5.5 SOF Detection and Wakeup Configuration

The USART start-of-frame (SOF) detector can wake up the CPU from Standby Sleep mode when it detects a Start bit. In Standby Sleep mode, the internal fast start-up oscillator must be selected as the GCLK_SERCOMx_CORE source. The application enters the Standby Sleep mode, and the PC key press character will wake up the device from the Sleep mode and displays the character on the terminal.

Note: Click here to access the source code for this application configuration. Alternatively, it is also available in the GitHub reference_apps repository.
Figure 5-63. Block diagram (SAM D21 Curiosity Nano Evaluation Kit)

This application uses only one SAM D21 Curiosity Nano Evaluation Kit connected to the PC terminal through EDBG. In this section, only SOF detection and wakeup configuration part will be explained whereas the remaining availabe in the Basic Configuration section.

To add and configure MPLAB Harmony components using the MCC, follow these steps:

  1. To create the project, see SAM D21 Curiosity Nano Evaluation Kit.
  2. Click and expand SERCOM5 and then select Enable Run in Standby, and then click Generate.
    Figure 5-64. Configuration Options
  3. Add the following code outside the main() function:
    #define RX_BUFFER_SIZE 1 
volatile bool rx_done = false;
uint8_t edbg_rx_data;
volatile bool tx_done = false;

void APP_SERCOM_5_WriteCallback(uintptr_t context)
{
   tx_done = true;
}

void APP_SERCOM_5_ReadCallback(uintptr_t context)
{
    if((SERCOM5_REGS->USART_INT.SERCOM_INTFLAG & SERCOM_USART_INT_INTFLAG_RXS_Msk) == SERCOM_USART_INT_INTFLAG_RXS_Msk)
    {
        SERCOM5_REGS->USART_INT.SERCOM_INTFLAG |= (uint8_t)SERCOM_USART_INT_INTFLAG_RXS_Msk;
        
    }  
    
    rx_done = true;
} 
void usart_send_string(const char *str)
{
    SERCOM5_USART_Write((void *)&str[0], strlen(str));
}
  4. In the following code example, the tx_done becomes true once the write function (which is happening in APP_SERCOM_5_Writecallback () is completed and rx_done becomes true once read function (APP_SERCOM_5_Raedcallback ()is completed. Also, set the RXS flag if the value of RXS_Msk and SERCOM_INTFLAG equals RXS_Msk. The function usart_send_string() is used to send strings to the console.
    Figure 5-65. Implementation of SOF Detection and Wakeup Configuration - Example 1
  5. Add the following code inside the main() function:
    /* Initialize all modules */
    SYS_Initialize ( NULL );
    SERCOM5_REGS->USART_INT.SERCOM_INTENSET =  (uint8_t)SERCOM_USART_INT_INTENSET_RXS_Msk;
    
    // EDBG SERCOM Read and Write Callback
    SERCOM5_USART_ReadCallbackRegister(APP_SERCOM_5_ReadCallback, 0);
    SERCOM5_USART_WriteCallbackRegister(APP_SERCOM_5_WriteCallback, 0);
    
    // Read request for EDBG
    SERCOM5_USART_Read(&edbg_rx_data, RX_BUFFER_SIZE);
  6. In the following figure, set the particular RXS interrupt, and the callback registers are called as seen in the basic configuration. Also, a read request is also given.
    Figure 5-66. Implementation of SOF Detection and Wakeup Configuration - Example 2
    Note: In SOF detection and wakeup configuration only SERCOM5 (EDBG) is used.
  7. Add the following code inside the while loop:
    if (!rx_done)
        {
            tx_done = false;
            usart_send_string("\r\n Device entered into standby sleep mode");
            while(!(SERCOM5_REGS->USART_INT.SERCOM_INTFLAG & SERCOM_USART_INT_INTFLAG_TXC_Msk));
            
            // Enters standby sleep mode.
            PM_StandbyModeEnter();
        }
        while(!rx_done);
        
        tx_done = false;
        usart_send_string("\r\n Character received after wakeup :");
        while(!tx_done);
        
        tx_done = false;
        SERCOM5_USART_Write(&edbg_rx_data, RX_BUFFER_SIZE);
        while(!tx_done);
        
        rx_done = false;
        SERCOM5_USART_Read(&edbg_rx_data, RX_BUFFER_SIZE);
        SYS_Tasks ( );
    }
  8. In the following figure, when a character is entered, it wakes up from Sleep mode and print the character that is entered. Until the TXC is false (i.e., until the transmission is complete) it loops in the while loop itself. When it becomes true (i.e., once the transmission is complete) it enters Standby Sleep mode.
    Figure 5-67. Implementation of SOF Detection and Wakeup Configuration - Example 3
    Figure 5-68. SERCOM USART – SOF Detection and Wake-up Configuration Output
    Note: The SOF detection and wakeup configuration is not available in the SAM E70/S70/V7x family of devices.