/* p4_4.c USART1 echo at 9600 Baud
 *
 * This program receives a character from USART1 receiver
 * then sends it back through USART1 transmitter.
 * USART1 is connected to PA9-Tx and PA10-Rx.
 * A 3.3V signal level to USB cable is used to connect PA9/PA10
 * to the host PC COM port.
 * PA9  - USART1 TX (AF1)
 * PA10 - USART1 RX (AF1)
 * 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 8 MHz.
 * The UART2 is configured for 9600 Baud.
 *
 * This program was tested with Keil uVision v5.23 with DFP v2.0.0
 */
#include "stm32F0xx.h"

void USART1_init(void);
void USART1_write(int c);
char USART1_read(void);

/*----------------------------------------------------------------------------
  MAIN function
 *----------------------------------------------------------------------------*/
int main (void) {
    char c;
    USART1_init();

    while (1) {
        c = USART1_read();
        USART1_write(c);
    }
}

/*----------------------------------------------------------------------------
  Initialize UART pins, Baudrate
 *----------------------------------------------------------------------------*/
void USART1_init (void) {
    RCC->AHBENR |= RCC_AHBENR_GPIOAEN;          /* Enable GPIOA clock */
    RCC->APB2ENR |= RCC_APB2ENR_USART1EN;    /* Enable USART1 clock */

    /* Configure PA9 for USART1_TX */
    GPIOA->AFR[1] &= ~0x00F0;
    GPIOA->AFR[1] |=  0x0010;   /* alt1 for USART1 */
    GPIOA->MODER  &= ~0xC0000;
    GPIOA->MODER  |=  0x80000;   /* enable alternate function for PA9 */
    /* Configure PA10 for USART1_RX */
    GPIOA->AFR[1] &= ~0x0F00;
    GPIOA->AFR[1] |=  0x0100;   /* alt1 for USART1 */
    GPIOA->MODER  &= ~0x300000;
    GPIOA->MODER  |=  0x200000;   /* enable alternate function for PA10 */

    USART1->BRR = 0x0341;       /* 9600 baud @ 8 MHz */
    USART1->CR1 = 0x000C;       /* enable RxTx , 8-bit data */
    USART1->CR2 = 0x0000;       /* 1 stop bit */
    USART1->CR3 = 0x0000;       /* no flow control */
    USART1->CR1 |= 0x0001;      /* enable USART1 */
}

/* Read a character from USART1 */
char USART1_read(void) {
    while (!(USART1->ISR & USART_ISR_RXNE)) {}   // wait until char arrives
    return USART1->RDR;
}

/* Write a character to USART1 */
void USART1_write (int ch) {
    while (!(USART1->ISR & USART_ISR_TXE)) {}   // wait until Tx buffer empty
    USART1->TDR = (ch & 0xFF);
}