
index b4ebfc92e..cd8904042 100644
--- a/src/class/cdc/cdc_device.c
+++ b/src/class/cdc/cdc_device.c
@@ -77,8 +77,8 @@ static void _prep_out_transaction (uint8_t itf)
 {
   cdcd_interface_t* p_cdc = &_cdcd_itf[itf];
 
-  // skip if previous transfer not complete
-  if ( usbd_edpt_busy(TUD_OPT_RHPORT, p_cdc->ep_out) ) return;
+  // claim endpoint
+  TU_VERIFY( usbd_edpt_claim(TUD_OPT_RHPORT, p_cdc->ep_out), );
 
   // Prepare for incoming data but only allow what we can store in the ring buffer.
   uint16_t max_read = tu_fifo_remaining(&p_cdc->rx_ff);
@@ -161,17 +161,29 @@ uint32_t tud_cdc_n_write_flush (uint8_t itf)
 {
   cdcd_interface_t* p_cdc = &_cdcd_itf[itf];
 
-  // skip if previous transfer not complete yet
-  TU_VERIFY( !usbd_edpt_busy(TUD_OPT_RHPORT, p_cdc->ep_in), 0 );
+  // No data to send
+  if ( !tu_fifo_count(&p_cdc->tx_ff) ) return 0;
 
-  uint16_t count = tu_fifo_read_n(&p_cdc->tx_ff, p_cdc->epin_buf, sizeof(p_cdc->epin_buf));
-  if ( count )
+  uint8_t const rhport = TUD_OPT_RHPORT;
+
+  // claim the endpoint first
+  TU_VERIFY( usbd_edpt_claim(rhport, p_cdc->ep_in), 0 );
+
+  // we can be blocked by another write()/write_flush() from other thread.
+  // causing us to attempt to transfer on an busy endpoint.
+  uint16_t const count = tu_fifo_read_n(&p_cdc->tx_ff, p_cdc->epin_buf, sizeof(p_cdc->epin_buf));
+
+  if ( count && tud_cdc_n_connected(itf) )
   {
-    TU_VERIFY( tud_cdc_n_connected(itf), 0 ); // fifo is empty if not connected
-    TU_ASSERT( usbd_edpt_xfer(TUD_OPT_RHPORT, p_cdc->ep_in, p_cdc->epin_buf, count), 0 );
+    TU_ASSERT( usbd_edpt_xfer(rhport, p_cdc->ep_in, p_cdc->epin_buf, count), 0 );
+    return count;
+  }else
+  {
+    // Release endpoint since we don't make any transfer
+    // Note: data is dropped if terminal is not connected
+    usbd_edpt_release(rhport, p_cdc->ep_in);
+    return 0;
   }
-
-  return count;
 }
 
 uint32_t tud_cdc_n_write_available (uint8_t itf)
@@ -194,7 +206,7 @@ void cdcd_init(void)
     p_cdc->wanted_char = -1;
 
     // default line coding is : stop bit = 1, parity = none, data bits = 8
-    p_cdc->line_coding.bit_rate = 115200;
+    p_cdc->line_coding.bit_rate  = 115200;
     p_cdc->line_coding.stop_bits = 0;
     p_cdc->line_coding.parity    = 0;
     p_cdc->line_coding.data_bits = 8;
@@ -421,10 +433,10 @@ bool cdcd_xfer_cb(uint8_t rhport, uint8_t ep_addr, xfer_result_t result, uint32_
 
     if ( 0 == tud_cdc_n_write_flush(itf) )
     {
-      // There is no data left, a ZLP should be sent if
+      // If there is no data left, a ZLP should be sent if
       // xferred_bytes is multiple of EP size and not zero
       // FIXME CFG_TUD_CDC_EP_BUFSIZE is not Endpoint packet size
-      if ( xferred_bytes && (0 == (xferred_bytes % CFG_TUD_CDC_EP_BUFSIZE)) )
+      if ( !tu_fifo_count(&p_cdc->tx_ff) && xferred_bytes && (0 == (xferred_bytes % CFG_TUD_CDC_EP_BUFSIZE)) )
       {
         usbd_edpt_xfer(rhport, p_cdc->ep_in, NULL, 0);
       }
diff --git a/src/device/usbd.c b/src/device/usbd.c
index 6b91048b6..25d3748e1 100644
--- a/src/device/usbd.c
+++ b/src/device/usbd.c
@@ -63,9 +63,11 @@ typedef struct
   {
     volatile bool busy    : 1;
     volatile bool stalled : 1;
+    volatile bool claimed : 1;
 
     // TODO merge ep2drv here, 4-bit should be sufficient
   }ep_status[8][2];
+
 }usbd_device_t;
 
 static usbd_device_t _usbd_dev;
@@ -237,6 +239,13 @@ static inline usbd_class_driver_t const * get_driver(uint8_t drvid)
 OSAL_QUEUE_DEF(OPT_MODE_DEVICE, _usbd_qdef, CFG_TUD_TASK_QUEUE_SZ, dcd_event_t);
 static osal_queue_t _usbd_q;
 
