/* p4_3.c Send a string "YES" to UART4
 *
 * UART4 Tx signal is connected to pin PA0. To see the output of UART4
 * on a PC, you need to use a USB-serial module. Connect the Rx pin of
 * the module to the PA0 pin of the STM32F446 Nucleo board. Make sure
 * the USB-serial module you use has a 3.3V interface.
 *
 * By default, the clock is running at 16 MHz.
 * The UART is configured for 9600 Baud.
 * PA0 - UART4 TX (AF8)
 *
 * This program was tested with Keil uVision v5.24a with DFP v2.11.0
 */
#include "stm32F4xx.h"

void UART4_init(void);
void UART4_write(int c);
void delayMs(int);

/*----------------------------------------------------------------------------
  MAIN function
 *----------------------------------------------------------------------------*/
int main (void) {
    char message[] = "Hello\r\n";
    int i;

    UART4_init();

    while (1) {
        for (i = 0; i < 7; i++) {
            UART4_write(message[i]); /* send a char */
        }
        delayMs(10);                /* leave a gap between messages */
    }
}

/*----------------------------------------------------------------------------
  Initialize UART pins, Baudrate
 *----------------------------------------------------------------------------*/
void UART4_init (void) {
    RCC->AHB1ENR |= 1;          /* Enable GPIOA clock */
    RCC->APB1ENR |= 0x80000;    /* Enable UART4 clock */

    /* Configure PA0 for UART4 TX */
    GPIOA->AFR[0] &= ~0x000F;
    GPIOA->AFR[0] |=  0x0008;   /* alt8 for UART4 */
    GPIOA->MODER  &= ~0x0003;
    GPIOA->MODER  |=  0x0002;   /* enable alternate function for PA0 */

    UART4->BRR = 0x0683;       /* 9600 baud @ 16 MHz */
    UART4->CR1 = 0x0008;       /* enable Tx, 8-bit data */
    UART4->CR2 = 0x0000;       /* 1 stop bit */
    UART4->CR3 = 0x0000;       /* no flow control */
    UART4->CR1 |= 0x2000;      /* enable UART4 */
}


/* Write a character to UART4 */
void UART4_write (int ch) {
    while (!(UART4->SR & 0x0080)) {}   // wait until Tx buffer empty
    UART4->DR = (ch & 0xFF);
}

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