text: /**
   * \file
   *
   * \brief SAM Quick Start Guide for Using Usart driver with DMA
   *
   * Copyright (C) 2014-2016 Atmel Corporation. All rights reserved.
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  /**
   * \page asfdoc_sam0_sercom_usart_dma_use_case Quick Start Guide for Using DMA with SERCOM USART
   *
   * The supported board list:
   *    - SAM D21 Xplained Pro
   *    - SAM R21 Xplained Pro
   *    - SAM D11 Xplained Pro
   *    - SAM DA1 Xplained Pro
   *    - SAM HA1G16A Xplained Pro
   *    - SAM L21 Xplained Pro
   *    - SAM L22 Xplained Pro
   *    - SAM C21 Xplained Pro
   *
   * This quick start will receive eight bytes of data from the PC terminal and transmit back the string
   * to the terminal through DMA. In this use case the USART will be configured with the following
   * settings:
   * - Asynchronous mode
   * - 9600 Baudrate
   * - 8-bits, No Parity and one Stop Bit
   * - TX and RX enabled and connected to the Xplained Pro Embedded Debugger virtual COM port
   *
   * \section asfdoc_sam0_sercom_usart_dma_use_case_setup Setup
   *
   * \subsection asfdoc_sam0_sercom_usart_dma_use_case_prereq Prerequisites
   * There are no special setup requirements for this use-case.
   *
   * \subsection asfdoc_sam0_usart_dma_use_case_setup_code Code
   * Add to the main application source file, outside of any functions:
   * \snippet qs_usart_dma_use.c module_inst
   * \snippet qs_usart_dma_use.c dma_resource
   * \snippet qs_usart_dma_use.c usart_buffer
   * \snippet qs_usart_dma_use.c transfer_descriptor
   *
   * Copy-paste the following setup code to your user application:
   * \snippet qs_usart_dma_use.c setup
   *
   * Add to user application initialization (typically the start of \c main()):
   * \snippet qs_usart_dma_use.c setup_init
   *
   * \subsection asfdoc_sam0_usart_dma_use_case_setup_flow Workflow
   *
   * \subsubsection asfdoc_sam0_usart_dma_use_case_setup_flow_inst Create variables
   * -# Create a module software instance structure for the USART module to store
   *    the USART driver state while it is in use.
   *    \snippet qs_usart_dma_use.c module_inst
   *    \note This should never go out of scope as long as the module is in use.
   *          In most cases, this should be global.
   *
   * -# Create module software instance structures for DMA resources to store
   *    the DMA resource state while it is in use.
   *    \snippet qs_usart_dma_use.c dma_resource
   *    \note This should never go out of scope as long as the module is in use.
   *          In most cases, this should be global.
   *
   * -# Create a buffer to store the data to be transferred /received.
   *    \snippet qs_usart_dma_use.c usart_buffer
   * -# Create DMA transfer descriptors for RX/TX.
   *    \snippet qs_usart_dma_use.c transfer_descriptor
   *
   * \subsubsection asfdoc_sam0_usart_dma_use_case_setup_flow_usart Configure the USART
   * -# Create a USART module configuration struct, which can be filled out to
   *    adjust the configuration of a physical USART peripheral.
   *    \snippet qs_usart_dma_use.c setup_config
   * -# Initialize the USART configuration struct with the module's default values.
   *    \snippet qs_usart_dma_use.c setup_config_defaults
   *    \note This should always be performed before using the configuration
   *          struct to ensure that all values are initialized to known default
   *          settings.
   *
   * -# Alter the USART settings to configure the physical pinout, baudrate, and
   *    other relevant parameters.
   *    \snippet qs_usart_dma_use.c setup_change_config
   * -# Configure the USART module with the desired settings, retrying while the
   *    driver is busy until the configuration is stressfully set.
   *    \snippet qs_usart_dma_use.c setup_set_config
   * -# Enable the USART module.
   *    \snippet qs_usart_dma_use.c setup_enable
   *
   * \subsubsection asfdoc_sam0_usart_dma_use_case_setup_flow_dma Configure DMA
   * -# Create a callback function of receiver done.
