MMC Reader

The accelerometer I built uses a Multimediacard to store the data. After the data is collected you need a way to read the contents of the card.

The following images show a simple MMC card reader that hooks to a parallel port. The circuit contains an MMC card socket, a coin-sized Lithium battery, male DB-25 connector, 74AHC244 chip used for level shifting, one resistor, one capacitor and a switch.

WARNING: I honestly don't know what I'm doing and this thing could easily fry your PC's parallel port. Mine has survived so far but you shouldn't use this circuit if you care about your port. Get someone more knowledgeable to design a better one for you!!!

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The battery is used to supply the relatively high current needed by the MMC when it is writing a block. You might be able to get enough current from some parallel port pins but I didn't want to risk blowing something up. The 74AHC244 is used as a level shifter between the 5v present on my PC's parallel port pins and the 3.3 volts needed to run the MMC card. This chip must be from the 74AHC family, 74LS, 74HC and all other 74 families will not work, at least not for long.

This unit has one quirk (well, at least I only know of the one). After plugging it into the parallel port, the circuit gets powered through some of the 74AHC244 pins and the circuit basically starts running at 5v. This wouldn't be good for an MMC which has a 3.6v limit. Because of this, you have to power the reader on after plugging it into the PC but before plugging the card into it. Like I said, don't use this circuit if you value your PC's parallel port. I can almost guarantee it will destroy it eventually.

If you haven't been scared off yet, here's the schematic to this little port killer and links to "C" routines to read the data and convert it to accelerations. This was written for a machine running Linux.

Data download routines. Gzipped tar file, 3410 Bytes.

Acceleration conversion routine.

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