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Performance of SPI SD Controller

Honored Contributor II



I am planning on using SD card and FAT library with my Altera design. SPI mode seems the simplest variant of all, so google'd for some benchmarking of this mode. The only one I could find was the one at this page (http://www.alterawiki.com/wiki/mmc/sd#mmc_spi_connection_on_de2) (this benchmark report (http://www.fps-tech.net/doc/sd_spi_comparison.pdf)).  


Can anyone give me a clue why the SPI performance is that low? 75 kB/s is really disturbing... The SPI clock set there is 15 MHz. Yes, I can't expect 15 Mbps, but I can't imagine where the performance drop occurs. Can anyone please explain me this? 


Thank you!
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Honored Contributor II

If I recall, SD over SPI is pretty slow because it is not streaming. Add to that the FAT protocol overhead and formatting. See if you can increase the block size to get a better data:overhead ratio. You may want to look into 4-bit SD mode with DMA engines instead.