main.c
// Sorry for my poor English (LOL)

#include <avr/io.h>
#include <avr/interrupt.h>
#define F_CPU 16000000UL
#define SCL 100000UL
#define LCD_ADDRESS 0x20
#define RFID_ADDRESS 0x48
#define BUTTON_UP 0x08
#define BUTTON_DOWN 0x04
#define BUTTON_LEFT 0x10
#define BUTTON_RIGHT 0x02
#define BUTTON_SELECT 0x01
// up : mask to check which button was pushed :)

ISR(TWI_vect) { //CPU jumps here when TWINT = 0
	uint8_t status = TWSR & (~0x7);
	if (status != 0x08 || status != 0x18 || status != 0x28 || status != 0x30) {
		/*
		 * Normally check if there is no error.
		 * Normally doing a sort of routine but I did not decide what to do yet.
		 * So I'm screwed xD
		 */
	}
	TWCR |= (1 << TWINT); // put the interrupt flag off (not always clean by hardware)
	//also I think TWDR (so data register) is writable only when TWINT = 0, not sure xD
}

void twi_init(void) {
	DDRC &= ~((1 << PC5) | (1 << PC4)); //force it to be an input
	PORTC |= (1 << PC5) | (1 << PC4); //internal pull-up activated
	TWBR = ((F_CPU / SCL) - 16) / 2; //clock frequency
	TWCR |= (1 << TWIE) | (1 << TWEN) | (1 << TWEA) | (1 << TWINT);
	/*
	 * Enable TWI, enable TWI interrupt and set to 1 (so unused)
	 * Enable ACK control when in master receiver mode,
	 * but disable it when the last byte is received, master have to put a NACK bit
	 * but I don't know how to do it (so I'm screwed, for the second time LOL.)
	 */
	sei();
	//set global interrupt enable in SREG register
}

void twi_start(void) {
	TWCR |= (1 << TWSTA); // put a start condition on the line
	while ((TWCR & (1 << TWINT)) == 0)
		//wait to see if start condition is sent correctly
		// then it check in the interrupt function the status (TWSR)
		;
	TWCR &= ~(1 << TWSTA); // Have to do it or start condition will be sent again
}
void twi_write(uint8_t slave_address, uint8_t data) { // write = 0, read = 1
	TWDR = (slave_address << 1) & 0xFE; //put slave address and w/r bit next to it :)
	while ((TWCR & (1 << TWINT)) == 0)
		// wait until the flag is equal to 0 (so it is on for the TWI)
		;
	TWDR = data; //put data to transmit into buffer
	while ((TWCR & (1 << TWINT)) == 0)
		//wait until the flag is == 0
		;
}

void twi_read(uint8_t slave_address) { //didn't finish this
	TWDR = (slave_address << 1) | 1; /* have to scroll it left 'cause
	 it's a seven bit address and the bit 0 is for R/W order
	 */
	while ((TWCR & (1 << TWINT)) == 0)
		// wait for status again and blablabla
		;
}

void twi_stop(void) { //stops communication but not I2C function on the two pins
	TWCR |= (1 << TWSTO);
	while ((TWCR & (1 << TWINT)) == 0)
		;
	TWCR &= ~(1 << TWSTO); //same reason as the start condition
}

int main(void) {
	/*
	 * To use the LCD, you have to ask him (address) something,
	 * so a command and then you can read or write to thing
	 * you wanted to change
	 */
	twi_init();
	twi_start();
	char salut[] = { "Hello" };
	for (int8_t i = 0; i < 6; i++) {
		twi_write(LCD_ADDRESS, salut[i]); //doesn't work :'(
	}
	for (;;) { //infinite loop :D
	}

}