/**
  @page AUDIO_Standalone USB Device AUDIO application
  
  @verbatim
  ******************** (C) COPYRIGHT 2015 STMicroelectronics *******************
  * @file    USB_Device/AUDIO_Standalone/readme.txt 
  * @author  MCD Application Team
  * @version V1.0.2
  * @date    18-November-2015 
  * @brief   Description of the USB Device AUDIO application.
  ******************************************************************************
  *
  * Licensed under MCD-ST Liberty SW License Agreement V2, (the "License");
  * You may not use this file except in compliance with the License.
  * You may obtain a copy of the License at:
  *
  *        http://www.st.com/software_license_agreement_liberty_v2
  *
  * Unless required by applicable law or agreed to in writing, software 
  * distributed under the License is distributed on an "AS IS" BASIS, 
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  *
  ******************************************************************************
  @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 STM32F746xx 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 STM32F746xx USB OTG Device peripheral and SAI peripheral to 
stream audio data from USB Host to the audio codec implemented on the STM32746G-Discovery 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 216 MHz. The Full Speed (FS) USB module uses
internally a 48-MHz clock which is coming from a specific output of two PLLs: main PLL or PLL SAI.

The 48 MHz clock for the USB FS can be derived from one of the two following sources:
  – PLL clock (clocked by the HSE): If the USB uses the PLL as clock source, the PLL clock must be programmed
    to output 48 MHz frequency (USBCLK = PLLVCO/PLLQ).
  – PLLSAI clock (clocked by the HSE): If the USB uses the PLLSAI as clock source, the PLLSAI clock must be programmed
    to output 48 MHz frequency (USBCLK = PLLSAIVCO/PLLSAIP).

Please note, in this application the Audio clock is generated from an integrated PLL (PLLI2S).

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 speed-up 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


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_stm32f7xx.c      STM32F7xx system clock configuration file
  - USB_Device/AUDIO_Standalone/Src/stm32f7xx_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/stm32f7xx_it.h          Interrupt handlers header file
  - USB_Device/AUDIO_Standalone/Inc/stm32f7xx_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 STM32F746xx devices.
    
  - This application has been tested with STMicroelectronics STM32746G-Discovery
    boards and can be easily tailored to any other supported device and development
    board.

  - STM32746G-Discovery RevB Set-up
    - Connect the STM32746G-Discovery RevB board to the PC through 'USB micro A-Male 
      to A-Male' cable to the connector CN13 to use USB Full Speed (FS)
    - Plug an external headset into CN10 connector.
    
@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>
 */
