====== T04 - UART communication ======

Write a program for [[courses:be2m37mam:hardware:start|Nucleo STM32F401RE]] to send and receive characters over USART. 

===== Instructions: =====

  - Send single character from Nucleo to PC, see template.
  - Modify template to send string (word or sentence).
  - Modify template to receive character (or more) and display it on Experimental shield. 
  - Use [[https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.html|Putty]] to communicate with Nucleo

===== Lecture =====

{{ :courses:be2m37mam:tasks:stm32-uart.pdf |}}

===== Template =====

Project: {{ :courses:be2m37mam:tasks:usart.zip |}}

==== GPIO settings ====

USART needs two of GPIO pins to serve as transmit and receive line. To enable it, GPIO pins mode shall be set like an alternate function. 

  * GPIOx MODER: p. 157 of reference manual
  * GPIOx AFR:   p. 162 of reference manual

<code c>
void gpio_init (void)
{
  setbit(RCC->AHB1ENR, 0);     // Enable AHB1 clock
  setbit(GPIOA->MODER, 5);     // PA2 alternate function, 
  setbit(GPIOA->MODER, 7);     // PA3 alternate function

  GPIOA->AFR[0] |= 0x0700;     // alternate function, PA2 will be USART2_TX
  GPIOA->AFR[0] |= 0x7000;     // alternate function, PA3 will be USART2_RX                             
                               // see p. 45 of STM32F401xE datasheet
}                           
</code>

==== USART ====

<code c>
/*
 * USART2 init
 * USART2 is connected to APB1 bus
 * USART2 uses PA2 as TX and PA3 as RX
 */
void usart_init (void)
{        
  setbit(RCC->APB1ENR, 17);    // Enable APB1 clock
  setbit(USART2->CR1, 13);     // Enable USART2
  setbit(USART2->CR1, 2);      // USART2 - enable receive
  setbit(USART2->CR1, 3);      // USART2 - enable transmit
  USART2->BRR = (104 << 4);    // USART2 baudrate,  first 4 bits are dedicated to fraction
/*
 * STM32F401 default clock is 16MHz HSI 
 * BRR ~ fclk/(8*(OVER8-2)*baudrate)
 * OVER8 reset state is 0
 * see p. 514 of STM32F401 reference manual
 */
}
</code>

==== USART - transmit byte ====

<code c>
void usart_tx (unsigned char ch)  {
  // wait till transmit register is empty 7th bit of SR
  while (!readbit(USART2->SR, 7));
  USART2->DR = ch;
}
</code>

==== USART - receive byte ====

<code c>
int usart_rx (unsigned char *x)
{
   // read from DR while receive register is not empty - 5th bit of SR
   if (getbit(USART2->SR, 5)) {
	   *x = USART2->DR;
	   return 1;
   }
   return 0;
}
</code>

==== main program ====

<code c>
int main(void)
{
    char mess[20];
    char a;
    int i = 0;

    gpio_init();
    usart_init();

    while (1)
    {
        if (usart_rx (&a))
        {
            if (a != '\r' && a != '\n')
                mess[i++] = a;
            else 
            {
                send_string(mess, i);
                i = 0;
            }
        }
    }
}
</code>