diff --git a/MicroDexed.ino b/MicroDexed.ino
index 843c251..1c45004 100644
--- a/MicroDexed.ino
+++ b/MicroDexed.ino
@@ -961,7 +961,6 @@ void handleSystemReset(void)
 /******************************************************************************
    MIDI HELPER
  ******************************************************************************/
-
 bool checkMidiChannel(byte inChannel)
 {
   // check for MIDI channel
diff --git a/config.h b/config.h
index b089fe7..d7b3fbb 100644
--- a/config.h
+++ b/config.h
@@ -219,7 +219,6 @@
 #endif
 
 #if defined(__IMXRT1062__) //Teensy-4.0
-#undef MIDI_DEVICE_USB_HOST
 #undef MIDI_DEVICE_USB
 #define MAX_NOTES 16
 #endif
diff --git a/source_microdexed.h b/source_microdexed.h
index bcd6b81..f0cb8f2 100644
--- a/source_microdexed.h
+++ b/source_microdexed.h
@@ -12,10 +12,20 @@ class AudioSourceMicroDexed : public AudioStream, public Dexed {
          AudioSourceMicroDexed(int sample_rate) : AudioStream(0, NULL), Dexed(sample_rate) { 
             };   
          void update(void) {
+            if (in_update) {
+               xrun++;
+	             return;
+	          }
+            else in_update = true;
+		
             elapsedMicros render_time;
             audio_block_t *lblock;
             lblock = allocate();
-            if (!lblock) return;
+            if (!lblock) {
+              in_update = false;
+              return;
+              
+            }
             getSamples(AUDIO_BLOCK_SAMPLES, lblock->data);
             if (render_time > audio_block_time_us) // everything greater 2.9ms is a buffer underrun!
               xrun++;
@@ -24,6 +34,8 @@ class AudioSourceMicroDexed : public AudioStream, public Dexed {
            
             transmit(lblock, 0);
             release(lblock);
+            in_update = false;
         };                                                           
 private:
+        volatile bool in_update = false;
 };