/**
  @page AUDIO_Standalone USB Device AUDIO application
  
  @verbatim
  ******************** (C) COPYRIGHT 2017 STMicroelectronics *******************
  * @file    USB_Device/AUDIO_Standalone/Readme.txt 
  * @author  MCD Application Team
  * @brief   Description of the USB Device AUDIO application.
  ******************************************************************************
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  *
  * <h2><center>&copy; Copyright © 2017 STMicroelectronics International N.V. 
  * All rights reserved.</center></h2>
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  ******************************************************************************
  @endverbatim

@par Application Description 

This application is a part of the USB Device Library package using STM32Cube firmware. It describes how to 
use USB device application based on the AUDIO Class implementation of an audio streaming 
(Out: Speaker/Headset) capability on the STM32F2xx devices.

It follows the "Universal Serial Bus Device Class Definition for Audio Devices Release 1.0 March 18, 
1998" defined by the USB Implementers Forum for reprogramming an application through USB-FS-Device. 
Following this specification, it is possible to manage only Full Speed USB mode (High Speed is not supported).
This class is natively supported by most Operating Systems: no need for specific driver setup.

This is a typical application on how to use the STM32F2xx USB OTG Device peripheral and I2S peripheral to 
stream audio data from USB Host to the audio codec implemented on the STM322xG-EVAL board.

At the beginning of the main program the HAL_Init() function is called to reset all the peripherals,
initialize the Flash interface and the systick. The user is provided with the SystemClock_Config()
function to configure the system clock (SYSCLK) to run at 120 MHz. The Full Speed (FS) USB module uses
internally a 48-MHz clock, which is generated from an integrated PLL.
                         
The device supports the following audio features:
  - Pulse Coded Modulation (PCM) format
  - sampling rate: 48KHz. 
  - Bit resolution: 16
  - Number of channels: 2
  - No volume control
  - Mute/Unmute capability
  - Asynchronous Endpoints

In order to overcome the difference between USB clock domain and STM32 clock domain,
the Add-Remove mechanism is implemented at class driver level.
This is a basic solution that doesn't require external components. It is based
on adding/removing one sample at a periodic basis to speedup or slowdown
the audio output process. This allows to resynchronize it with the input flow.

@note Care must be taken when using HAL_Delay(), this function provides accurate delay (in milliseconds)
      based on variable incremented in SysTick ISR. This implies that if HAL_Delay() is called from
      a peripheral ISR process, then the SysTick interrupt must have higher priority (numerically lower)
      than the peripheral interrupt. Otherwise the caller ISR process will be blocked.
      To change the SysTick interrupt priority you have to use HAL_NVIC_SetPriority() function.
      
@note The application needs to ensure that the SysTick time base is always set to 1 millisecond
      to have correct HAL operation.
      
For more details about the STM32Cube USB Device library, please refer to UM1734 
"STM32Cube USB Device library".
      

@par USB Library Configuration

To select the appropriate USB Core to work with, user must add the following macro defines within the
compiler preprocessor (already done in the preconfigured projects provided with this application):
      - "USE_USB_FS" when using USB Full Speed (FS) Core 

It is possible to fine tune needed USB Device features by modifying defines values in USBD configuration
file “usbd_conf.h” available under the project includes directory, in a way to fit the application
requirements, such as:      
 - USBD_AUDIO_FREQ, specifying the sampling rate conversion from original audio file sampling rate to the
   sampling rate supported by the device.   
    
         
@par Directory contents

  - USB_Device/AUDIO_Standalone/Src/main.c                  Main program
  - USB_Device/AUDIO_Standalone/Src/system_stm32f2xx.c      STM32F2xx system clock configuration file
  - USB_Device/AUDIO_Standalone/Src/stm32f2xx_it.c          Interrupt handlers
  - USB_Device/AUDIO_Standalone/Src/usbd_audio_if.c         USBD Audio interface
  - USB_Device/AUDIO_Standalone/Src/usbd_conf.c             General low level driver configuration
  - USB_Device/AUDIO_Standalone/Src/usbd_desc.c             USB device AUDIO descriptor                                    
  - USB_Device/AUDIO_Standalone/Inc/main.h                  Main program header file
  - USB_Device/AUDIO_Standalone/Inc/stm32f2xx_it.h          Interrupt handlers header file
  - USB_Device/AUDIO_Standalone/Inc/stm32f2xx_hal_conf.h    HAL configuration file
  - USB_Device/AUDIO_Standalone/Inc/usbd_conf.h             USB device driver Configuration file
  - USB_Device/AUDIO_Standalone/Inc/usbd_desc.h             USB device AUDIO descriptor header file
  - USB_Device/AUDIO_Standalone/Inc/usbd_audio_if.h         USBD Audio interface header file  


@par Hardware and Software environment

  - This application runs on STM32F207xx/STM32F217xx devices.
    
  - This application has been tested with STMicroelectronics STM322xG-EVAL 
    evaluation boards and can be easily tailored to any other supported device 
    and development board.

  - STM322xG-EVAL Set-up
    - Connect the STM322xG-EVAL board to the PC for audio streaming through 
     'USB micro A-Male to A-Male' cable to the connector:
      - CN8 : to use USB Full Speed (FS)      
    - Use CN11 connector to connect the board to external headset
	

@par How to use it ? 

In order to make the program work, you must do the following :
 - Open your preferred toolchain 
 - Rebuild all files and load your image into target memory
 - Run the application
 - Open an audio player application (Windows Media Player) and play music on the PC host

 * <h3><center>&copy; COPYRIGHT STMicroelectronics</center></h3>
 */
