USART receive character and echo back via interrupts

In this use case, the USART module is configured for:

• Using USART0
• Baudrate: 9600
• Character length: 8 bit
• Parity mode: Disabled
• Stop bit: None
• RS232 mode

The use case waits for a received character on the configured USART and echoes the character back to the same USART. The character reception is performed via an interrupt handler, rather than the polling method used in USART receive character and echo back.

Setup steps

Prerequisites

1. System Clock Management (sysclock)
2. Parallel Input/Output Controller (pio)
3. Power Management Controller (pmc)

Example code

The following configuration must be added to the project (typically to a conf_usart.h file, but it can also be added to your main application file.):

#define USART_SERIAL                 USART0
#define USART_SERIAL_ID              ID_USART0  //USART0 for sam4l
#define USART_SERIAL_ISR_HANDLER     USART0_Handler
#define USART_SERIAL_BAUDRATE        9600
#define USART_SERIAL_CHAR_LENGTH     US_MR_CHRL_8_BIT
#define USART_SERIAL_PARITY          US_MR_PAR_NO
#define USART_SERIAL_STOP_BIT        US_MR_NBSTOP_1_BIT

A variable for the received byte must be added:

uint32_t received_byte;

Add to application initialization:

    sysclk_init();

    board_init();

    const sam_usart_opt_t usart_console_settings = {
        USART_SERIAL_BAUDRATE,
        USART_SERIAL_CHAR_LENGTH,
        USART_SERIAL_PARITY,
        USART_SERIAL_STOP_BIT,
        US_MR_CHMODE_NORMAL
    };

#if SAM4L
  sysclk_enable_peripheral_clock(USART_SERIAL);
#else
    sysclk_enable_peripheral_clock(USART_SERIAL_ID);
#endif

    usart_init_rs232(USART_SERIAL, &usart_console_settings,
            sysclk_get_main_hz());
    usart_enable_tx(USART_SERIAL);
    usart_enable_rx(USART_SERIAL);

    usart_enable_interrupt(USART_SERIAL, US_IER_RXRDY);
    NVIC_EnableIRQ(USART_SERIAL_IRQ);

Workflow

1. Initialize system clock:

   sysclk_init();

2. Configure the USART Tx and Rx pins by call the board init function:

   board_init();

   Note

   Set the following define in conf_board.h file to enable COM port,it will be used in board_init() function to set up IOPorts for the USART pins. For SAM4L:

   #define CONF_BOARD_COM_PORT

   For other SAM devices:

   #define CONF_BOARD_UART_CONSOLE

3. Create USART options struct:

   const sam_usart_opt_t usart_console_settings = {
        USART_SERIAL_BAUDRATE,
        USART_SERIAL_CHAR_LENGTH,
        USART_SERIAL_PARITY,
        USART_SERIAL_STOP_BIT,
        US_MR_CHMODE_NORMAL
   };

4. Enable the clock to the USART module:

   #if SAM4L
     sysclk_enable_peripheral_clock(USART_SERIAL);
   #else
       sysclk_enable_peripheral_clock(USART_SERIAL_ID);
   #endif

5. Initialize the USART module in RS232 mode:

   usart_init_rs232(USART_SERIAL, &usart_console_settings,
           sysclk_get_main_hz());

6. Enable the Rx and Tx modes of the USART module:

   usart_enable_tx(USART_SERIAL);
   usart_enable_rx(USART_SERIAL);

7. Enable the USART character reception interrupt, and general interrupts for the USART module.

   usart_enable_interrupt(USART_SERIAL, US_IER_RXRDY);
   NVIC_EnableIRQ(USART_SERIAL_IRQ);

Usage steps

Example code

Add to your main application C-file the USART interrupt handler:

void USART_SERIAL_ISR_HANDLER(void)
{
   uint32_t dw_status = usart_get_status(USART_SERIAL);

   if (dw_status & US_CSR_RXRDY) {
       uint32_t received_byte;

       usart_read(USART_SERIAL, &received_byte);
       usart_write(USART_SERIAL, received_byte);
   }
}

Workflow

1. When the USART ISR fires, retrieve the USART module interrupt flags:

   uint32_t dw_status = usart_get_status(USART_SERIAL); 

2. Check if the USART Receive Character interrupt has fired:

   if (dw_status & US_CSR_RXRDY) 

3. If a character has been received, fetch it into a temporary variable:

   usart_read(USART_SERIAL, &received_byte); 

4. Echo the character back:

   usart_write(USART_SERIAL, received_byte);