diff --git a/module02/ex00/main.c b/module02/ex00/main.c
index b456e75..8cddfff 100644
--- a/module02/ex00/main.c
+++ b/module02/ex00/main.c
@@ -104,7 +104,8 @@ void uart_tx(char c) {
 	UDR0 = (unsigned char) c;													// 20.11.1 : Put data into buffer, UDRn – USART I/O Data Register (read and write)
 }
 
-// screen /dev/ttyUSB0 115200
+// write Z on serial port, at a !Hz frequency
+// `screen /dev/ttyUSB0 115200`
 int main() {
 	uart_init();
 	while (1) {
diff --git a/module02/ex01/main.c b/module02/ex01/main.c
index eb5f809..9f9244f 100644
--- a/module02/ex01/main.c
+++ b/module02/ex01/main.c
@@ -62,29 +62,29 @@
 
 // USART
 // Table 20-1 : Baud Rate Calculation
-#define USART_BAUDRATE			115200
-#define BAUD_PRESCALER			(DIV_ROUND_CLOSEST(F_CPU, (16 * USART_BAUDRATE)) - 1)
+#define USART_BAUDRATE					115200
+#define BAUD_PRESCALER					(DIV_ROUND_CLOSEST(F_CPU, (16 * USART_BAUDRATE)) - 1)
 // Table 20-8 : Mode Selection (USART Mode SELect)
-#define ASYNCHRONOUS			(0<<UMSEL01 | 0<<UMSEL00)
-#define SYNCHRONOUS				(0<<UMSEL01 | 1<<UMSEL00)
+#define ASYNCHRONOUS					(0<<UMSEL01 | 0<<UMSEL00)
+#define SYNCHRONOUS						(0<<UMSEL01 | 1<<UMSEL00)
 // Table 20-9 : Parity Bit Selection (USART Parity Mode)
-#define PARITY_DISABLED			(0<<UPM01 | 0<<UPM00)
-#define PARITY_EVEN				(1<<UPM01 | 0<<UPM00)
-#define PARITY_ODD				(1<<UPM01 | 1<<UPM00)
+#define PARITY_DISABLED					(0<<UPM01 | 0<<UPM00)
+#define PARITY_EVEN						(1<<UPM01 | 0<<UPM00)
+#define PARITY_ODD						(1<<UPM01 | 1<<UPM00)
 // Table 20-10 : Stop Bit Selection (USART Stop Bit Select)
-#define STOP_ONE_BIT			(0<<USBS0)
-#define STOP_TWO_BIT			(1<<USBS0)
+#define STOP_ONE_BIT					(0<<USBS0)
+#define STOP_TWO_BIT					(1<<USBS0)
 // Table 20-11 : Data Bit Selection (USART Character SiZe)
-#define DATA_FIVE_BIT			(0<<UCSZ02 | 0<<UCSZ01 | 0<<UCSZ00)
-#define DATA_SIX_BIT			(0<<UCSZ02 | 0<<UCSZ01 | 1<<UCSZ00)
-#define DATA_SEVEN_BIT			(0<<UCSZ02 | 1<<UCSZ01 | 0<<UCSZ00)
-#define DATA_EIGHT_BIT			(0<<UCSZ02 | 1<<UCSZ01 | 1<<UCSZ00)
-#define DATA_NINE_BIT			(1<<UCSZ02 | 1<<UCSZ01 | 1<<UCSZ00)
+#define DATA_FIVE_BIT					(0<<UCSZ02 | 0<<UCSZ01 | 0<<UCSZ00)
+#define DATA_SIX_BIT					(0<<UCSZ02 | 0<<UCSZ01 | 1<<UCSZ00)
+#define DATA_SEVEN_BIT					(0<<UCSZ02 | 1<<UCSZ01 | 0<<UCSZ00)
+#define DATA_EIGHT_BIT					(0<<UCSZ02 | 1<<UCSZ01 | 1<<UCSZ00)
+#define DATA_NINE_BIT					(1<<UCSZ02 | 1<<UCSZ01 | 1<<UCSZ00)
 // 20.11.3 : USART Control and Status Register B (UCSRnB)
-#define RECEIVER_DISABLED		(0<<RXEN0)
-#define RECEIVER_ENABLED		(1<<RXEN0)
-#define TRANSMITTER_DISABLED	(0<<TXEN0)
-#define TRANSMITTER_ENABLED		(1<<TXEN0)
+#define RECEIVER_DISABLED				(0<<RXEN0)
+#define RECEIVER_ENABLED				(1<<RXEN0)
+#define TRANSMITTER_DISABLED			(0<<TXEN0)
+#define TRANSMITTER_ENABLED				(1<<TXEN0)
 
 // TIMER
 #define PERIOD							2000
@@ -134,7 +134,7 @@ void uart_printstr(const char* str) {
 	}
 }
 
-// print hello world, on serial port, every 2 seconds, with empty infinite loop
+// print "Hello World!\n", on serial port, every 2 seconds, with empty infinite loop
 // `screen /dev/ttyUSB0 115200`
 int main() {
 	uart_init();
@@ -154,5 +154,5 @@ int main() {
 
 // ISR interrupt macro : https://www.nongnu.org/avr-libc/user-manual/group__avr__interrupts.html
 ISR(TIMER1_COMPA_vect) {							// Table 12-7 : we select the code for timer 1 on channel A
-	uart_printstr("Hello World!");
+	uart_printstr("Hello World!\r\n");				// \n and \r : https://stackoverflow.com/questions/7812142/how-to-toggle-cr-lf-in-gnu-screen
 }
diff --git a/module02/ex04/main.c b/module02/ex04/main.c
index 9252415..d059e96 100644
--- a/module02/ex04/main.c
+++ b/module02/ex04/main.c
@@ -142,7 +142,13 @@ void uart_tx(char c) {
 ISR(USART_RX_vect) {																// Table 12-7 : we select the code for USART Receive
 	// char received_char = uart_rx();
 	char received_char = UDR0;				// Read received character
-	uart_tx(SWITCH_CASE(received_char));	// Toggle case and send back
+	if (received_char == '\b' || received_char == 127) {  // If backspace is received
+		uart_tx('\b');  // Move cursor back
+		uart_tx(' ');   // Erase the character on screen
+		uart_tx('\b');  // Move cursor back again
+	} else {
+		uart_tx(SWITCH_CASE(received_char)); // Toggle case and send back
+	}
 }
 
 // send back caracters received on serial port with case toggling, using interupt and empty infinite loop