/*
 * Initialize LCD controller and flash "HELLO" on LCD
 *
 * The LCD controller of EduBase board is connected to
 * a shift register.
 * SCLK - PA5
 * MOSI - PA7
 * PA12 is used for slave select of the shift register.
 *
 * The connections between the shift register and LCD are
 * Bit 0 - RS (register select)
 * Bit 1 - E (enable)
 *
 * Bit 4 - Data 4
 * Bit 5 - Data 5
 * Bit 6 - Data 6
 * Bit 7 - Data 7
 *
 * For simplicity, all delay below 1 ms uses 1 ms.
 */

#include "stm32f4xx.h"

#define RS 1    /* BIT0 mask for reg select */
#define EN 2    /* BIT1 mask for E */

void delayMs(int n);
void LCD_nibble_write(char data, unsigned char control);
void LCD_command(unsigned char command);
void LCD_data(char data);
void LCD_init(void);
void SPI1_write(unsigned char data);

int main(void) {
    /* initialize SPI1 that connects to the shift registers */
    LCD_init();

    while(1) {
        /* Write "HELLO" on LCD */
        LCD_data('H');
        LCD_data('E');
        LCD_data('L');
        LCD_data('L');
        LCD_data('O');
        delayMs(1000);

        /* clear LCD display */
        LCD_command(1);
        delayMs(1000);
    }
}

/* configure SPI1 and the associated GPIO pins */
void LCD_init(void) {
    RCC->AHB1ENR |= 1;              /* enable GPIOA clock */
    RCC->AHB1ENR |= 4;              /* enable GPIOC clock */
    RCC->APB2ENR |= 0x1000;         /* enable SPI1 clock */

    /* PORTA 5, 7 for SPI1 MOSI and SCLK */
    GPIOA->MODER &= ~0x0000CC00;    /* clear pin mode */
    GPIOA->MODER |=  0x00008800;    /* set pin alternate mode */
    GPIOA->AFR[0] &= ~0xF0F00000;   /* clear alt mode */
    GPIOA->AFR[0] |=  0x50500000;   /* set alt mode SPI1 */

    /* PA12 as GPIO output for SPI slave select */
    GPIOA->MODER &= ~0x03000000;    /* clear pin mode */
    GPIOA->MODER |=  0x01000000;    /* set pin output mode */

    /* initialize SPI1 module */
    SPI1->CR1 = 0x31F;
    SPI1->CR2 = 0;
    SPI1->CR1 |= 0x40;              /* enable SPI1 module */

    /* LCD controller reset sequence */
    delayMs(20);
    LCD_nibble_write(0x30, 0);
    delayMs(5);
    LCD_nibble_write(0x30, 0);
    delayMs(1);
    LCD_nibble_write(0x30, 0);
    delayMs(1);
    LCD_nibble_write(0x20, 0);  /* use 4-bit data mode */
    delayMs(1);
    LCD_command(0x28);          /* set 4-bit data, 2-line, 5x7 font */
    LCD_command(0x06);          /* move cursor right */
    LCD_command(0x01);          /* clear screen, move cursor to home */
    LCD_command(0x0F);          /* turn on display, cursor blinking */
}

void LCD_nibble_write(char data, unsigned char control) {
    data &= 0xF0;       /* clear lower nibble for control */
    control &= 0x0F;    /* clear upper nibble for data */
    SPI1_write (data | control);           /* RS = 0, R/W = 0 */
    SPI1_write (data | control | EN);      /* pulse E */
    delayMs(0);
    SPI1_write (data);
}

void LCD_command(unsigned char command) {
    LCD_nibble_write(command & 0xF0, 0);    /* upper nibble first */
    LCD_nibble_write(command << 4, 0);      /* then lower nibble */

    if (command < 4)
        delayMs(2);         /* command 1 and 2 needs up to 1.64ms */
    else
        delayMs(1);         /* all others 40 us */
}

void LCD_data(char data) {
    LCD_nibble_write(data & 0xF0, RS);      /* upper nibble first */
    LCD_nibble_write(data << 4, RS);        /* then lower nibble */

    delayMs(1);
}

/* This function enables slave select, writes one byte to SPI1, */
/* wait for transmit complete and deassert slave select. */
void SPI1_write(unsigned char data) {
    while (!(SPI1->SR & 2)) {}      /* wait until Transfer buffer Empty */
    GPIOA->BSRR = 0x10000000;       /* assert slave select */
    SPI1->DR = data;                /* write data */
    while (SPI1->SR & 0x80) {}      /* wait for transmission done */
    GPIOA->BSRR = 0x00001000;       /* deassert slave select */
}

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