USBD_Audio_DrvStreamPlaybackTx

Description

Provide a ready playback buffer to codec.

Files

usbd_audio_drv_<codec-name>.h / usbd_audio_drv_<codec-name>.c

Prototype

static  void  USBD_Audio_DrvStreamPlaybackTx (USBD_AUDIO_DRV  *p_audio_drv,
                                              CPU_INT08U       terminal_id_link,
                                              void            *p_buf,
                                              CPU_INT16U       buf_len,
                                              USBD_ERR        *p_err);

Arguments

p_audio_drv

Pointer to audio driver structure.

terminal_id_link

Terminal ID associated to this stream.

p_buf

Pointer to ready playback buffer.

buf_len

Buffer length in bytes.

p_err

Pointer to variable that will receive the return error code from this function.

USBD_ERR_NONE

USBD_ERR_TX

Returned Value

DEF_OK, if NO error(s) occurred and request is supported.

DEF_FAIL, otherwise.

Callers

Audio Class.

Implementation guidelines

This listing shows an example usage of this function.

Listing - USBD_Audio_DrvStreamPlaybackTx() Example Usage

#define  DMA_CH_0                                  0u 
#define  DMA_MAX_NBR_CH_USED                       2u
static   CPU_INT08U  DMA_AsHandleTbl[DMA_MAX_NBR_CH_USED];
 
static  void  USBD_Audio_DrvStreamPlaybackTx (USBD_AUDIO_DRV  *p_audio_drv,
                                              CPU_INT08U       terminal_id_link,
                                              void            *p_buf,
                                              CPU_INT16U       buf_len,
                                              USBD_ERR        *p_err)
{
    /* $$$$ Store ready playback buffer. */					                  (1)
    /* $$$$ Prepare DMA transfer for initial buffer. */					      (2)
}

(1) The ready buffer provided by the audio class could be stored internally into the codec driver using any buffer memory management method (for example a circular buffer). Then the stored ready buffer could be retrieved by the audio peripheral ISR.

The Audio Peripheral Driver should support at least double-buffering to optimize the playback streaming.

(2) If the Audio Peripheral driver support buffer queuing, you may accumulate a certain number of ready buffers (for instance, 2 buffers should be enough). When the buffers accumulation is done, you should prepare the initial DMA transfer. When the stream is launched, the next DMA transfer may be prepared in an interrupt context when the current DMA transfer completes.

If the peripheral managing audio data transfer can work with a DMA controller, we recommend using DMA transfers. It will improve overall audio performance by offloading the CPU. As opposed to CPU copies involved for FIFO transfers that would be time-consuming.

(3) In case of error, you may return the error code USBD_ERR_TX.