36 lines
1.5 KiB
Plaintext
36 lines
1.5 KiB
Plaintext
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SD cards have very complex controllers and they are evolving in a manner that
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make them difficult to use with the Arduino.
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Higher end cards often perform poorly on the Arduino. Here is a bit of
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background.
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First, SPI mode is not used in most devices so the SPI controller is not
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very good.
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The flash erase groups in SD cards are very large, 128 KB is common. This
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means that rewriting file structures can result in a huge amount of data
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being moved.
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This is what is happening when you see long clock activity. The card
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indicates busy by holding data out low. The spec allow a card to go busy
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for up to 250 ms.
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Cards have two write modes, single block random mode and multiple block
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sequential mode. For file writes I must use single block mode. This is
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always slow but often extremely slow in high end cards. microSD cards also
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have poor support for this mode.
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I have several applications that use multiple block mode and they run much
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faster. The binaryLogger.pde example can log 40,000 16-bit samples per
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second without dropping data on a good SanDisk video card.
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Looks like I should develop a Serial logger using this method. It requires
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allocating a huge contiguous file and writing it using the multiple block
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sequential mode. This makes the app more complex and not so easy to
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understand.
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The write time for a 512 byte block in sequential mode is about 850
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microseconds with occasional busy times of 2 - 3 ms on a SanDisk Extreme
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30 MB/sec card. In random mode this card is often slower than five year
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old class 2 cards.
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