/* 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.
 *
 * I2C1SCL P6.5
 * I2C1SDA P6.4
 *
 * Built and tested with MSP432P401R Rev. C, Keil MDK-ARM v5.24a and MSP432P4xx_DFP v3.1.0
 */

#include "msp.h"

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

#define SLAVE_ADDR 0x68     // 1101 000.

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

    P2->DIR |= 0x30;     /* P2.5, P2.4 set as output for LEDs */
    P3->DIR |= 0x0C;     /* P3.3, P3.2 set as output for LEDs */

    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

        display(readback);
        delayMs(200);
    }
}

/* configure UCB1 as I2C */
void I2C1_init(void) {
    extern uint32_t SystemCoreClock;
    EUSCI_B1->CTLW0 |= 1;           /* disable UCB1 during config */
    EUSCI_B1->CTLW0 = 0x0F81;       /* 7-bit slave addr, master, I2C, synch mode, use SMCLK */
    EUSCI_B1->BRW = 30;             /* set clock prescaler for 100kHz */
    P6->SEL0 |= 0x30;               /* P6.5, P6.4 for UCB1 */
    P6->SEL1 &= ~0x30;
    EUSCI_B1->CTLW0 &= ~1;          /* enable UCB1 after config */
}

// Read one byte of data at memAddr
unsigned char I2C1_byteRead(int slaveAddr, unsigned char memAddr, unsigned char* data) {
    EUSCI_B1->CTLW1 &= ~0x000C;       /* no automatic stop generation */
    EUSCI_B1->I2CSA = slaveAddr;      /* send slave address */
    EUSCI_B1->CTLW0 |= 0x0010;        /* enable transmitter */
    EUSCI_B1->CTLW0 |= 0x0002;        /* generate start */
    while((EUSCI_B1->CTLW0 & 2));   /* wait until slave address is sent */
    EUSCI_B1->TXBUF = memAddr;        /* send memory address to slave */
    while(!(EUSCI_B1->IFG & 2));      /* wait till it's ready to transmit */
    EUSCI_B1->CTLW0 &= ~0x0010;       /* enable receiver */
    EUSCI_B1->CTLW0 |= 0x0002;        /* generate restart */
    while(EUSCI_B1->CTLW0 & 2);       /* wait till restart is finished */
    
    /* receive one byte data */
    EUSCI_B1->CTLW0 |= 0x0004;    /* send STOP */
    while(!(EUSCI_B1->IFG & 1));  /* wait till data is received */
    *data++ = EUSCI_B1->RXBUF;
    while(EUSCI_B1->CTLW0 & 4) ;      /* wait until stop is sent */

    return 0;                   /* no error */
}

/* system clock at 3 MHz, MSP432P401R Rev. C, Start-up v2.2.1 */
void delayMs(int n) {
    int i, j;
    
    for (j = 0; j < n; j++)
        for (i = 750; i > 0; i--);      /* Delay */
}

void display(int data) {		
    if (data & 8)              /* bit 3 -> LED3 */
        P2->OUT |= 0x10;
    else
        P2->OUT &= ~0x10;
    
    if (data & 4)              /* bit 2 -> LED2 */
        P2->OUT |= 0x20;
    else
        P2->OUT &= ~0x20;
    
    if (data & 2)              /* bit 1 -> LED1 */
        P3->OUT |= 8;
    else
        P3->OUT &= ~8;
    
    if (data & 1)              /* bit 0 -> LED0 */
        P3->OUT |= 4;
    else
        P3->OUT &= ~4;        
}