/* p4_4.c UART4 echo at 9600 Baud
 *
 * This program receives a character from UART4 receiver
 * then sends it back through UART4 transmitter.
 * UART4 is connected to PA0-Tx and PA1-Rx.
 * A 3.3V signal level to USB cable is used to connect PA0/PA1
 * to the host PC COM port.
 * PA0 - UART4 TX (AF8)
 * PA1 - UART4 RX (AF8)
 * Use Tera Term on the host PC to send keystrokes and observe the display
 * of the characters echoed.
 *
 * By default, the clock is running at 16 MHz.
 *
 * 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);
char UART4_read(void);

int main (void) {
    char c;
    UART4_init();

    while (1) {
        c = UART4_read();
        UART4_write(c);
    }
}

/* initialize UART4 to transmit at 9600 Baud */
void UART4_init (void) {
    RCC->AHB1ENR |= 1;          /* Enable GPIOA clock */
    RCC->APB1ENR |= 0x80000;    /* Enable UART4 clock */

    /* Configure PA0, PA1 for UART4 TX, RX */
    GPIOA->AFR[0] &= ~0x00FF;
    GPIOA->AFR[0] |=  0x0088;   /* alt8 for UART4 */
    GPIOA->MODER  &= ~0x000F;
    GPIOA->MODER  |=  0x000A;   /* enable alternate function for PA0, PA1 */

    UART4->BRR = 0x0683;        /* 9600 baud @ 16 MHz */
    UART4->CR1 = 0x000C;        /* enable Tx, Rx, 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);
}

/* Read a character from UART4 */
char UART4_read(void) {
    while (!(UART4->SR & 0x0020)) {}   // wait until char arrives
    return UART4->DR;
}