#include <stm32f4xx.h>
#include <stm32f4xx_i2c.h>
#define SLAVE_ADDRESS 0x3D // the slave address (example)
void init_I2C1(void){

GPIO_InitTypeDef GPIO_InitStruct;
I2C_InitTypeDef I2C_InitStruct;

// enable APB1 peripheral clock for I2C1
RCC_APB1PeriphClockCmd(RCC_APB1Periph_I2C1, ENABLE);
// enable clock for SCL and SDA pins
RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOB, ENABLE);

/* setup SCL and SDA pins
  * You can connect I2C1 to two different
  * pairs of pins:
  * 1. SCL on PB6 and SDA on PB7
  * 2. SCL on PB8 and SDA on PB9
  */
GPIO_InitStruct.GPIO_Pin = GPIO_Pin_6 | GPIO_Pin_7; // we are going to use PB6 and PB7
GPIO_InitStruct.GPIO_Mode = GPIO_Mode_AF;   // set pins to alternate function
GPIO_InitStruct.GPIO_Speed = GPIO_Speed_50MHz;  // set GPIO speed
GPIO_InitStruct.GPIO_OType = GPIO_OType_OD;   // set output to open drain --> the line has to be only pulled low, not driven high
GPIO_InitStruct.GPIO_PuPd = GPIO_PuPd_UP;   // enable pull up resistors
GPIO_Init(GPIOB, &GPIO_InitStruct);	 // init GPIOB

// Connect I2C1 pins to AF 
GPIO_PinAFConfig(GPIOB, GPIO_PinSource6, GPIO_AF_I2C1); // SCL
GPIO_PinAFConfig(GPIOB, GPIO_PinSource7, GPIO_AF_I2C1); // SDA

// configure I2C1
I2C_InitStruct.I2C_ClockSpeed = 100000;   // 100kHz
I2C_InitStruct.I2C_Mode = I2C_Mode_I2C;   // I2C mode
I2C_InitStruct.I2C_DutyCycle = I2C_DutyCycle_2; // 50% duty cycle --> standard
I2C_InitStruct.I2C_OwnAddress1 = 0x00;   // own address, not relevant in master mode
I2C_InitStruct.I2C_Ack = I2C_Ack_Disable;  // disable acknowledge when reading (can be changed later on)
I2C_InitStruct.I2C_AcknowledgedAddress = I2C_AcknowledgedAddress_7bit; // set address length to 7 bit addresses
I2C_Init(I2C1, &I2C_InitStruct);	// init I2C1

// enable I2C1
I2C_Cmd(I2C1, ENABLE);
}
/* This function issues a start condition and
* transmits the slave address + R/W bit
*
* Parameters:
*   I2Cx --> the I2C peripheral e.g. I2C1
*   address --> the 7 bit slave address
*   direction --> the tranmission direction can be:
*	   I2C_Direction_Tranmitter for Master transmitter mode
*	   I2C_Direction_Receiver for Master receiver
*/
void I2C_start(I2C_TypeDef* I2Cx, uint8_t address, uint8_t direction){
// wait until I2C1 is not busy anymore
while(I2C_GetFlagStatus(I2Cx, I2C_FLAG_BUSY));
 
// Send I2C1 START condition
I2C_GenerateSTART(I2Cx, ENABLE);
  
// wait for I2C1 EV5 --> Slave has acknowledged start condition
while(!I2C_CheckEvent(I2Cx, I2C_EVENT_MASTER_MODE_SELECT));
 
// Send slave Address for write
I2C_Send7bitAddress(I2Cx, address, direction);
  
/* wait for I2C1 EV6, check if
  * either Slave has acknowledged Master transmitter or
  * Master receiver mode, depending on the transmission
  * direction
  */
if(direction == I2C_Direction_Transmitter){
  while(!I2C_CheckEvent(I2Cx, I2C_EVENT_MASTER_TRANSMITTER_MODE_SELECTED));
}
else if(direction == I2C_Direction_Receiver){
  while(!I2C_CheckEvent(I2Cx, I2C_EVENT_MASTER_RECEIVER_MODE_SELECTED));
}
}
/* This function transmits one byte to the slave device
* Parameters:
*  I2Cx --> the I2C peripheral e.g. I2C1
*  data --> the data byte to be transmitted
*/
void I2C_write(I2C_TypeDef* I2Cx, uint8_t data)
{
I2C_SendData(I2Cx, data);
// wait for I2C1 EV8_2 --> byte has been transmitted
while(!I2C_CheckEvent(I2Cx, I2C_EVENT_MASTER_BYTE_TRANSMITTED));
}
/* This function reads one byte from the slave device
* and acknowledges the byte (requests another byte)
*/
uint8_t I2C_read_ack(I2C_TypeDef* I2Cx){
// enable acknowledge of recieved data
I2C_AcknowledgeConfig(I2Cx, ENABLE);
// wait until one byte has been received
while( !I2C_CheckEvent(I2Cx, I2C_EVENT_MASTER_BYTE_RECEIVED) );
// read data from I2C data register and return data byte
uint8_t data = I2C_ReceiveData(I2Cx);
return data;
}
/* This function reads one byte from the slave device
* and doesn't acknowledge the recieved data
*/
uint8_t I2C_read_nack(I2C_TypeDef* I2Cx){
// disabe acknowledge of received data
// nack also generates stop condition after last byte received
// see reference manual for more info
I2C_AcknowledgeConfig(I2Cx, DISABLE);
I2C_GenerateSTOP(I2Cx, ENABLE);
// wait until one byte has been received
while( !I2C_CheckEvent(I2Cx, I2C_EVENT_MASTER_BYTE_RECEIVED) );
// read data from I2C data register and return data byte
uint8_t data = I2C_ReceiveData(I2Cx);
return data;
}
/* This funtion issues a stop condition and therefore
* releases the bus
*/
void I2C_stop(I2C_TypeDef* I2Cx){
// Send I2C1 STOP Condition
I2C_GenerateSTOP(I2Cx, ENABLE);
}
int main(void){

init_I2C1(); // initialize I2C peripheral

uint8_t received_data[2];

while(1){
 
  I2C_start(I2C1, SLAVE_ADDRESS<<1, I2C_Direction_Transmitter); // start a transmission in Master transmitter mode
  I2C_write(I2C1, 0x20); // write one byte to the slave
  I2C_write(I2C1, 0x03); // write another byte to the slave
  I2C_stop(I2C1); // stop the transmission
 
  I2C_start(I2C1, SLAVE_ADDRESS<<1, I2C_Direction_Receiver); // start a transmission in Master receiver mode
  received_data[0] = I2C_read_ack(I2C1); // read one byte and request another byte
  received_data[1] = I2C_read_nack(I2C1); // read one byte and don't request another byte, stop transmission
}
}