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A single SD/MMC volume is opened as shown in Listing 14-2 . The file system initialization (FS_Init()) function must have previously been called.
ROM/RAM characteristics and performance benchmarks of the SD/MMC driver can be found in section 11-1-1 “Driver Characterization” Driver Characterization. The SD/MMC driver also provides interface functions to get low-level card information and read the Card ID and Card-Specific Data registers (see section A-12 “FAT System Driver Functions”).
Listing 14-2 Opening a SD/MMC device volume
L14-2(1) Register the SD/MMC SPI device driver FSDev_SD_SPI.
L14-2(2) FSDev_Open() opens/initializes a file system device. The parameters are the device name (1a) and a pointer to a device driver-specific configuration structure (1b). The device name (1a) is composed of a device driver name (“sd”), a single colon, an ASCII-formatted integer (the unit number) and another colon. Since the SD/MMC SPI driver requires no configuration, the configuration structure (1b) should be passed a NULL pointer.
Since SD/MMC are often removable media, it is possible for the device to not be present when FSDev_Open() is called. The device will still be added to the file system and a volume opened on the (not yet present) device. When the volume is later accessed, the file system will attempt to refresh the device information and detect a file system (see section 7-2 “Using Devices” for more information).
L14-2(3) FSVol_Open() opens/mounts a volume. The parameters are the volume name (2a), the device name (2b) and the partition that will be opened (2c). There is no restriction on the volume name (2a); however, it is typical to give the volume the same name as the underlying device. If the default partition is to be opened, or if the device is not partition, then the partition number (2c) should be zeroFAT System Driver Functions).
If the SD/MMC initialization succeeds, the file system will produce the trace output as shown in Figure 14-10 (if a sufficiently high trace level is configured). See section E-9 “Trace Configuration” about configuring the trace level.