diff --git a/hardware/arduino/avr/cores/arduino/wiring.c b/hardware/arduino/avr/cores/arduino/wiring.c
index b956f787e0a..d70bf8c910a 100644
--- a/hardware/arduino/avr/cores/arduino/wiring.c
+++ b/hardware/arduino/avr/cores/arduino/wiring.c
@@ -37,7 +37,7 @@
 
 volatile unsigned long timer0_overflow_count = 0;
 volatile unsigned long timer0_millis = 0;
-static unsigned char timer0_fract = 0;
+volatile unsigned char timer0_fract = 0;
 
 #if defined(__AVR_ATtiny24__) || defined(__AVR_ATtiny44__) || defined(__AVR_ATtiny84__)
 ISR(TIM0_OVF_vect)