/* Initialize LCD controller and flash HELLO on LCD.
 *
 * The LCD controller of Wytec EduPad board is connected to
 * a shift register on SSI2 of Tiva LaunchPad.  PORTC 6 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 2 - 
 * Bit 4 - Data 4
 * Bit 5 - Data 5
 * Bit 6 - Data 6
 * Bit 7 - Data 7
 *
 * Built and tested with Keil MDK-ARM v5.24a and TM4C_DFP v1.1.0
 */

#include "TM4C123.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 SSI2_Write(unsigned char data);

int main(void) {
    LCD_init();     // initialize LCD controller

    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);
    }
}

// initialize SSI2 then initialize LCD controller
void LCD_init(void) {
    SYSCTL->RCGCSSI |= 0x04;    // enable clock to SSI2
    SYSCTL->RCGCGPIO |= 0x02;   // enable clock to GPIOB
    SYSCTL->RCGCGPIO |= 0x04;   // enable clock to GPIOC

    // PORTB 7, 4 for SSI2 TX and SCLK
    GPIOB->AMSEL &= ~0x90;      // turn off analog of PORTB 7, 4
    GPIOB->AFSEL |= 0x90;       // PORTB 7, 4 for alternate function
    GPIOB->PCTL &= ~0xF00F0000; // clear functions for PORTB 7, 4
    GPIOB->PCTL |= 0x20020000;  // PORTB 7, 4 for SSI2 function
    GPIOB->DEN |= 0x90;         // PORTB 7, 4 as digital pins

    // PORTC 6 for SSI2 slave select
    GPIOC->AMSEL &= ~0x40;      // disable analog
    GPIOC->DATA |= 0x40;        // set PORTC6 idle high
    GPIOC->DIR |= 0x40;         // set PORTC6 as output for CS
    GPIOC->DEN |= 0x40;         // set PORTC6 as digital pins

    SSI2->CR1 = 0;              // make it master
    SSI2->CC = 0;               // use system clock
    SSI2->CPSR = 16;            // clock prescaler divide by 16 gets 1 MHz clock
    SSI2->CR0 = 0x0007;         // clock rate div by 1, phase/polarity 0 0, mode freescale, data size 8
    SSI2->CR1 = 2;              // enable SSI2

    delayMs(20);                // LCD controller reset sequence
    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 SSI2_Write(unsigned char data) {
    GPIOC->DATA &= ~0x40;       // assert chip select
    SSI2->DR = data;            // write data
    while (SSI2->SR & 0x10) ;   // wait for transmit done
    GPIOC->DATA |= 0x40;        // deassert chip select
}

void LCD_nibble_write(char data, unsigned char control) {
    data &= 0xF0;       // clear lower nibble for control
    control &= 0x0F;    // clear upper nibble for data
    SSI2_Write (data | control);           // RS = 0, R/W = 0
    SSI2_Write (data | control | EN);      // pulse E
    delayMs(0);
    SSI2_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);
}

/* delay n milliseconds (50 MHz CPU clock) */
void delayMs(int n) {
    int i, j;
    for(i = 0 ; i< n; i++)
        for(j = 0; j < 6265; j++)
            {}  /* do nothing for 1 ms */
}