diff --git a/gpio.h b/gpio.h
--- a/gpio.h
+++ b/gpio.h
@@ -1,6 +1,7 @@
 #ifndef GPIO_H
 #define GPIO_H
 
+#include <inttypes.h>
 
 #define CHANGE_PERIOD_MS 100
 #define CHANGE_ELAPSED (ticks - change_time_reset) > CHANGE_PERIOD_MS