Has anyone here played with MIFARE DESFIRE cards? They’re programmable RFID cards, like smart cards it seems. Does anyone know where I can purchase a devkit for them?
I haven’t used them, but Futurlec carries the cards and reader/writer: http://www.futurlec.com/RFID_Cards.shtml
Edit: Just checked again and I’m not sure if they are the DESFIRE cards, so the link may not be helpful.
Well, it might help if I tell why I’m using it. I’m working on a device, where if I put a bag of RFID sensors on top of a specific area it’ll give me the list of RFID tags on top of it. I’m thinking the number of tags will be from 1-10. I know with most standard RFID tags it wont work because they’ll step on each other. Since the MIFare DESFire cards are programmable. I’m guessing I can inquire if a card is near the sensor. Something like “Are cards 55, 78, or 22 here?” and only those three cards would respond. I want to read some documentation on one of these cards before I pour several hundred dollars into it.
Ah, yes. Your description was helpful and your analysis correct about the tags colliding with each other. I looked into this exact problem a while back and I couldn’t find a good solution for the price-point that our customers would need. I think you are on the right track in that the cards need some type of strategy for collision avoidance. I know that it is possible to do as the PASS system to read passport RFID tags needs to process up to 55 tags at a time with a passive RFID tag. You may find this [RFID paper helpful as it describes various collision avoidance strategies. Although it seems that you would need to pick your reader and algorithms, it is possible to achieve your application without a custom tag, and even if want to roll your own tag, the paper should still be useful to you.
It was interesting to note that one method described in the paper was similar to an approach that the Dallas/Maxim one-wire devices use to identify themselves while attached to the same one-wire network.](http://www.vs.inf.ethz.ch/edu/SS2004/DS/reports/06_rfid-mac_report.pdf)
The Desfire cards may not be the right approach to this if security is not required. Anti Collision is a requirement of the ISO 15693 protocol. This allows you to read many tags simultaneously. Reading 10 tags is no problem depending on your antenna size and range requirement. ISO 15693 operates at 13.56MHz and is better suited for general purpose applications where security is not required. The tags can be in a variety of form factors. How large of an area do you need to cover? How large can the tags be?
Description of Anti Collision Sequence in ISO 15693 (TI TRF7960)
http://focus.ti.com/lit/an/sloa138/sloa138.pdf
One thing to consider also is the seperation between tags. If tags are directly on top of each other, this can cause mutual detuning of the antennas. A couple mm seperation usually helps. Smaller tags seem to be less affected by mutual detuning.
DLP Design makes a nice ISO 15693 reader with a USB interface. They also have an 8 channel version that can multiplex 8 antennas.
Another approach would be the selective addressing feature of the TI low frequency RFID tags like MUSA and SAMPT. The MUSA can be selectively addressed and works quite well when a small tag size(12mm) is required.