diff --git a/Makefile b/Makefile
index 98461ad..881a5dc 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,6 @@
 PREFIX := avr-
 CFLAGS_COMMON := -I/usr/avr/include/
-CFLAGS_CPU := -DF_CPU=20000000UL -mmcu=atmega328p -Os
+CFLAGS_CPU := -DF_CPU=16000000UL -mmcu=atmega328p -Os
 CFLAGS := $(CFLAGS_COMMON) $(CFLAGS_CPU)
 LDFLAGS_COMMON := -I/usr/avr/include/
 LDFLAGS_CPU := -mmcu=atmega328p