diff --git a/ports/atmel-samd/usb.c b/ports/atmel-samd/usb.c
index 740486aa1e..6ce8aeea7e 100644
--- a/ports/atmel-samd/usb.c
+++ b/ports/atmel-samd/usb.c
@@ -227,17 +227,17 @@ void init_usb(void) {
 }
 
 static bool cdc_enabled(void) {
-    if (mp_cdc_enabled) {
-        return true;
-    }
     if (!cdcdf_acm_is_enabled()) {
+        mp_cdc_enabled = false;
         return false;
     }
-    cdcdf_acm_register_callback(CDCDF_ACM_CB_READ, (FUNC_PTR)read_complete);
-    cdcdf_acm_register_callback(CDCDF_ACM_CB_WRITE, (FUNC_PTR)write_complete);
-    cdcdf_acm_register_callback(CDCDF_ACM_CB_STATE_C, (FUNC_PTR)usb_device_cb_state_c);
-    cdcdf_acm_register_callback(CDCDF_ACM_CB_LINE_CODING_C, (FUNC_PTR)usb_device_cb_line_coding_c);
-    mp_cdc_enabled = true;
+    if (!mp_cdc_enabled) {
+        cdcdf_acm_register_callback(CDCDF_ACM_CB_READ, (FUNC_PTR)read_complete);
+        cdcdf_acm_register_callback(CDCDF_ACM_CB_WRITE, (FUNC_PTR)write_complete);
+        cdcdf_acm_register_callback(CDCDF_ACM_CB_STATE_C, (FUNC_PTR)usb_device_cb_state_c);
+        cdcdf_acm_register_callback(CDCDF_ACM_CB_LINE_CODING_C, (FUNC_PTR)usb_device_cb_line_coding_c);
+        mp_cdc_enabled = true;
+    }
 
     return true;
 }