diff --git a/module04/ex01/main.c b/module04/ex01/main.c
index 886108e..84fa8fd 100644
--- a/module04/ex01/main.c
+++ b/module04/ex01/main.c
@@ -70,7 +70,6 @@ typedef enum {
 } Slope;
 
 volatile uint8_t duty_cycle = T1_DUTY_CYCLE_PERCENT;
-volatile Slope slope = UP;
 
 // using TIMER 1
 void blink_led_1() {
@@ -119,6 +118,7 @@ int main() {
 }
 
 ISR(TIMER0_COMPA_vect) {
+	volatile static Slope slope = UP;
 	if (slope == UP) {
 		if (duty_cycle >= 100) {
 			duty_cycle = 100;
diff --git a/module04/ex02/Makefile b/module04/ex02/Makefile
new file mode 100644
index 0000000..2e40cba
--- /dev/null
+++ b/module04/ex02/Makefile
@@ -0,0 +1 @@
+include ../../Makefile
\ No newline at end of file
diff --git a/module04/ex02/bitmanip.h b/module04/ex02/bitmanip.h
new file mode 100644
index 0000000..0808aec
--- /dev/null
+++ b/module04/ex02/bitmanip.h
@@ -0,0 +1,61 @@
+#ifndef BITMANIP_H
+#define BITMANIP_H
+
+#include "utils.h"
+
+// Bit operations 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)))
+
+// Get arguments from tuple-like definitions
+#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 elements
+// #define SET_ELEM(...)			SET(CONCAT(PORT, GET_PORT(__VA_ARGS__)), GET_BIT(__VA_ARGS__)) // version for "LED1 B, D1" without parenthesis
+#define SET_ELEM(elem)			SET(CONCAT(PORT, GET_PORT(elem)), GET_BIT(elem))
+#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)
+#define PULLUP_ON(elem)			SET(CONCAT(PORT, GET_PORT(elem)), GET_BIT(elem))
+#define PULLUP_OFF(elem)		CLEAR(CONCAT(PORT, GET_PORT(elem)), GET_BIT(elem))
+
+// Bit definitions (<pin number> // <pin name>)
+#define D1 0	// PB0
+#define D2 1	// PB1
+#define D3 2	// PB2
+#define D4 4	// PB4
+#define SW1 2	// PD2
+#define SW2 4	// PD4
+#define D5B 3	// PD3
+#define D5R 5	// PD5
+#define D5G 6	// PD6
+
+// Elements (port, bit)
+#define LED1		(B, D1)
+#define LED2		(B, D2)
+#define LED3		(B, D3)
+#define LED4		(B, D4)
+#define BUTTON1		(D, SW1)
+#define BUTTON2		(D, SW2)
+#define RGB5_BLUE	(D, D5B)
+#define RGB5_RED	(D, D5R)
+#define RGB5_GEEN	(D, D5G)
+
+#endif // BITMANIP_H
\ No newline at end of file
diff --git a/module04/ex02/header.h b/module04/ex02/header.h
new file mode 100644
index 0000000..b4d8b56
--- /dev/null
+++ b/module04/ex02/header.h
@@ -0,0 +1,20 @@
+#ifndef HEADER_H
+#define HEADER_H
+
+#include <avr/io.h>
+#include <util/delay.h>
+#include <avr/interrupt.h>
+
+#include "utils.h"
+#include "bitmanip.h"
+#include "interrupt.h"
+#include "timer.h"
+
+//
+// PROTOTYPES
+//
+// main.c
+void blink_led_1();
+void vary_duty();
+
+#endif // HEADER_H
\ No newline at end of file
diff --git a/module04/ex02/interrupt.h b/module04/ex02/interrupt.h
new file mode 100644
index 0000000..9668ef7
--- /dev/null
+++ b/module04/ex02/interrupt.h
@@ -0,0 +1,8 @@
+#ifndef INTERRUPT_H
+#define INTERRUPT_H
+
+// 7.3.1 : SREG – AVR Status Register
+#define ENABLE_GLOBAL_INTERRUPT		(1<<SREG_I)
+#define DISABLE_GLOBAL_INTERRUPT	(0<<SREG_I)
+
+#endif // INTERRUPT_H
\ No newline at end of file
diff --git a/module04/ex02/main.c b/module04/ex02/main.c
new file mode 100644
index 0000000..e3a85c3
--- /dev/null
+++ b/module04/ex02/main.c
@@ -0,0 +1,72 @@
+#include "header.h"
+
+typedef struct {
+    int *value;
+    int max;
+	int min;
+} IncrementParams;
+
+void print_binary(int value) {
+	int mask_3 = 0b1000;
+	int bit_at_3 = value & mask_3;
+	int bit_at_4 = bit_at_3 << 1;
+	PORTB = (value & 0b111) + bit_at_4;
