title: Multibyte UART messages of different lengths using UartLite
body: <p>I have an Arty Board using XUartLite to communicate with my laptop.</p>
  
  <p>I want to receive multibyte messages of differing lengths. The second byte contains the message type and I can then determine how many more bytes remain in the buffer to retrieve.</p>
  
  <p>I have done this successfully after adapting the code found <a href="https://lancesimms.com/Xilinx/VivadoFiles/VC707_Microblaze_UART_to_LED/main.c" rel="nofollow noreferrer">here</a> (Thank you Lance Simms).</p>
  
  <p>My issue is that, if I use the  <code>XUartLite_RecvByte</code> method inside my interrupt routine to request the correct number of bytes, because it is a blocking call, if something goes wrong then I will be stuck waiting for a byte which never arrived - and then when the next message comes it will be looking for the wrong data.</p>
  
  <p>Alternatively, if I use<code>XUartLite_Recv</code> method, which is non-blocking, and specify the number of bytes unless I debug and step through it doesn't retrieve the bytes, and doesn't trigger another interrupt for those bytes.</p>
  
  <p>Ideally, I would like the interrupt to trigger the <code>RecvHandler</code> when the message has completed being received as each multi-byte message is being sent every 1/2 second. Messages can be 7,8,10 or 13 bytes long.</p>
  
  <p>Or set the specified number of bytes to be the maximum (13) and have it timeout if it's not filled after a certain period from receiving the first byte. </p>
  
  <p>In the example below I am sending the message "foobar". The interrupt calls the <code>RecvHandler</code> method after the first byte "f" and then it manually calls 5 times to get "oobar". Then it echos it back.</p>
  
  <pre><code>//NAME:
  //  main.c
  //
  //PURPOSE:
  //  This C file is meant to be built with the Xilinx SDK and run as an
  //  ELF file on a Microblaze processor.
  //
  //AUTHOR:
  //  Lance Simms
  //  Note: Code uses Xilinx examples included with Vivado.
  // https://lancesimms.com/Xilinx/VivadoFiles/VC707_Microblaze_UART_to_LED/main.c
  //
  //DATE:
  //  01/09/17
  //
  /***************************** Include Files *********************************/
  
  #include "xparameters.h"
  #include &lt;xil_printf.h&gt;
  #include &lt;stdio.h&gt;
  #include "xil_exception.h"
  #include "xuartlite.h"
  #include "xintc.h"
  #include "xgpio_l.h"
  #include "xenv.h"
  
  /************************** Constant Definitions *****************************/
  
  /*
   * The following constants map to the XPAR parameters created in the
   * xparameters.h file. They are defined here such that a user can easily
   * change all the needed parameters in one place.
   */
  #define UARTLITE_DEVICE_ID      XPAR_UARTLITE_0_DEVICE_ID
  #define INTC_DEVICE_ID          XPAR_INTC_0_DEVICE_ID
  #define UARTLITE_INT_IRQ_ID     XPAR_INTC_0_UARTLITE_0_VEC_ID
  #define GPIO_REG_BASEADDR       0x40000000
  
  /*
   * The following constant controls the length of the buffers to be sent
   * and received with the UartLite device.
   */
  #define TEST_BUFFER_SIZE        500
  
  /************************** Function Prototypes ******************************/
  
  int SetupUartLite(u16 DeviceId);
  
  int SetupInterruptSystem(XUartLite *UartLitePtr);
  
  void SendHandler(void *CallBackRef, unsigned int EventData);
  
  void RecvHandler(void *CallBackRef, unsigned int EventData);
  
  void resetBuffer();
  
  void printBuffer();
  
  /************************** Variable Definitions *****************************/
  
   XUartLite UartLite;             /* The instance of the UartLite Device */
   XUartLite_Config *UartLite_Cfg; /* The instance of the UartLite Config */
   XIntc InterruptController;      /* The instance of the Interrupt Controller */
  
   /*
    * The following buffers are used in this example to send and receive data
    * with the UartLite.
    */
   u8 SendBuffer[TEST_BUFFER_SIZE];
   u8 ReceiveBuffer[TEST_BUFFER_SIZE];
  
   /* Here are the pointers to the buffer */
   u8* ReceiveBufferPtr = &amp;ReceiveBuffer[0];
  
   /*
    * The following counters are used to determine when the entire buffer has
    * been sent and received.
    */
   static volatile int TotalReceivedCount;
   static volatile int TotalSentCount;
  
  
  int main()
  {
      //Variable definitions
      int Status=0;
  
