/* P9_1.c - I2C byte write to a DS3231
 *
 * This program initializes the I2C and sends a
 * command to the DS3231 to turn on the 1 Hz output.
 * 1 Hz output is an open-drain output. It needs a pull-up
 * to observe the signal.
 * 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_byteWrite(char saddr, char maddr, char data);

int Main(void)
{
    I2C1_init();

    I2C1_byteWrite(SLAVE_ADDR, 0x0E, 0x00);

    while(1)
    {
    }
}

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 function writes a byte of data to 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_byteWrite(char saddr, char maddr, char data) {
    while (I2C1->ISR & 0x8000);             /* wait until bus not busy */

    I2C1->CR2 = 0x02002000| (2 << 16)       /* generate start, autoend, byte count 2 */
                          | (saddr << 1);   /* and send slave address */

    while (!(I2C1->ISR & 0x02));            /* wait until TXIS is set */
    I2C1->TXDR = maddr;                     /* send register address */

    while (!(I2C1->ISR & 0x02));            /* wait until TXIS is set */
    I2C1->TXDR = data;                      /* send data */

    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;
}