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SD AdaptorThis SD Adaptor board allows a Micro SD card to be used with Teensy, or almost any microcontroller with a SPI port. Only 4 SPI signals, plus power and ground are required. A 3.3 volt regulator and buffer chip allows using Micro SD cards with 5 volt systems.
DimensionsThe board size is 0.7 by 0.52 inch. The pins are 0.1 (2.54 mm) spacing, compatible the solderless breadboard and most electronic prototyping materials. The two 5-pin rows are 0.6 inch apart.
Teensy++ ConnectionsThe SD adaptor can be used on Teensy++, with 4 pins and 2 wires.
Software SupportTeensyduino 0.9 (currently in experimental alpha release) supports SD cards for USB communicatoin. Select it from the Tools->USB Type menu after installing version 0.9.
Several Arduino libraries support access to SD cards.
The SdFat library has been tested and is known to work on Teensy 2.0 and Teensy++ 2.0 with this SD adaptor.
Switch (SW) PinThe flash socket has a switch that can detect when a card is inserted. Normally SW is not used. Most projects don't need this pin, and nearly all SD libraries just check for the card by communicating with it rather than requiring an extra pin.
But if you design your code for a switch, or use someone else's code that requires a switch, the option is there. The switch connects to an an unpopulated resistor, so a low-value resistor or jumper wire needs to be soldered in place to enable access to the switch.
The switch might also be useful in special cases, like perhaps a battery powered project that waits for a card to be inserted. The pin change interrupts can wake the processor from a deep sleep mode, which would save battery life compared to regularly polling to see if a card has appeared.