/* Seven-segment display counter
 *
 * This program counts number 0-3 on the seven segment display.
 * The seven segment display is driven by a shift register which is
 * connected to SSI2 in SPI mode.
 *
 * Built and tested with Keil MDK-ARM v5.28 and TM4C_DFP v1.1.0
 */

#include "TM4C123.h"

void delayMs(int n);
void sevenseg_init(void);
void SSI2_write(unsigned char data);

int main(void) {
    const static unsigned char digitPattern[] = {0xB0, 0xA4, 0xF9, 0xC0};
    int i;

    sevenseg_init();    // initialize SSI2 that connects to the shift registers

    while(1) {
        SSI2_write(digitPattern[i]);    // write digit pattern to the seven segments
		SSI2_write((1 << i));  			// select digit
        if (++i > 3)
            i = 0;
        delayMs(4);                     // 1000 / 60 / 4 = 4.17
    }
}

// enable SSI2 and associated GPIO pins
void sevenseg_init(void) {
    SYSCTL->RCGCGPIO |= 0x02;   // enable clock to GPIOB
    SYSCTL->RCGCGPIO |= 0x04;   // enable clock to GPIOC
    SYSCTL->RCGCSSI |= 0x04;    // enable clock to SSI2

    // 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 7 for SSI2 slave select
    GPIOC->AMSEL &= ~0x80;      // disable analog of PORTC 7
    GPIOC->DATA |= 0x80;        // set PORTC 7 idle high
    GPIOC->DIR |= 0x80;         // set PORTC 7 as output for SS
    GPIOC->DEN |= 0x80;         // set PORTC 7 as digital pin

    SSI2->CR1 = 0;              // turn off SSI2 during configuration
    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 as master
}

// This function enables slave select, writes one byte to SSI2,
// wait for transmit complete and deassert slave select.
void SSI2_write(unsigned char data) {
    GPIOC->DATA &= ~0x80;       // assert slave select
    SSI2->DR = data;            // write data
    while (SSI2->SR & 0x10) {}  // wait for transmit done
    GPIOC->DATA |= 0x80;        // deassert slave select
}

/* 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 */
}