/* p4_2.c UART on USART2 Receive at 115200 Baud
 *
 * Receive key strokes from terminal emulator (TeraTerm) of the
 * host PC to the USART2 of the Nucleo-F446RE board. 
 * USART2 is connected to the ST-Link virtual COM port.
 * Launch Tera Term on a PC and hit any key.
 * The LED program from P3_5 of Chapter 3 is used to blink the green LED
 * according to the key received. 
 * You need to wait till the blinking stops before hitting another key.
 * Received key is not echoed back to the terminal, so you will not see
 * the character displayed.
 *
 * By default, the clock is running at 16 MHz.
 * The UART2 is configured for 115200 Baud.
 * PA3 - USART2 RX (AF7)
 *
 * This program was tested with Keil uVision v5.24a with DFP v2.11.0
 */
#include "stm32F4xx.h"

void USART2_init(void);
char USART2_read(void);
void LED_blink(int value);
void delayMs(int);

/*----------------------------------------------------------------------------
  MAIN function
 *----------------------------------------------------------------------------*/
int main (void) {
    char c;

    RCC->AHB1ENR |=  1;             /* enable GPIOA clock */
    GPIOA->MODER &= ~0x00000C00;    /* clear pin mode */
    GPIOA->MODER |=  0x00000400;    /* set pin to output mode */

    USART2_init();                  /* initialize USART2 */

    while(1) {                      /* Loop forever */
        c = USART2_read();          /* wait for a character received */
        LED_blink(c);               /* blink the LED */
    }
}

/*----------------------------------------------------------------------------
  Initialize UART pins, Baudrate
 *----------------------------------------------------------------------------*/
void USART2_init (void) {
    RCC->AHB1ENR |= 1;          /* Enable GPIOA clock */
    RCC->APB1ENR |= 0x20000;    /* Enable USART2 clock */

    /* Configure PA3 for USART2 RX */
    GPIOA->AFR[0] &= ~0xF000;
    GPIOA->AFR[0] |=  0x7000;   /* alt7 for USART2 */
    GPIOA->MODER  &= ~0x00C0;
    GPIOA->MODER  |=  0x0080;   /* enable alternate function for PA3 */

    USART2->BRR = 0x008B;       /* 115200 baud @ 16 MHz */
    USART2->CR1 = 0x0004;       /* enable Rx, 8-bit data */
    USART2->CR2 = 0x0000;       /* 1 stop bit */
    USART2->CR3 = 0x0000;       /* no flow control */
    USART2->CR1 |= 0x2000;      /* enable USART2 */
}

/* Read a character from USART2 */
char USART2_read(void) {
    while (!(USART2->SR & 0x0020)) {}   // wait until char arrives
    return USART2->DR;
}

/* turn on or off the LEDs according to the value */
void LED_blink(int value) {
    value %= 16;                    /* cap the max count at 15 */

    for (; value > 0; value--) {
        GPIOA->BSRR = 0x00000020;   /* turn on LED */
        delayMs(200);
        GPIOA->BSRR = 0x00200000;   /* turn off LED */
        delayMs(200);
    }
    delayMs(800);
}

void delayMs(int n) {
    int i;
    for (; n > 0; n--)
        for (i = 0; i < 2000; i++) ;
}