diff --git a/src/usb/usb.c b/src/usb/usb.c
index 1137fbf..0115072 100644
--- a/src/usb/usb.c
+++ b/src/usb/usb.c
@@ -40,9 +40,6 @@
 // MACRO TYPEDEF CONSTANT ENUM DECLARATION
 //--------------------------------------------------------------------+
 
-// Serial string using unique Device ID
-extern char               usb_desc_str_serial[1+16];
-
 /* tinyusb function that handles power event (detected, ready, removed)
  * We must call it within SD's SOC event handler, or set it as power event handler if SD is not enabled. */
 extern void tusb_hal_nrf_power_event(uint32_t event);
@@ -90,12 +87,9 @@ void usb_init(bool cdc_only)
     tusb_hal_nrf_power_event(NRFX_POWER_USB_EVT_READY);
   }
 
-  usb_desc_set_mode(cdc_only);
+  usb_desc_init(cdc_only);
 
-  // Create Serial string descriptor
-  sprintf(usb_desc_str_serial, "%08lX%08lX", NRF_FICR->DEVICEID[1], NRF_FICR->DEVICEID[0]);
-
-  // Init tusb stack
+  // Init TinyUSB stack
   tusb_init();
 }
 
diff --git a/src/usb/usb_desc.c b/src/usb/usb_desc.c
index 5ca1856..69bc652 100644
--- a/src/usb/usb_desc.c
+++ b/src/usb/usb_desc.c
@@ -33,8 +33,12 @@ enum {
     ITF_STR_MSC
 };
 
+// CDC + MSC or CDC only mode
 static bool _cdc_only = false;
 
+// Serial is 64-bit DeviceID -> 16 chars len
+static char desc_str_serial[1+16];
+
 //--------------------------------------------------------------------+
 // Device Descriptor
 //--------------------------------------------------------------------+
@@ -112,7 +116,7 @@ uint8_t const * tud_descriptor_configuration_cb(uint8_t index)
 }
 
 // Enumerate as CDC + MSC or CDC only
-void usb_desc_set_mode(bool cdc_only)
+void usb_desc_init(bool cdc_only)
 {
   _cdc_only = cdc_only;
 
@@ -121,6 +125,9 @@ void usb_desc_set_mode(bool cdc_only)
     // Change PID to CDC only
     desc_device.idProduct = USB_DESC_CDC_ONLY_PID;
   }
+
+  // Create Serial string descriptor
+  sprintf(desc_str_serial, "%08lX%08lX", NRF_FICR->DEVICEID[1], NRF_FICR->DEVICEID[0]);
 }
 
 //--------------------------------------------------------------------+
@@ -149,8 +156,6 @@ void usb_desc_set_mode(bool cdc_only)
 }
 #endif
 
-// Serial is 64-bit DeviceID -> 16 chars len
-char usb_desc_str_serial[1+16];
 
 // array of pointer to string descriptors
 char const* string_desc_arr [] =
@@ -158,7 +163,7 @@ char const* string_desc_arr [] =
   (const char[]) { 0x09, 0x04 }, // 0: is supported language is English (0x0409)
   "Adafruit Industries",         // 1: Manufacturer
   "Bluefruit DFU",               // 2: Product
-  usb_desc_str_serial,           // 3: Serials, should use chip ID
+  desc_str_serial,               // 3: Serials, should use chip ID
   "Bluefruit Serial",            // 4: CDC Interface
   "Bluefruit UF2",               // 5: MSC Interface
 };
diff --git a/src/usb/usb_desc.h b/src/usb/usb_desc.h
index da292ff..8f83d0e 100644
--- a/src/usb/usb_desc.h
+++ b/src/usb/usb_desc.h
@@ -28,7 +28,7 @@
 #include "tusb.h"
 #include "boards.h"
 
-void usb_desc_set_mode(bool cdc_only);
+void usb_desc_init(bool cdc_only);
 
 #ifndef USB_DESC_VID
 #define USB_DESC_VID             0x239A