/*
 * Display number "2018" on the seven-segment LED of the EduBase board.
 * The four digit seven segment LED is driven by two shift registers,
 * one to control the anodes (segments) and the other the common cathodes
 * (digit select).  The shift registers are daisy-chained to SPI1.
 * The pin assignments are:
 * SCLK    - PA5
 * MOSI    - PA7
 * 7SEG_CS - PA9
 *
 * Each digit is enabled for 4 ms.  With four digits, the cycle time is
 * 4 ms * 4 = 16 ms that translates to 62.5 Hz.
 */

#include "stm32f4xx.h"

void delayMs(int n);
void SPI1_init(void);
void SPI1_write(unsigned char data);

int main(void) {
    /* initialize SPI1 that connects to the shift registers */
    SPI1_init();

    while(1) {
        SPI1_write(~0x5B);   /* write pattern 2 to the seven segments */
        SPI1_write(1 << 3);  /* select digit 3 */
        delayMs(4);

        SPI1_write(~0x3F);   /* write pattern 0 to the seven segments */
        SPI1_write(1 << 2);  /* select digit 2 */
        delayMs(4);

        SPI1_write(~0x06);   /* write pattern 1 to the seven segments */
        SPI1_write(1 << 1);  /* select digit 1 */
        delayMs(4);

        SPI1_write(~0x7F);   /* write pattern 8 to the seven segments */
        SPI1_write(1 << 0);  /* select digit 0 */
        delayMs(4);
    }
}

/* configure SPI1 and the associated GPIO pins */
void SPI1_init(void) {
    RCC->AHB1ENR |= 1;              /* enable GPIOA 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 */

    /* PA9 as GPIO output for SPI slave select */
    GPIOA->MODER &= ~0x000C0000;    /* clear pin mode */
    GPIOA->MODER |=  0x00040000;    /* set pin output mode */

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

/* 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 = 0x02000000;       /* assert slave select */
    SPI1->DR = data;                /* write data */
    while (SPI1->SR & 0x80) {}      /* wait for transmission done */
    GPIOA->BSRR = 0x00000200;       /* deassert slave select */
}

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