创建媒体驱动程序：读取扇区
[创建 FreeRTOS-Plus-FAT 媒体驱动程序]

每个 FreeRTOS-Plus-FAT 媒体驱动程序都需要一个函数来从存储嵌入式文件系统的媒体中读取扇区。读取函数的实际工作方式取决于媒体类型。例如，如果媒体是 RAM 磁盘，则可以使用 memcpy() 从 RAM 读取数据，但如果媒体是 SD 卡，则卡的命令接口必须通过 MMC 或 SPI 外围驱动器使用。

读取函数

读取函数可以具有任何名称，但必须具有以下原型:



int32_t prvFFRead( uint8_t *pucDestination, /* Destination for data being read. */

                   uint32_t ulSectorNumber, /* Sector from which to start reading data. */

                   uint32_t ulSectorCount,  /* Number of sectors to read. */

                   FF_Disk_t *pxDisk );     /* Describes the disk being read from. */



The prototype of a function used to read sectors from the media holding the embedded file system

示例大致描述了 FreeRTOS-Plus-FAT RAM 磁盘驱动器中使用的读取函数。完整版本包含输入参数检查，， /FreeRTOS-Plus/Source/FreeRTOS-Plus-FAT/portable/common/ff_ramdisk.c。



/* Each sector is 512 bytes. */

#define ramSECTOR_SIZE    512



static int32_t prvReadRAM( uint8_t *pucDestination,

                           uint32_t ulSectorNumber,

                           uint32_t ulSectorCount,

                           FF_Disk_t *pxDisk )

{

uint8_t *pucSource;



    /* The FF_Disk_t structure describes the media being accessed.  Attributes that

    are common to all media types are stored in the structure directly.  The pvTag

    member of the structure is used to add attributes that are specific to the media

    actually being accessed.  In the case of the RAM disk the pvTag member is just

    used to point to the RAM buffer being used as the disk. */

    pucSource = ( uint8_t * ) pxDisk->pvTag;



    /* Move to the start of the sector being read. */

    pucSource += ( ramSECTOR_SIZE * ulSectorNumber );



    /* Copy the data from the disk.  As this is a RAM disk data can be copied

    using memcpy(). */

    memcpy( ( void * ) pucDestination,

            ( void * ) pucSource,

            ( size_t ) ( ulSectorCount * ramSECTOR_SIZE ) );



    return FF_ERR_NONE;

}



The read function used by the RAM disk driver - for clarity input parameter checking is not shown

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创建媒体驱动程序： 读取扇区 [创建 FreeRTOS-Plus-FAT 媒体驱动程序]

读取函数

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创建媒体驱动程序：读取扇区
[创建 FreeRTOS-Plus-FAT 媒体驱动程序]