   *    \snippet qs_usart_dma_use.c transfer_done_rx
   *
   * -# Create a callback function of transmission done.
   *    \snippet qs_usart_dma_use.c transfer_done_tx
   *
   * -# Create a DMA resource configuration structure, which can be filled out to
   *    adjust the configuration of a single DMA transfer.
   *    \snippet qs_usart_dma_use.c setup_rx_1
   *
   * -# Initialize the DMA resource configuration struct with the module's
   *    default values.
   *    \snippet qs_usart_dma_use.c setup_rx_2
   *    \note This should always be performed before using the configuration
   *          struct to ensure that all values are initialized to known default
   *          settings.
   *
   * -# Set extra configurations for the DMA resource. It is using peripheral
   *    trigger. SERCOM TX empty trigger causes a beat transfer in
   *    this example.
   *    \snippet qs_usart_dma_use.c setup_rx_3
   *
   * -# Allocate a DMA resource with the configurations.
   *    \snippet qs_usart_dma_use.c setup_rx_4
   *
   * -# Create a DMA transfer descriptor configuration structure, which can be
   *    filled out to adjust the configuration of a single DMA transfer.
   *    \snippet qs_usart_dma_use.c setup_rx_5
   *
   * -# Initialize the DMA transfer descriptor configuration struct with the module's
   *    default values.
   *    \snippet qs_usart_dma_use.c setup_rx_6
   *    \note This should always be performed before using the configuration
   *          struct to ensure that all values are initialized to known default
   *          settings.
   *
   * -# Set the specific parameters for a DMA transfer with transfer size, source
   *    address, and destination address.
   *    \snippet qs_usart_dma_use.c setup_rx_7
   *
   * -# Create the DMA transfer descriptor.
   *    \snippet qs_usart_dma_use.c setup_rx_8
   *
   * -# Create a DMA resource configuration structure for TX, which can be filled
   *    out to adjust the configuration of a single DMA transfer.
   *    \snippet qs_usart_dma_use.c setup_tx_1
   *
   * -# Initialize the DMA resource configuration struct with the module's
   *    default values.
   *    \snippet qs_usart_dma_use.c setup_tx_2
   *    \note This should always be performed before using the configuration
   *          struct to ensure that all values are initialized to known default
   *          settings.
   *
   * -# Set extra configurations for the DMA resource. It is using peripheral
   *    trigger. SERCOM RX Ready trigger causes a beat transfer in
   *    this example.
   *    \snippet qs_usart_dma_use.c setup_tx_3
   *
   * -# Allocate a DMA resource with the configurations.
   *    \snippet qs_usart_dma_use.c setup_tx_4
   *
   * -# Create a DMA transfer descriptor configuration structure, which can be
   *    filled out to adjust the configuration of a single DMA transfer.
   *    \snippet qs_usart_dma_use.c setup_tx_5
   *
   * -# Initialize the DMA transfer descriptor configuration struct with the module's
   *    default values.
   *    \snippet qs_usart_dma_use.c setup_tx_6
   *    \note This should always be performed before using the configuration
   *          struct to ensure that all values are initialized to known default
   *          settings.
   *
   * -# Set the specific parameters for a DMA transfer with transfer size, source
   *    address, and destination address.
   *    \snippet qs_usart_dma_use.c setup_tx_7
   *
   * -# Create the DMA transfer descriptor.
   *    \snippet qs_usart_dma_use.c setup_tx_8
   *
   * \section asfdoc_sam0_usart_dma_use_case_main Use Case
   *
   * \subsection asfdoc_sam0_usart_dma_use_case_main_code Code
   * Copy-paste the following code to your user application:
   * \snippet qs_usart_dma_use.c main
   *
   * \subsection asfdoc_sam0_usart_dma_use_case_main_flow Workflow
   * -# Wait for receiving data.
   *    \snippet qs_usart_dma_use.c main_1
   *
   * -# Enter endless loop.
   *    \snippet qs_usart_dma_use.c endless_loop
   */
  