+}
+
+void increment_int(IncrementParams *params) {
+	int *value = params->value;
+	int max = params->max;
+	*value = (*value >= max) ? max : ++(*value);
+}
+
+void decrement_int(IncrementParams *params) {
+	int *value = params->value;
+	int min = params->min;
+    *value = (*value <= min) ? min : --(*value);
+}
+
+void increment_led(void *param) {
+	IncrementParams *params = (IncrementParams *)param;
+	increment_int(params);
+	print_binary(*(params->value));
+}
+
+void decrement_led(void *param) {
+	IncrementParams *params = (IncrementParams *)param;
+	decrement_int(params);
+	print_binary(*(params->value));
+}
+
+void on_press(int bit, void (*action)(void*), void *params) {
+	if ((TEST(PIND, bit)) == 0) {
+	    action(params);
+	    _delay_ms(20);
+	    while ((TEST(PIND, bit)) == 0) {
+	        continue;
+	    }
+	    _delay_ms(20);
+	}
+}
+
+// write a program that increments and decrements a binary number using the buttons, and displays the result on the LEDs
+int main() {
+	MODE_OUTPUT(LED1);
+	MODE_OUTPUT(LED2);
+	MODE_OUTPUT(LED3);
+	MODE_OUTPUT(LED4);
+	MODE_INPUT(BUTTON1);
+	MODE_INPUT(BUTTON2);
+	
+	int value = 0;
+    int max = 15;
+	int min = 0;
+    IncrementParams params = {&value, max};
+
+	while(1) {
+		on_press(SW1, increment_led, &params);
+		on_press(SW2, decrement_led, &params);
+	}
+}
+
+ISR(TIMER0_COMPA_vect) {
+}
diff --git a/module04/ex02/timer.h b/module04/ex02/timer.h
new file mode 100644
index 0000000..df4eaaa
--- /dev/null
+++ b/module04/ex02/timer.h
@@ -0,0 +1,37 @@
+#ifndef TIMER_H
+#define TIMER_H
+
+// table 15-9 : timer 0 prescale sets
+#define T0_PRESCALE_SET(value)  \
+									((value) == 1   ? (0<<CS02 | 0<<CS01 | 1<<CS00) : \
+									 (value) == 8   ? (0<<CS02 | 1<<CS01 | 0<<CS00) : \
+									 (value) == 64  ? (0<<CS02 | 1<<CS01 | 1<<CS00) : \
+									 (value) == 256 ? (1<<CS02 | 0<<CS01 | 0<<CS00) : \
+									 (value) == 1024? (1<<CS02 | 0<<CS01 | 1<<CS00) : \
+													  (0<<CS02 | 0<<CS01 | 0<<CS00))
+#define TIME_MS(ms, prescale_value)	(((F_CPU / prescale_value) * ms) / 1000)
+
+// table 16-5 : timer 1 prescale sets
+#define T1_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))
+#define TIME_MS(ms, prescale_value)	(((F_CPU / prescale_value) * ms) / 1000)
+
+// 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)
+
+#endif // TIMER_H
\ No newline at end of file
diff --git a/module04/ex02/usart.h b/module04/ex02/usart.h
new file mode 100644
index 0000000..b930036
--- /dev/null
+++ b/module04/ex02/usart.h
@@ -0,0 +1,31 @@
+#ifndef USART_H
+#define USART_H
+
+#define BAUD_PRESCALER(usart_baudrate)	(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)
+
+#endif // USART_H
\ No newline at end of file
diff --git a/module04/ex02/utils.h b/module04/ex02/utils.h
new file mode 100644
index 0000000..1dfefcb
--- /dev/null
+++ b/module04/ex02/utils.h
@@ -0,0 +1,23 @@
+#ifndef UTILS_H
+#define UTILS_H
+
+// 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)
+
+// mathematics
+#define PERCENT(percent, total)		(((float)percent / 100) * total)
+#define DIV_ROUND_CLOSEST(n, d)		((((n) < 0) == ((d) < 0)) ? (((n) + (d)/2)/(d)) : (((n) - (d)/2)/(d)))
+
+// text
+#define SWITCH_CASE(ch) (((ch) >= 'A' && (ch) <= 'Z') || ((ch) >= 'a' && (ch) <= 'z') ? ((ch) ^ (1 << 5)) : (ch))
+
+// boolean
+#define TRUE						1
+#define FALSE						0
+
+#endif // UTILS_H
\ No newline at end of file