I am running a test using the fatfs third-party code supplied in Stellarisware and the specific LM3S9B96 port.
The code starts up with an SPI clockrate of 400K as mandated by the SD specification for MMC support. Once the card type is determined it ramps the SPI clock up to 12.5MHz (set_max_speed). This speed is according to the spec for SDHC Normal Speed ( https://www.sdcard.org/developers/overview/bus_speed )
If the SPI was used exclusively for data transfer that would translate to a rate of 1.25MB/second. In practice I cannot achieve a rate better than 0.38MB/second. Is this reasonable? I used a read buffer of multiples of 512 to avoid the overhead of moving partial data out of sectors.
If my understanding is correct, there is no way that I can achieve the speed rating of the MSD card e.g. 4MByte/sec for a class 4 card using simple SPI. So what do I need to do?