/* p4_1.c Send a string "YES" to USART2
 *
 * USART2 is connected to the ST-Link virtual COM port.
 * Use Tera Term to see the message "YES" on a PC.
 *
 * By default, the clock is running at 16 MHz.
 * The UART2 is configured for 9600 Baud.
 * PA2 - USART2 TX (AF7)
 *
 * This program was tested with Keil uVision v5.24a with DFP v2.11.0
 */
#include "stm32F4xx.h"

void USART2_init(void);
void USART2_write(int c);
void delayMs(int);

/*----------------------------------------------------------------------------
  MAIN function
 *----------------------------------------------------------------------------*/
int main (void) {
    USART2_init();          /* initialize USART2 */

    while(1) {              /* Loop forever */
        USART2_write('Y');
        USART2_write('e');
        USART2_write('s');
        delayMs(10);        /* leave a gap between messages */
    }
}

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

    /* Configure PA2 for USART2_TX */
    GPIOA->AFR[0] &= ~0x0F00;
    GPIOA->AFR[0] |=  0x0700;   /* alt7 for USART2 */
    GPIOA->MODER  &= ~0x0030;
    GPIOA->MODER  |=  0x0020;   /* enable alternate function for PA2 */

    USART2->BRR = 0x0683;       /* 9600 baud @ 16 MHz */
    USART2->CR1 = 0x0008;       /* enable Tx, 8-bit data */
    USART2->CR2 = 0x0000;       /* 1 stop bit */
    USART2->CR3 = 0x0000;       /* no flow control */
    USART2->CR1 |= 0x2000;      /* enable USART2 */
}


/* Write a character to USART2 */
void USART2_write (int ch) {
    while (!(USART2->SR & 0x0080)) {}   // wait until Tx buffer empty
    USART2->DR = (ch & 0xFF);
}

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