/* P9_2.c - I2C byte read from a DS3231
 *
 * This program initializes the I2C and continuously reads
 * the register 0 of the DS3231.
 * Register 0 contains the second count. The clock is powered-up
 * enabled and the second count is incrementing. The bit 0 is
 * used to turn on and off the LED of the Nucleo board. The LED
 * should blink every second.
 * No errors or acknowledgment are checked.
 *
 * The connections
 * I2C1_SCL - PB8
 * I2C1_SDA - PB9
 *
 * This program was tested with Keil uVision v5.23 with DFP v2.0.0
 */

#include "stm32f0xx.h"
#define SLAVE_ADDR 0x68    /* 1101 000.    DS3231 */

void delayMs(int n);

void I2C1_init(void);
int I2C1_byteRead(char saddr, char maddr, char* data);
void delayMs(int n);

int Main(void)
{
    char data;

    I2C1_init();

    /* configure PA5 for the green LED (LD2) */
    RCC->AHBENR |=  RCC_AHBENR_GPIOAEN; /* enable GPIOA clock */
    GPIOA->MODER &= ~0x00000C00;        /* clear pin mode */
    GPIOA->MODER |=  0x00000400;        /* set pin to output mode */

    while (1) {
        I2C1_byteRead(SLAVE_ADDR, 0, &data);
        if (data & 1)
            GPIOA->ODR |=  0x00000020;  /* turn on LED */
        else
            GPIOA->ODR &= ~0x00000020;  /* turn off LED */

        delayMs(10);
    }
}

void I2C1_init(void) {
    RCC->AHBENR  |=  0x00040000;		/* Enable GPIOB clock */
    RCC->APB1ENR |=  0x00200000;		/* Enable I2C1 clock */

    /* configure PB8, PB9 pins for I2C1 */
    GPIOB->MODER    &= ~0x000F0000;     /* PB8, PB9 use alternate function */
    GPIOB->MODER    |=  0x000A0000;
    GPIOB->AFR[1]   &= ~0x000000FF;     /* PB8, PB9 I2C1 SCL, SDA */
    GPIOB->AFR[1]   |=  0x00000011;
    GPIOB->OTYPER   |=  0x00000300;     /* output open-drain */
    GPIOB->PUPDR    &= ~0x000F0000;     /* with pull-ups */
    GPIOB->PUPDR    |=  0x00050000;

    I2C1->CR1       = 0;                /* software reset I2C1 */
    I2C1->TIMINGR   = 0x10420F13;       /* 100 KHz, peripheral clock is 8 MHz */
    I2C1->CR1       = 0x00000001;       /* enable I2C1 module */
}

/* this funtion reads a byte of data from the memory location
 * maddr of a device with I2C slave device address saddr.
 * For simplicity, no error checking or error report is done.
 */
int I2C1_byteRead(char saddr, char maddr, char* data) {
    while (I2C1->ISR & 0x8000);             /* wait until bus not busy */

    I2C1->CR2 = 0x00012000 | (saddr << 1);  /* generate start and send slave address */
    while (!(I2C1->ISR & 0x2));             /* wait until start flag is set */

    I2C1->TXDR = maddr;                     /* send memory address */
    while (!(I2C1->ISR & 0x40));            /* wait until transfer complete */

    /* generate restart, send slave address, read 1 byte, set for auto end */
    I2C1->CR2 = 0x02002400 | (1 << 16) | (saddr << 1);

    while (!(I2C1->ISR & 0x4));             /* Wait until RXNE flag is set */
    *data = I2C1->RXDR;                     /* Read data from DR */

    while (!(I2C1->ISR & 0x20));            /* wait until stop flag is set */

    I2C1->ICR = 0x20;                       /* clear stop flag */
    I2C1->CR2 = 0x02000000;                 /* clear configuration register 2 */

    return 0;
}

/* 8 MHz SYSCLK */
void delayMs(int n) {
    int i;
    for (; n > 0; n--)
        for (i = 0; i < 1142; i++) ;
}