diff --git a/module02/ex03/main.c b/module02/ex03/main.c
index 27b858b..9252415 100644
--- a/module02/ex03/main.c
+++ b/module02/ex03/main.c
@@ -154,4 +154,3 @@ int main() {
 
 	while(1);
 }
-
diff --git a/module02/ex04/Makefile b/module02/ex04/Makefile
new file mode 100644
index 0000000..2e40cba
--- /dev/null
+++ b/module02/ex04/Makefile
@@ -0,0 +1 @@
+include ../../Makefile
\ No newline at end of file
diff --git a/module02/ex04/main.c b/module02/ex04/main.c
new file mode 100644
index 0000000..9252415
--- /dev/null
+++ b/module02/ex04/main.c
@@ -0,0 +1,156 @@
+#include <avr/io.h>
+#include <util/delay.h>
+#include <avr/interrupt.h>		// https://www.nongnu.org/avr-libc/user-manual/group__avr__interrupts.html
+
+// mathematics 
+#define DIV_ROUND_CLOSEST(n, d)	((((n) < 0) == ((d) < 0)) ? (((n) + (d)/2)/(d)) : (((n) - (d)/2)/(d))) // https://stackoverflow.com/a/18067292
+
+// stringify
+#define STRINGIFY_HELPER(x)		#x
+#define STRINGIFY(x)			STRINGIFY_HELPER(x)
+
+// concatenate
+#define CONCAT_HELPER(x, y)		x ## y
+#define CONCAT(x, y)			CONCAT_HELPER(x, y)
+
+// get argument
+#define ARG_1(v1, v2)			v1
+#define ARG_2(v1, v2)			v2
+#define GET_PORT(args)			ARG_1 args
+#define GET_BIT(args)			ARG_2 args
+// // version with "LED1 B, D1" without parenthesis
+// #define ARG_1(v1, ...)			v1
+// #define ARG_2(v1, v2, ...)		v2
+// #define GET_PORT(...)			ARG_1(__VA_ARGS__)
+// #define GET_BIT(...)			ARG_2(__VA_ARGS__)
+
+// actions on registers
+#define SET(register, bit)		register |= 1 << bit
+#define CLEAR(register, bit)	register &= ~(1 << bit)
+#define TEST(register, bit)		(register & 1 << bit)
+#define TOGGLE(register, bit)	register ^= 1 << bit
+
+// actions on elements
+#define SET_ELEM(elem)			SET(CONCAT(PORT, GET_PORT(elem)), GET_BIT(elem))
+// #define SET_ELEM(...)			SET(CONCAT(PORT, GET_PORT(__VA_ARGS__)), GET_BIT(__VA_ARGS__)) // version for "LED1 B, D1" without parenthesis
+#define CLEAR_ELEM(elem)		CLEAR(CONCAT(PORT, GET_PORT(elem)), GET_BIT(elem))
+#define TEST_ELEM(elem)			TEST(CONCAT(PORT, GET_PORT(elem)), GET_BIT(elem))
+#define TOGGLE_ELEM(elem)		TOGGLE(CONCAT(PORT, GET_PORT(elem)), GET_BIT(elem))
+#define MODE_OUTPUT(elem)		SET(CONCAT(DDR, GET_PORT(elem)), GET_BIT(elem))
+#define MODE_INPUT(elem)		CLEAR(CONCAT(DDR, GET_PORT(elem)), GET_BIT(elem))
+#define TOGGLE_PIN(elem)		SET(CONCAT(PIN, GET_PORT(elem)), GET_BIT(elem))
+#define TEST_PIN(elem)			(TEST(CONCAT(PIN, GET_PORT(elem)), GET_BIT(elem)))
+#define IS_PIN_SET(elem)		(TEST_PIN(elem) == 0)
+#define IS_PIN_CLEAR(elem)		(TEST_PIN(elem) == 1)
+
+// bits
+#define D1	0
+#define D2	1
+#define D3	2
+#define D4	4
+#define SW1	2
+#define SW2	4
+
+// elements (port, bit)
+// #define LED1	B, D1
+#define LED1	(B, D1)
+#define LED2	(B, D2)
+#define LED3	(B, D3)
+#define LED4	(B, D4)
+#define BUTTON1	(D, SW1)
+#define BUTTON2	(D, SW2)
+
+// USART
+// Table 20-1 : Baud Rate Calculation
+#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)
+// 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)
+// Table 20-10 : Stop Bit Selection (USART Stop Bit Select)
+#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)
