-       /* Macros: */
-               #if !defined(__DOXYGEN__)
-                       #define _GETADCMUXMASK2(x, y)       x ## y
-                       #define _GETADCMUXMASK(x, y)        _GETADCMUXMASK2(x, y)
+               #if defined(USB_SERIES_4_AVR) && ((VTARGET_ADC_CHANNEL == 2) || (VTARGET_ADC_CHANNEL == 3)) && !defined(NO_VTARGET_DETECT)
+                       #error The U4 AVR chips do not contain ADC channels 2 or 3. Please change VTARGET_ADC_CHANNEL or define NO_VTARGET_DETECT in the makefile.
+               #endif
+
+               #if defined(VTARGET_USE_INTERNAL_REF)
+                       #undef  VTARGET_REF_VOLTS
+                       #define VTARGET_REF_VOLTS 2.56
+
+                       #define VTARGET_REF_MASK ADC_REFERENCE_INT2560MV
+               #else
+                       #define VTARGET_REF_MASK ADC_REFERENCE_AVCC