diff --git a/module01/ex00/main.c b/module01/ex00/main.c
index 6c0f9b1..94d3932 100644
--- a/module01/ex00/main.c
+++ b/module01/ex00/main.c
@@ -54,10 +54,10 @@ int main() {
 
 	TCCR1B |= (PRESCALE_SET);				// set timer with prescale
 											//   -> set timer with bits CS10-12, in register TCCR1B : 16.4 Timer/Counter Clock Sources
-											//   -> prescale values : 16.11.2 TCCR1B – Timer/Counter1 Control Register B, table 16-5. Clock Select Bit Description
+											//   -> prescale values : table 16-5. Clock Select Bit Description
 
 	while(1) {
-		if ( TCNT1 >= TIME_MS(500)) {		// read timer with register TCNT1 (read/write allowed), that combines two 8 bits registers : 16.11.4 TCNT1H and TCNT1L – Timer/Counter1
+		if (TCNT1 >= TIME_MS(500)) {		// read timer with register TCNT1 (read/write allowed), that combines two 8 bits registers : 16.11.4 TCNT1H and TCNT1L – Timer/Counter1
 			TOGGLE_LED(D2);
 			TCNT1 = 0;						// reset timer value, also in register TCNT1
 		}
diff --git a/module01/ex01/main.c b/module01/ex01/main.c
index 8fa7bad..4b0669a 100644
--- a/module01/ex01/main.c
+++ b/module01/ex01/main.c
@@ -1,5 +1,4 @@
 #include <avr/io.h>
-#include <util/delay.h>
 
 // #define FIRST_LETTER_IMPL(x) #x[0]
 // #define FIRST_LETTER(x) FIRST_LETTER_IMPL(x)
@@ -26,7 +25,7 @@
 #define TURN_OFF_LED(bit)			CLEAR(PORTB, bit)
 #define TOGGLE_LED(led)				TOGGLE_PIN(B, led)
 
-
+// ELEMENTS
 #define D1 0
 #define D2 1
 #define D3 2
@@ -34,19 +33,42 @@
 #define SW1 2
 #define SW2 4
 
+// TIME
 #define PRESCALE_VALUE				1024
-#define TIME(ms)					(((F_CPU / PRESCALE_VALUE) * ms) / 1000)
+#if (PRESCALE_VALUE == 1)
+	#define PRESCALE_SET			(1 << CS10)
+#elif (PRESCALE_VALUE == 8)
+	#define PRESCALE_SET			(1 << CS11)
+#elif (PRESCALE_VALUE == 64)
+	#define PRESCALE_SET			(1 << CS10) | (1 << CS11)
+#elif (PRESCALE_VALUE == 256)
+	#define PRESCALE_SET			(1 << CS12)
+#elif (PRESCALE_VALUE == 1024)
+	#define PRESCALE_SET			(1 << CS10) | (1 << CS12)
+#endif
+#define TIME_MS(ms)					(((F_CPU / PRESCALE_VALUE) * ms) / 1000)
 
 int main() {
 	MODE_OUTPUT(B, D2);
-	TURN_OFF_LED(D2);
 
-	TCCR1B |= ((1 << CS10 ) | (1 << CS12 ) ); // set timer with prescale
+	TCCR1B |= (1 << WGM12);						// set timer in CTC (Clear Time on Compare) mode
+												//   -> use bit WGM12 to set mode ctc : Table 16-4. Waveform Generation Mode Bit Description
+												//   -> bit WGM12 is located in register TCCR1B : 16.11.2 TCCR1B Timer/Counter1 Control Register B
+
+	OCR1A = TIME_MS(500);						// set CTC compare value
+												//   -> the value to reset the timer is the Output Compare Registers OCR1A : Table 16-4. Waveform Generation Mode Bit Description
+
+	TCCR1B |= (PRESCALE_SET);
 
 	while(1) {
-		if ( TCNT1 >= TIME(500)) {
-			TOGGLE_LED(D2);
-			TCNT1 = 0; // reset timer value
+		if (TIFR1 & (1 << OCF1A)) {				// when comparison has occured, bit OCF1A is set
+												//    -> "The compare match event will also set the Compare Match Flag (OCF1A/B) which can be used to generate an Output Compare interrupt request" : 16.2.1 Register
+												//    -> bit OCF1A is in register TIFR1 : 16.11.9 TIFR1 – Timer/Counter1 Interrupt Flag Register
+
+			TOGGLE_LED(D2);						// toggle using PINx register
+
+			TIFR1 = (1 << OCF1A);				// clear the CTC flag
+												//    -> "OCF1A can be cleared by writing a logic one to its bit location" : 16.11.9 TIFR1 – Timer/Counter1 Interrupt Flag Register
 		}
 	}
 }