      //Set up the UART and configure the interrupt handler for bytes in RX buffer
      Status = SetupUartLite(UARTLITE_DEVICE_ID);
  
      //Get a reference pointer to the Uart Configuration
      UartLite_Cfg = XUartLite_LookupConfig(UARTLITE_DEVICE_ID);
  
      //Print out the info about our XUartLite instance
      xil_printf("\n\r");
      xil_printf("Serial Port Properties ------------------\n\r");
      xil_printf("Device ID : %d\n\r", UartLite_Cfg-&gt;DeviceId);
      xil_printf("Baud Rate : %d\n\r", UartLite_Cfg-&gt;BaudRate);
      xil_printf("Data Bits : %d\n\r", UartLite_Cfg-&gt;DataBits);
      xil_printf("Base Addr : %08X\n\r", UartLite_Cfg-&gt;RegBaseAddr);
      xil_printf("\n\r");
  
  
      // Run
      while (1){};
  
      //End of program
      return Status;
  }
  
  /****************************************************************************/
  /**
  *
  * This function does a minimal test on the UartLite device and driver as a
  * design example. The purpose of this function is to illustrate
  * how to use the XUartLite component.
  *
  * This function sends data and expects to receive the same data through the
  * UartLite. The user must provide a physical loopback such that data which is
  * transmitted will be received.
  *
  * This function uses interrupt driver mode of the UartLite device. The calls
  * to the UartLite driver in the handlers should only use the non-blocking
  * calls.
  *
  * @param    DeviceId is the Device ID of the UartLite Device and is the
  *       XPAR_&lt;uartlite_instance&gt;_DEVICE_ID value from xparameters.h.
  *
  * @return   XST_SUCCESS if successful, otherwise XST_FAILURE.
  *
  * @note
  *
  * This function contains an infinite loop such that if interrupts are not
  * working it may never return.
  *
  ****************************************************************************/
  int SetupUartLite(u16 DeviceId)
  {
      int Status;
  
      /*
       * Initialize the UartLite driver so that it's ready to use.
       */
      Status = XUartLite_Initialize(&amp;UartLite, DeviceId);
      if (Status != XST_SUCCESS) {
          return XST_FAILURE;
      }
  
      /*
       * Perform a self-test to ensure that the hardware was built correctly.
       */
      Status = XUartLite_SelfTest(&amp;UartLite);
      if (Status != XST_SUCCESS) {
          return XST_FAILURE;
      }
  
      /*
       * Connect the UartLite to the interrupt subsystem such that interrupts can
       * occur. This function is application specific.
       */
      Status = SetupInterruptSystem(&amp;UartLite);
      if (Status != XST_SUCCESS) {
          return XST_FAILURE;
      }
  
      /*
       * Setup the handlers for the UartLite that will be called from the
       * interrupt context when data has been sent and received, specify a
       * pointer to the UartLite driver instance as the callback reference so
       * that the handlers are able to access the instance data.
       */
      XUartLite_SetSendHandler(&amp;UartLite, SendHandler, &amp;UartLite);
      XUartLite_SetRecvHandler(&amp;UartLite, RecvHandler, &amp;UartLite);
  
      /*
       * Enable the interrupt of the UartLite so that interrupts will occur.
       */
      XUartLite_EnableInterrupt(&amp;UartLite);
      XUartLite_Recv(&amp;UartLite, ReceiveBufferPtr, 1);
  
