/*
* LM75_Tempsensor.c
* Temperaturmessung (5x pro Sekunde) und Ausgabe der Temperatur als Binärwert am PORTD
* Eine negative Temperatur wird zusätzlich durch eine LED an PB0 angezeigt
* CPU: ATmega88
* FCPU: 8MHz
* I²C CLK Frequency = 100kHz, CLK = 8000000/(16 + 2*(0x50)*1)
*/
#include <avr/io.h>
#include <util/delay.h>

/*** Statusregister codes and other defines ***/
#define START           0x08    // A START condition has been transmitted
#define R_START         0x10    // A REPEATED START condition has been transmitted
#define SLA_W           0x90    // Slave Address & Write 1001 0000
#define SLA_R           0x91    // Slave Address & Read 1001 0001
#define MT_SLA_ACK      0x18    // Master Transmit SLA+W has been transmitted & ACK has been received
#define MR_SLA_ACK      0x40    // Master Receive SLA + R has been transmitted & ACK has been received
#define MT_DATA_ACK     0x28    // Master Transmit Data byte has been transmitted & ACK has been received
#define MR_DATA_NACK    0x58    // Master Receive Data byte has been transmitted & NACK has been returned
#define BIT_RATE        0x50    // Set value for the bit rate register TWBR
#define POINTER_ADDRESS 0x00    // LM75 pointer address for temperature register
void ERROR(void);               // Prototyping of function "ERROR"

/*** Function to send a START Condition ***/
void TWI_START(void)
{
    TWCR = (1<<TWINT)|(1<<TWSTA)|(1<<TWEN);
    // Wait for TWINT Flag set. This indicates that the START condition has been transmitted
    while (!(TWCR & (1<<TWINT)));
    // Check value of TWI statusregister. Mask prescaler bits. If status different from START go to ERROR if ((TWSR & 0xF8) != START)
    ERROR();
}

/*** Function to send the Slave Address with ACK in Master Transmit Mode ***/
void TWI_MT_SLA_ACK(void)
{
    // Load SLA_W into TWDR Register
    TWDR = SLA_W;
    // Clear TWINT bit in TWCR to start transmission of address
    TWCR = (1<<TWINT) | (1<<TWEN);
    // Wait for TWINT Flag set. This indicates that the SLA+W has been transmitted, and ACK/NACK has been received.
    while (!(TWCR & (1<<TWINT)));
    // Check value of TWI status register. Mask prescaler bits. If status different from MT_SLA_ACK go to ERROR
    if ((TWSR & 0xF8) != MT_SLA_ACK)
    ERROR();
}

/*** Function to send 8Bit of data with ACK in Master Transmit Mode **/
void TWI_MT_DATA_ACK(void)
{
    // Load DATA into TWDR register
    TWDR = POINTER_ADDRESS;
    // Clear TWINT bit in TWCR to start transmission of data
    TWCR = (1<<TWINT) | (1<<TWEN);
    // Wait for TWINT flag set. This indicates that the DATA has been transmitted, and ACK/NACK has been received.
    while (!(TWCR & (1<<TWINT)));
    // Check value of TWI status register. Mask prescaler bits. If status different from MT_DATA_ACK go to ERROR
    if ((TWSR & 0xF8) != MT_DATA_ACK)
    ERROR();
}

/*** Function to send a REPEATED START condition **/
void TWI_R_START(void)
{
    // Send a START condition
    TWCR = (1<<TWINT)|(1<<TWSTA)|(1<<TWEN);
    // Wait for TWINT Flag set. This indicates that the START condition has been transmitted
    while (!(TWCR & (1<<TWINT)));
    // Check value of TWI statusregister. Mask prescaler bits. If status different from R_START go to ER-ROR
    if ((TWSR & 0xF8) != R_START)
    ERROR();
}

/*** Function to send the Slave Address in Master Receive Mode with Acknowledge **/
void TWI_MR_SLA_ACK(void)
{
    // Load SLA_R into TWDR Register
    TWDR = SLA_R;
    // Clear TWINT bit in TWCR to start transmission of address
    TWCR = (1<<TWINT) | (1<<TWEN);
    // Wait for TWINT Flag set. This indicates that the SLA+W has been transmitted, and ACK/NACK has been received.
    while (!(TWCR & (1<<TWINT)));
    // Check value of TWI status register. Mask prescaler bits. If status different from MR_SLA_ACK go to ERROR
    if ((TWSR & 0xF8) != MR_SLA_ACK)
    ERROR();
}

/*** Function to read one Databyte in Master Receive Mode with NACK ***/
uint8_t TWI_READ_DATABYTE_NACK(void)
{
    // Read one data byte
    TWCR = (1<<TWINT)| (1<<TWEN);
    // Wait for TWINT flag set. This indicates that the DATA has been transmitted, and ACK/NACK has been received.
    while (!(TWCR & (1<<TWINT)));
    // Check value of TWI status register. Mask prescaler bits. If status different from MR_DATA_NACK go to ERROR
    if ((TWSR & 0xF8) != MR_DATA_NACK)
    ERROR();
    return TWDR; // Return the value of data register
}

/*** function to send a STOP condition ***/
void TWI_STOP(void)
{
    TWCR = (1<<TWINT)|(1<<TWEN)|(1<<TWSTO); // Transmit STOP condition
}

/*** function to show a bus error ***/
void ERROR(void)
{
    PORTB |= (1<<PB1); // ERROR LED ON
}

uint8_t LSByte, MSByte;
uint16_t data_16bit, temperature, high_byte, low_byte;

int main(void)
{
    DDRD = 0xFF;        // PORTD = output
    DDRC = 0x3F;        // PC0 ... PC5 = output
    DDRB = 0x03;        // PB0 and PB1 = output
    TWBR = BIT_RATE;    // Set the TWI clock-frequency in bit rate register
    while(1)
    {
        TWI_START();                        // Send START condition
        TWI_MT_SLA_ACK();                   // Master Transmit Slave Address with ACK
        TWI_MT_DATA_ACK();                  // Master Transmit Data with ACK
        TWI_R_START();                      // Send REPEATED START condition
        TWI_MR_SLA_ACK();                   // Send the Slave Address in Master Receive Mode with ACK
        MSByte = TWI_READ_DATABYTE_NACK();  // Read one Databyte without ACK
        LSByte = TWI_READ_DATABYTE_NACK();  // Read one Databyte without ACK
        TWI_STOP();                         // Send STOP condition
        low_byte = LSByte >> 5;               // Shift LSByte 5 digits right
        high_byte = MSByte << 8;          // Shift MSByte 8 digits left
        data_16bit = high_byte + low_byte;  // Store 16bit-result of temp-register in data
        temperature = data_16bit >> 8;        // Shift data 8 digits right to get a 8bit value

        if (MSByte & 0x80)                  // if bit7 = 1 --> negative Temperature
        {
            MSByte = ~MSByte + 1;           // build 2´s complement
            temperature = MSByte >> 3;        // divide by 8
            PORTD = temperature;            // Show negative temperature on PORTD
            PORTB |= (1<<PB0);                // LED for negative Temperature ON
        }

        else                                // if bit7 = 0 --> positive Temperature
        {
            PORTB &= ~(1<<PB0);               // LED for negative Temperature OFF
            PORTD = temperature;            // Show positive temperature on PORTD
        }
        _delay_ms(200);                     // wait 200ms for new readout
    }
}