diff --git a/hardware/arduino/cores/arduino/USBCore.cpp b/hardware/arduino/cores/arduino/USBCore.cpp
index 8bcf66c54..decfd6bfb 100644
--- a/hardware/arduino/cores/arduino/USBCore.cpp
+++ b/hardware/arduino/cores/arduino/USBCore.cpp
@@ -638,7 +638,11 @@ void USBDevice_::attach()
 	_usbConfiguration = 0;
 	UHWCON = 0x01;						// power internal reg
 	USBCON = (1<<USBE)|(1<<FRZCLK);		// clock frozen, usb enabled
+#if F_CPU == 16000000UL
 	PLLCSR = 0x12;						// Need 16 MHz xtal
+#elif F_CPU == 8000000UL
+	PLLCSR = 0x02;						// Need 8 MHz xtal
+#endif
 	while (!(PLLCSR & (1<<PLOCK)))		// wait for lock pll
 		;