/*
 * This program reads the second register of a DS3231 RTC and displays
 * the least significant digit on the four LEDs of the EduBase board.
 * The DS3231 Real Time Clock chip is not included in the Edubase board but
 * a connector is provided for a DS3231 module.
 *
 * The DS3231 is connected via I2C. The I2C connections are:
 * I2C1_SCL - PB08
 * I2C1_SDA - PB09
 *
 * The four LEDs are:
 * LED3 - PB7
 * LED2 - PB6
 * LED1 - PB5
 * LED0 - PB4
 */

#include "stm32f4xx.h"

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

#define SLAVE_ADDR 0x68     // 1101 000.

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

    RCC->AHB1ENR |=  2;             /* enable GPIOB clock */
    /* configure LED pins */
    GPIOB->MODER &= ~0x0000ff00;    /* clear pin mode */
    GPIOB->MODER |=  0x00005500;    /* set pins to output mode */

    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) {
    RCC->AHB1ENR |=  2;                     /* Enable GPIOB clock */
    RCC->APB1ENR |=  0x00200000;	        /* Enable I2C1 clock */

    /* configure PB8, PB9 pins for I2C1 */
    GPIOB->AFR[1]   &= ~0x000000FF;         /* PB8, PB9 I2C1 SCL, SDA */
    GPIOB->AFR[1]   |=  0x00000044;
    GPIOB->MODER    &= ~0x000F0000;         /* PB8, PB9 use alternate function */
    GPIOB->MODER    |=  0x000A0000;
    GPIOB->OTYPER   |=  0x00000300;         /* output open-drain */
    GPIOB->PUPDR    &= ~0x000F0000;         /* with pull-ups */
    GPIOB->PUPDR    |=  0x00050000;

    I2C1->CR1       =   0x8000;             /* software reset I2C1 */
    I2C1->CR1       &= ~0x8000;             /* out of reset */
    I2C1->CR2       =   0x0010;             /* peripheral clock is 16 MHz */
    I2C1->CCR       =   80;                 /* standard mode, 100kHz clock */
    I2C1->TRISE     =   17;                 /* maximum rise time */
    I2C1->CR1       |=  0x0001;             /* 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.
 */
char I2C1_byteRead(char saddr, char maddr, char* data)
    {
    volatile int tmp;

    while (I2C1->SR2 & 2);                  /* wait until bus not busy */

    I2C1->CR1 |=  0x100;                    /* generate start */
    while (!(I2C1->SR1 & 1));               /* wait until start flag is set */

    I2C1->DR = saddr << 1;                  /* transmit slave address + Write */
    while (!(I2C1->SR1 & 2));               /* wait until addr flag is set */
    tmp = I2C1->SR2;                        /* clear addr flag */

    while (!(I2C1->SR1 & 0x80));            /* wait until data register empty */
    I2C1->DR = maddr;                       /* send memory address */

    while (!(I2C1->SR1 & 0x80));            /* wait until data register empty */

    I2C1->CR1 |= 0x100;                     /* generate restart */
    while (!(I2C1->SR1 & 1));               /* wait until start flag is set */
    I2C1->DR = saddr << 1 | 1;              /* transmit slave address + Read */

    while (!(I2C1->SR1 & 2));               /* wait until addr flag is set */
    I2C1->CR1 &= ~0x400;                    /* Disable Acknowledge */
    tmp = I2C1->SR2;                        /* clear addr flag */

    I2C1->CR1 |= 0x200;                     /* generate stop after data received */

    while (!(I2C1->SR1 & 0x40));            /* Wait until RXNE flag is set */
    *data++ = I2C1->DR;                     /* Read data from DR */

    return 0;
}

void display(int data) {
    GPIOB->BSRR = 0x00F00000;
    GPIOB->BSRR = data << 4;
}

/* 16 MHz SYSCLK */
void delayMs(int n) {
    int i;
    for (; n > 0; n--)
        for (i = 0; i < 3195; i++) ;
}