This example code shows how to use the STM32F407′s USART. It initializes the USART1 in receive and transmit mode an receives a data string via an interrupt.

The USART allows us to send data to a PC or other device via the serial interface (RS232). We can also receive data from a PC or another device. However please note that the STM32F407 works at 3.3V and most other devices operate at 5V or even the standard +-12V for RS232.

In order to connect to these devices (particularly the RS232 devices) you need to use a level shifter like the MAX232 or a serial to USB converter like the FT232.

If you wonder where to find all the values and properties for the InitStruct’s have a look at the include files located in the STM32F4 library by ST under Libraries/STM32F4xx_StdPeriph_Driver/ in the inc and src folders

Everything is explained in the comments. Enjoy the code!

#include <stm32f4xx.h>
#include <misc.h>			 // I recommend you have a look at these in the ST firmware folder
#include <stm32f4xx_usart.h> // under Libraries/STM32F4xx_StdPeriph_Driver/inc and src

#define MAX_STRLEN 12 // this is the maximum string length of our string in characters
volatile char received_string[MAX_STRLEN+1]; // this will hold the recieved string

void Delay(__IO uint32_t nCount) {
  while(nCount--) {
  }
}

/* This funcion initializes the USART1 peripheral
 *
 * Arguments: baudrate --> the baudrate at which the USART is
 * 						   supposed to operate
 */
void init_USART1(uint32_t baudrate){

	/* This is a concept that has to do with the libraries provided by ST
	 * to make development easier the have made up something similar to
	 * classes, called TypeDefs, which actually just define the common
	 * parameters that every peripheral needs to work correctly
	 *
	 * They make our life easier because we don't have to mess around with
	 * the low level stuff of setting bits in the correct registers
	 */
	GPIO_InitTypeDef GPIO_InitStruct; // this is for the GPIO pins used as TX and RX
	USART_InitTypeDef USART_InitStruct; // this is for the USART1 initilization
	NVIC_InitTypeDef NVIC_InitStructure; // this is used to configure the NVIC (nested vector interrupt controller)

	/* enable APB2 peripheral clock for USART1
	 * note that only USART1 and USART6 are connected to APB2
	 * the other USARTs are connected to APB1
	 */
	RCC_APB2PeriphClockCmd(RCC_APB2Periph_USART1, ENABLE);

	/* enable the peripheral clock for the pins used by
	 * USART1, PB6 for TX and PB7 for RX
	 */
	RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOB, ENABLE);

	/* This sequence sets up the TX and RX pins
	 * so they work correctly with the USART1 peripheral
	 */
	GPIO_InitStruct.GPIO_Pin = GPIO_Pin_6 | GPIO_Pin_7; // Pins 6 (TX) and 7 (RX) are used
	GPIO_InitStruct.GPIO_Mode = GPIO_Mode_AF; 			// the pins are configured as alternate function so the USART peripheral has access to them
	GPIO_InitStruct.GPIO_Speed = GPIO_Speed_50MHz;		// this defines the IO speed and has nothing to do with the baudrate!
	GPIO_InitStruct.GPIO_OType = GPIO_OType_PP;			// this defines the output type as push pull mode (as opposed to open drain)
	GPIO_InitStruct.GPIO_PuPd = GPIO_PuPd_UP;			// this activates the pullup resistors on the IO pins
	GPIO_Init(GPIOB, &GPIO_InitStruct);					// now all the values are passed to the GPIO_Init() function which sets the GPIO registers

	/* The RX and TX pins are now connected to their AF
	 * so that the USART1 can take over control of the
	 * pins
	 */
	GPIO_PinAFConfig(GPIOB, GPIO_PinSource6, GPIO_AF_USART1); //
	GPIO_PinAFConfig(GPIOB, GPIO_PinSource7, GPIO_AF_USART1);

	/* Now the USART_InitStruct is used to define the
	 * properties of USART1
	 */
	USART_InitStruct.USART_BaudRate = baudrate;				// the baudrate is set to the value we passed into this init function
	USART_InitStruct.USART_WordLength = USART_WordLength_8b;// we want the data frame size to be 8 bits (standard)
	USART_InitStruct.USART_StopBits = USART_StopBits_1;		// we want 1 stop bit (standard)
	USART_InitStruct.USART_Parity = USART_Parity_No;		// we don't want a parity bit (standard)
	USART_InitStruct.USART_HardwareFlowControl = USART_HardwareFlowControl_None; // we don't want flow control (standard)
	USART_InitStruct.USART_Mode = USART_Mode_Tx | USART_Mode_Rx; // we want to enable the transmitter and the receiver
	USART_Init(USART1, &USART_InitStruct);					// again all the properties are passed to the USART_Init function which takes care of all the bit setting


	/* Here the USART1 receive interrupt is enabled
	 * and the interrupt controller is configured
	 * to jump to the USART1_IRQHandler() function
	 * if the USART1 receive interrupt occurs
	 */
	USART_ITConfig(USART1, USART_IT_RXNE, ENABLE); // enable the USART1 receive interrupt

	NVIC_InitStructure.NVIC_IRQChannel = USART1_IRQn;		 // we want to configure the USART1 interrupts
	NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;// this sets the priority group of the USART1 interrupts
	NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;		 // this sets the subpriority inside the group
	NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;			 // the USART1 interrupts are globally enabled
	NVIC_Init(&NVIC_InitStructure);							 // the properties are passed to the NVIC_Init function which takes care of the low level stuff

	// finally this enables the complete USART1 peripheral
	USART_Cmd(USART1, ENABLE);
}

/* This function is used to transmit a string of characters via
 * the USART specified in USARTx.
 *
 * It takes two arguments: USARTx --> can be any of the USARTs e.g. USART1, USART2 etc.
 * 						   (volatile) char *s is the string you want to send
 *
 * Note: The string has to be passed to the function as a pointer because
 * 		 the compiler doesn't know the 'string' data type. In standard
 * 		 C a string is just an array of characters
 *
 * Note 2: At the moment it takes a volatile char because the received_string variable
 * 		   declared as volatile char --> otherwise the compiler will spit out warnings
 * */
void USART_puts(USART_TypeDef* USARTx, volatile char *s){

	while(*s){
		// wait until data register is empty
		while( !(USARTx->SR & 0x00000040) );
		USART_SendData(USARTx, *s);
		*s++;
	}
}

int main(void) {

  init_USART1(9600); // initialize USART1 @ 9600 baud

  USART_puts(USART1, "Init complete! Hello World!rn"); // just send a message to indicate that it works

  while (1){
    /*
     * You can do whatever you want in here
     */
  }
}

// this is the interrupt request handler (IRQ) for ALL USART1 interrupts
void USART1_IRQHandler(void){

	// check if the USART1 receive interrupt flag was set
	if( USART_GetITStatus(USART1, USART_IT_RXNE) ){

		static uint8_t cnt = 0; // this counter is used to determine the string length
		char t = USART1->DR; // the character from the USART1 data register is saved in t

		/* check if the received character is not the LF character (used to determine end of string)
		 * or the if the maximum string length has been been reached
		 */
		if( (t != 'n') && (cnt < MAX_STRLEN) ){
			received_string[cnt] = t;
			cnt++;
		}
		else{ // otherwise reset the character counter and print the received string
			cnt = 0;
			USART_puts(USART1, received_string);
		}
	}
}