      return XST_SUCCESS;
  }
  
  /*****************************************************************************/
  /**
  *
  * This function is the handler which performs processing to send data to the
  * UartLite. It is called from an interrupt context such that the amount of
  * processing performed should be minimized. It is called when the transmit
  * FIFO of the UartLite is empty and more data can be sent through the UartLite.
  *
  * This handler provides an example of how to handle data for the UartLite,
  * but is application specific.
  *
  * @param    CallBackRef contains a callback reference from the driver.
  *       In this case it is the instance pointer for the UartLite driver.
  * @param    EventData contains the number of bytes sent or received for sent
  *       and receive events.
  *
  * @return   None.
  *
  * @note     None.
  *
  ****************************************************************************/
  void SendHandler(void *CallBackRef, unsigned int EventData)
  {
      TotalSentCount = EventData;
  }
  /****************************************************************************/
  /**
  *
  * This function is the handler which performs processing to receive data from
  * the UartLite. It is called from an interrupt context such that the amount of
  * processing performed should be minimized.  It is called data is present in
  * the receive FIFO of the UartLite such that the data can be retrieved from
  * the UartLite. The size of the data present in the FIFO is not known when
  * this function is called.
  *
  * This handler provides an example of how to handle data for the UartLite,
  * but is application specific.
  *
  * @param    CallBackRef contains a callback reference from the driver, in
  *       this case it is the instance pointer for the UartLite driver.
  * @param    EventData contains the number of bytes sent or received for sent
  *       and receive events.
  *
  * @return   None.
  *
  * @note     None.
  *
  ****************************************************************************/
  void RecvHandler(void *CallBackRef, unsigned int EventData)
  {
      if(ReceiveBuffer[0] == 'f') //Look at the start of the message to determine the remaining bytes.
      {
          ReceiveBufferPtr++; // 1 Byte has been found so increment the buffer ptr.
          for(int i=1;i&lt;6;i++){
          ReceiveBuffer[i]=XUartLite_RecvByte(0x40600000U);
          }
          ReceiveBufferPtr+=5; //Non-blocking but it should be successful. so increment by 5.
      }
  
      printBuffer(); // print the contents of the buffer.
  
  
      XUartLite_ResetFifos(&amp;UartLite); // Reset the FIFOs
      resetBuffer(); // Reset the contents of the buffer.
  
      ReceiveBufferPtr = &amp;ReceiveBuffer[0]; // Reset the pointer back to the start of the buffer.
      XUartLite_Recv(&amp;UartLite, ReceiveBufferPtr, 1); // Get ready to receive another byte.
  }
  
  void resetBuffer()
  {
      for(int i=0;i&lt;TEST_BUFFER_SIZE;i++){
              ReceiveBuffer[i]=0;
          }
  }
  void printBuffer()
  {
      //return the message
      XUartLite_Send(&amp;UartLite,&amp;ReceiveBuffer[0],6);
  }
  /****************************************************************************/
  /**
  *
  * This function setups the interrupt system such that interrupts can occur
  * for the UartLite device. This function is application specific since the
  * actual system may or may not have an interrupt controller. The UartLite
  * could be directly connected to a processor without an interrupt controller.
  * The user should modify this function to fit the application.
  *
  * @param    UartLitePtr contains a pointer to the instance of the UartLite
  *           component which is going to be connected to the interrupt
  *           controller.
  *
  * @return   XST_SUCCESS if successful, otherwise XST_FAILURE.
  *
  * @note     None.
  *
  ****************************************************************************/
  int SetupInterruptSystem(XUartLite *UartLitePtr)
  {
  
      int Status;
  
      /*
       * Initialize the interrupt controller driver so that it is ready to
       * use.
       */
      Status = XIntc_Initialize(&amp;InterruptController, INTC_DEVICE_ID);
      if (Status != XST_SUCCESS) {
          return XST_FAILURE;
      }
  
      /*
       * Connect a device driver handler that will be called when an interrupt
       * for the device occurs, the device driver handler performs the
       * specific interrupt processing for the device.
       */
      Status = XIntc_Connect(&amp;InterruptController, UARTLITE_INT_IRQ_ID,
                 (XInterruptHandler)XUartLite_InterruptHandler,
                 (void *)UartLitePtr);
      if (Status != XST_SUCCESS) {
          return XST_FAILURE;
      }
  
      /*
       * Start the interrupt controller such that interrupts are enabled for
       * all devices that cause interrupts, specific real mode so that
       * the UartLite can cause interrupts through the interrupt controller.
       */
      Status = XIntc_Start(&amp;InterruptController, XIN_REAL_MODE);
      if (Status != XST_SUCCESS) {
          return XST_FAILURE;
      }
  
      /*
       * Enable the interrupt for the UartLite device.
       */
      XIntc_Enable(&amp;InterruptController, UARTLITE_INT_IRQ_ID);
  
      /*
       * Initialize the exception table.
       */
      Xil_ExceptionInit();
  
      /*
       * Register the interrupt controller handler with the exception table.
       */
      Xil_ExceptionRegisterHandler(XIL_EXCEPTION_ID_INT,
               (Xil_ExceptionHandler)XIntc_InterruptHandler,
               &amp;InterruptController);
  
      /*
       * Enable exceptions.
       */
      Xil_ExceptionEnable();
  
      return XST_SUCCESS;
  }
  </code></pre>
  
  <p>Thanks in advance.</p>