diff --git a/arch/arm/include/sam3u/irq.h b/arch/arm/include/sam3u/irq.h
index 16f7ba3a5a..b2df14c889 100755
--- a/arch/arm/include/sam3u/irq.h
+++ b/arch/arm/include/sam3u/irq.h
@@ -105,7 +105,7 @@
 
 /* External interrupts (vectors >= 16) */
 
-#define SAM3U_IRQ_EXTINT        (16)
+#define SAM3U_IRQ_EXTINT        (16) /* Vector number of the first external interrupt */
 #define SAM3U_IRQ_SUPC          (SAM3U_IRQ_EXTINT+SAM3U_PID_SUPC)   /* Supply Controller */
 #define SAM3U_IRQ_RSTC          (SAM3U_IRQ_EXTINT+SAM3U_PID_RSTC)   /* Reset Controller */
 #define SAM3U_IRQ_RTC           (SAM3U_IRQ_EXTINT+SAM3U_PID_RTC)    /* Real Time Clock */