+// Mutex for claiming endpoint, only needed when using with preempted RTOS
+#if CFG_TUSB_OS != OPT_OS_NONE
+static osal_mutex_def_t _ubsd_mutexdef;
+static osal_mutex_t _usbd_mutex;
+#endif
+
+
 //--------------------------------------------------------------------+
 // Prototypes
 //--------------------------------------------------------------------+
@@ -351,9 +360,15 @@ bool tud_init (void)
 
   tu_varclr(&_usbd_dev);
 
+#if CFG_TUSB_OS != OPT_OS_NONE
+  // Init device mutex
+  _usbd_mutex = osal_mutex_create(&_ubsd_mutexdef);
+  TU_ASSERT(_usbd_mutex);
+#endif
+
   // Init device queue & task
   _usbd_q = osal_queue_create(&_usbd_qdef);
-  TU_ASSERT(_usbd_q != NULL);
+  TU_ASSERT(_usbd_q);
 
   // Get application driver if available
   if ( usbd_app_driver_get_cb )
@@ -478,6 +493,7 @@ void tud_task (void)
         TU_LOG2("on EP %02X with %u bytes\r\n", ep_addr, (unsigned int) event.xfer_complete.len);
 
         _usbd_dev.ep_status[epnum][ep_dir].busy = false;
+        _usbd_dev.ep_status[epnum][ep_dir].claimed = 0;
 
         if ( 0 == epnum )
         {
@@ -1076,6 +1092,56 @@ bool usbd_edpt_open(uint8_t rhport, tusb_desc_endpoint_t const * desc_ep)
   return dcd_edpt_open(rhport, desc_ep);
 }
 
+bool usbd_edpt_claim(uint8_t rhport, uint8_t ep_addr)
+{
+  (void) rhport;
+
+  uint8_t const epnum = tu_edpt_number(ep_addr);
+  uint8_t const dir   = tu_edpt_dir(ep_addr);
+
+#if CFG_TUSB_OS != OPT_OS_NONE
+  osal_mutex_lock(_usbd_mutex, OSAL_TIMEOUT_WAIT_FOREVER);
+#endif
+
+  // can only claim the endpoint if it is not busy and not claimed yet.
+  bool ret = (_usbd_dev.ep_status[epnum][dir].busy == 0) && (_usbd_dev.ep_status[epnum][dir].claimed == 0);
+  if (ret)
+  {
+    _usbd_dev.ep_status[epnum][dir].claimed = 1;
+  }
+
+#if CFG_TUSB_OS != OPT_OS_NONE
+  osal_mutex_unlock(_usbd_mutex);
+#endif
+
+  return ret;
+}
+
+bool usbd_edpt_release(uint8_t rhport, uint8_t ep_addr)
+{
+  (void) rhport;
+
+  uint8_t const epnum = tu_edpt_number(ep_addr);
+  uint8_t const dir   = tu_edpt_dir(ep_addr);
+
+#if CFG_TUSB_OS != OPT_OS_NONE
+  osal_mutex_lock(_usbd_mutex, OSAL_TIMEOUT_WAIT_FOREVER);
+#endif
+
+  // can only release the endpoint if it is claimed and not busy
+  bool ret = (_usbd_dev.ep_status[epnum][dir].busy == 0) && (_usbd_dev.ep_status[epnum][dir].claimed == 1);
+  if (ret)
+  {
+    _usbd_dev.ep_status[epnum][dir].claimed = 0;
+  }
+
+#if CFG_TUSB_OS != OPT_OS_NONE
+  osal_mutex_unlock(_usbd_mutex);
+#endif
+
+  return ret;
+}
+
 bool usbd_edpt_xfer(uint8_t rhport, uint8_t ep_addr, uint8_t * buffer, uint16_t total_bytes)
 {
   uint8_t const epnum = tu_edpt_number(ep_addr);
@@ -1083,6 +1149,9 @@ bool usbd_edpt_xfer(uint8_t rhport, uint8_t ep_addr, uint8_t * buffer, uint16_t
 
   TU_LOG2("  Queue EP %02X with %u bytes ... ", ep_addr, total_bytes);
 
+  // Attempt to transfer on busy or not claimed (skip now) endpoint, sound like an race condition !
+  TU_ASSERT(_usbd_dev.ep_status[epnum][dir].busy == 0 /*&& _usbd_dev.ep_status[epnum][dir].claimed == 1*/);
+
   // Set busy first since the actual transfer can be complete before dcd_edpt_xfer() could return
   // and usbd task can preempt and clear the busy
   _usbd_dev.ep_status[epnum][dir].busy = true;
diff --git a/src/device/usbd_pvt.h b/src/device/usbd_pvt.h
index a5d223329..5fa9a72a5 100644
--- a/src/device/usbd_pvt.h
+++ b/src/device/usbd_pvt.h
@@ -70,6 +70,12 @@ void usbd_edpt_close(uint8_t rhport, uint8_t ep_addr);
 // Submit a usb transfer
 bool usbd_edpt_xfer(uint8_t rhport, uint8_t ep_addr, uint8_t * buffer, uint16_t total_bytes);
 
+// Claim an endpoint before submitting a transfer
+bool usbd_edpt_claim(uint8_t rhport, uint8_t ep_addr);
+
+// Release an endpoint without submitting a transfer
+bool usbd_edpt_release(uint8_t rhport, uint8_t ep_addr);
+
 // Check if endpoint transferring is complete
 bool usbd_edpt_busy(uint8_t rhport, uint8_t ep_addr);