+// 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 INTERRUPT_RECEIVER_DISABLED		(0<<RXCIE0)
+#define INTERRUPT_RECEIVER_ENABLED		(1<<RXCIE0)
+#define INTERRUPT_TRANSMITTER_DISABLED	(0<<TXCIE0)
+#define INTERRUPT_TRANSMITTER_ENABLED	(1<<TXCIE0)
+
+// TIMER
+#define PERIOD							2000
+#define TIME_MS(ms)						(((F_CPU / PRESCALE_VALUE) * ms) / 1000)
+#define PRESCALE_VALUE					1024		// can be 1, 8, 64, 256, 1024
+// table 16-5 : prescale sets
+#define PRESCALE_SET(value)  \
+										((value) == 1   ? (0<<CS12 | 0<<CS11 | 1<<CS10) : \
+										 (value) == 8   ? (0<<CS12 | 1<<CS11 | 0<<CS10) : \
+										 (value) == 64  ? (0<<CS12 | 1<<CS11 | 1<<CS10) : \
+										 (value) == 256 ? (1<<CS12 | 0<<CS11 | 0<<CS10) : \
+										 (value) == 1024? (1<<CS12 | 0<<CS11 | 1<<CS10) : \
+														  (0<<CS12 | 0<<CS11 | 0<<CS10))
+// Table 16-4 : Waveform Generation Mode Bit Description
+#define CTC_TOP_OCR1A_IN_TCCR1B			(0<<WGM13 | 1<<WGM12)
+#define CTC_TOP_OCR1A_IN_TCCR1A									(0<<WGM11 | 0<<WGM10)
+#define CTC_TOP_ICR1_IN_TCCR1B			(1<<WGM13 | 1<<WGM12)
+#define CTC_TOP_ICR1_IN_TCCR1A									(0<<WGM11 | 0<<WGM10)
+#define FAST_PWM_TOP_OCR1A_IN_TCCR1B	(1<<WGM13 | 1<<WGM12)
+#define FAST_PWM_TOP_OCR1A_IN_TCCR1A							(1<<WGM11 | 1<<WGM10)
+#define FAST_PWM_TOP_ICR1_IN_TCCR1B		(1<<WGM13 | 1<<WGM12)
+#define FAST_PWM_TOP_ICR1_IN_TCCR1A								(1<<WGM11 | 0<<WGM10)
+// 16.11.8 : Timer/Counter1 Interrupt Mask Register
+#define INTERRUPT_ENABLE_CHANNEL_A		(1 << OCIE1A)
+#define INTERRUPT_DISABLE_CHANNEL_A		(0 << OCIE1A)
+
+// text
+#define SWITCH_CASE(ch) (((ch) >= 'A' && (ch) <= 'Z') || ((ch) >= 'a' && (ch) <= 'z') ? ((ch) ^ (1 << 5)) : (ch))
+
+// END MACROS
+
+void uart_init() {
+	UBRR0H = (unsigned char) (BAUD_PRESCALER >> 8);									// 20.11.5 : UBRRnL and UBRRnH – USART Baud Rate Registers
+	UBRR0L = (unsigned char) BAUD_PRESCALER;
+
+	UCSR0C |= ASYNCHRONOUS | PARITY_DISABLED | STOP_ONE_BIT | DATA_EIGHT_BIT;		// 20.11.4 : set Frame Format
+	
+	UCSR0B |= RECEIVER_ENABLED | TRANSMITTER_ENABLED | INTERRUPT_RECEIVER_ENABLED;	// 20.11.3 : enable Receiver and Transmitter, and interrupt on receiver
+}
+
+// char uart_rx(void) {
+// 	while (TEST(UCSR0A, RXC0) == 0);												// 20.11.2 : do nothing until there are unread data in the receive buffer (UDRn), (RXCn flag in UCSRnA register set to 1 when buffer has data)
+// 	return UDR0;																	// 20.11.1 : get data in buffer, UDRn – USART I/O Data Register (read and write)
+// }
+
+void uart_tx(char c) {
+	while (TEST(UCSR0A, UDRE0) == 0);												// 20.11.2 : do nothing until UDRn buffer is empty, (UDREn flag in UCSRnA register set to 1 when buffer empty)
+	UDR0 = (unsigned char) c;														// 20.11.1 : Put data into buffer, UDRn – USART I/O Data Register (read and write)
+}
+
+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
+}
+
+// send back caracters received on serial port with case toggling, using interupt and empty infinite loop
+// `screen /dev/ttyUSB0 115200`
+int main() {
+	uart_init();
+
+	sei();																			// enable global interrupts
+
+	while(1);
+}