/* Read register 0 (second count) of DS3231
 *
 * This program reads register at address 0 in the DS3231, the second count, 
 * repetitively and dump the data to the LEDs.  Since the second count uses 
 * binary coded decimal format, the LEDs will count up from 0 to 9 in binary
 * and wrap around to 0.
 *
 * Built and tested with Keil MDK-ARM v5.24a and TM4C_DFP v1.1.0
 */

#include "TM4C123GH6PM.h"

void delayMs(int n);
void I2C1_init(void);
unsigned char I2C1_byteRead(int slaveAddr, unsigned char memAddr, unsigned char* data);

#define SLAVE_ADDR 0x68     // 1101 000.

int main(void) {
    unsigned char readback;
    unsigned char error;

    SYSCTL->RCGCGPIO |= 0x02;   // enable clock to GPIOB
    readback = SYSCTL->RCGCGPIO;
    // enable PORTB 3-0 as output for LEDs
    GPIOB->DIR |= 0x0F;         // set PORTB 3-0 as output pins
    GPIOB->DEN |= 0x0F;         // set PORTB 3-0 as digital pins

    I2C1_init();
    
    for (;;) {
        // use second count to turn LEDs on or off
        error = I2C1_byteRead(SLAVE_ADDR, 0, &readback);
        if (error)
            for(;;) ;   // replace with error handling

        GPIOB->DATA = readback;
        delayMs(200);
    }
}

void I2C1_init(void) {
    SYSCTL->RCGCI2C |= 0x02;    // enable clock to I2C1
    SYSCTL->RCGCGPIO |= 0x01;   // enable clock to GPIOA

    // PORTA 7, 6 for I2C1
    GPIOA->AFSEL |= 0xC0;       // PORTA 7, 6 for I2C1
    GPIOA->PCTL &= ~0xFF000000; // PORTA 7, 6 for I2C1
    GPIOA->PCTL |= 0x33000000;
    GPIOA->DEN |= 0xC0;         // PORTA 7, 6 as digital pins
	GPIOA->ODR |= 0x80;         // PORTA 7 as open drain

    I2C1->MCR = 0x10;           // master mode
    I2C1->MTPR = 24;            // 100 kHz @ 50 MHz
}

static int I2C1_wait_till_done(void) {
    while(I2C1->MCS & 1);       // wait until I2C1 master is not busy
    return I2C1->MCS & 0xE;     // return I2C1 error code
}

// Read one byte of data at memAddr
unsigned char I2C1_byteRead(int slaveAddr, unsigned char memAddr, unsigned char* data) {
    unsigned char error;

    // read: S-(addr+w)-ACK-memAddr-ACK-R-(addr+r)-ACK-data-NACK-P
    I2C1->MSA = slaveAddr << 1;
    I2C1->MDR = memAddr;
    I2C1->MCS = 3;                      // S-(addr+w)-ACK-memAddr-ACK
    error = I2C1_wait_till_done();
    if (error)
        return error;

    I2C1->MSA = (slaveAddr << 1) + 1;   // restart: -R-(addr+r)-ACK
    I2C1->MCS = 7;                      // -S-(addr+r)-NACK-P
    error = I2C1_wait_till_done();
    if (error)
        return error;

    *data = I2C1->MDR;

    while(I2C1->MCS & 0x40);            // wait until bus is not busy
    error = I2C1->MCS & 0xE;
    
    return error;
}

/* delay n milliseconds (50 MHz CPU clock) */
void delayMs(int n) {
    int i, j;
    for(i = 0 ; i< n; i++)
        for(j = 0; j < 6265; j++)
            {}  /* do nothing for 1 ms */
}