Create I2C/TWI library and enable it

i2c.c

@@ -0,0 +1,60 @@
+#include <avr/io.h>
+#include "i2c.h"
+
+void i2c_init(uint32_t bitrate) 
+{
+    uint8_t prescaler_options[] = {1, 4, 16, 64};
+    uint8_t prescaler_value = prescaler_options[TWSR & 0x03]; // 0x03 (hex) = 0b00000011 (bin)
+    TWBR = (uint8_t)((F_CPU / bitrate - 16) / (2 * prescaler_value));
+}
+
+void i2c_start(void) 
+{
+  TWCR = (1<<TWINT) | (1<<TWEN) | (1<<TWSTA);
+  while(!(TWCR & (1<<TWINT)));  
+}
+
+void i2c_stop(void)
+{
+  TWCR = (1<<TWINT) | (1<<TWEN) | (1<<TWSTO);
+  while(TWCR & (1<<TWSTO));  
+}
+
+uint8_t i2c_read(uint8_t ack) 
+{
+  TWCR = (1<<TWINT) | (ack<<TWEA) | (1<<TWEN);
+  while(!(TWCR & (1<<TWINT)));
+  return TWDR;
+}
+
+void i2c_write(uint8_t byte) 
+{
+  TWDR = byte;
+  TWCR = (1<<TWINT) | (1<<TWEN);
+  while(!(TWCR & (1<<TWINT)));
+}
+
+void i2c_readbuf(uint8_t sla, uint8_t adr, uint8_t len, uint8_t* buf) 
+{
+  i2c_start();
+  i2c_write(sla);
+  i2c_write(adr);
+  i2c_start();
+  i2c_write(sla + I2C_READ);
+  while(len--) *buf++ = i2c_read(len ? I2C_ACK : I2C_NACK);
+  i2c_stop();
+}
+
+void i2c_writebuf(uint8_t sla, uint8_t adr, uint8_t len, uint8_t* buf) 
+{
+  i2c_start();
+  i2c_write(sla);
+  i2c_write(adr);
+
+  while (len--) 
+  {
+    i2c_write(*buf++);
+  }
+
+  i2c_stop();
+}

i2c.h

@@ -0,0 +1,16 @@
+#ifndef __I2C_H__
+#define __I2C_H__
+
+#define I2C_ACK 1
+#define I2C_NACK 0
+#define I2C_READ 1
+
+void i2c_init(uint32_t bitrate);
+void i2c_start(void);
+void i2c_stop(void);
+uint8_t i2c_read(uint8_t ack);
+void i2c_write(uint8_t byte);
+void i2c_readbuf(uint8_t sla, uint8_t adr, uint8_t len, uint8_t* buf);
+void i2c_writebuf(uint8_t sla, uint8_t adr, uint8_t len, uint8_t* buf);
+
+#endif

main.c

@@ -1,9 +1,13 @@
 #include <avr/io.h>
 #include <avr/interrupt.h>
+#include "i2c.h"
 #include "led.h"
 
+#define I2C_BITRATE 100000UL // 100kHz
+
 int main() 
 {
+  i2c_init(I2C_BITRATE); 
   led_init();
 
   sei();