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SPI speedup is working!

December 13, 2013

terminalOK!  The SPI interface is stable at 400khz for reading the SD card.  The terminal image on the right is the same text as when I originally got it working but it popped up in a blink instead of a crawl.  Peak data transfer for reading is about 16,000 bytes per second and the sustained speed is about 10,000 bytes/second.  At 400khz it should be 25,000 bytes/sec peak but when reading you have to send a byte before you can read one and you have to wait two instruction times between the write and the read for the external shift to complete. I suspect there’s a clever way to tighten the loop a bit but I haven’t seen it yet.  The schematic is below:

shield6-2

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