diff --git a/atusb/fw/board_app.c b/atusb/fw/board_app.c
index 22b93c3..000d8eb 100644
--- a/atusb/fw/board_app.c
+++ b/atusb/fw/board_app.c
@@ -157,7 +157,7 @@ ISR(INT0_vect)
 {
 	if (eps[1].state == EP_IDLE) {
 		led(1);
-		irq_serial++;
+		irq_serial = (irq_serial+1) & 0x7f;
 		usb_send(&eps[1], &irq_serial, 1, done, NULL);
 	}
 }