The I2C Programmable NFC Tag

NFCNFC tags are cool, but programming them to do your bidding – whether unlocking your computer, making an Arduino vending machine, or a smart home application – requires using an NFC device to program the tag over the air. An NFC tag programmable with any ‘ol microcontroller would certainly have some interesting applications, and Elecfreaks’ DNFC tag is just the thing to test out these ideas.

While NFC tags are reprogrammable, reprogramming them requires an NFC controller, be that through a dedicated hardware, a phone, or an Arduino shield. The DNFC tag is reprogrammable with a microcontroller with an I2C interface thanks to TI’s RF430CL330H dynamic NFC transponder IC. It still does everything you would expect from a NFC tag – MIFARE compatible. NDEF reading and writing, and everything else – but you can program it through an Arduino, Pi, or any other board with an I2C interface.

TI has an app note on using the chip inside the DNFC for automatic Bluetooth pairing, and Elecfreaks themselves have a few use cases in mind that include putting WiFi credentials on an Arduino board without putting the SSID in code and other Internet of Things™ applications. We’re thinking this is one of those devices that is eminently useful, but for something we just can’t think of off the top of your head. If you’ve got an idea for how to use an I2C programmable NFC tag, drop a note in the comments.

Elecfreaks is doing an Indiegogo campaign for the DNFC, $13 for one. I picked one up, but it’s flexible funding, so buy it or don’t. I don’t care.


  1. Nice project!
    I just launched into production a very similar project… but much smaller and designed for the xNT implant and NFC rings (any backers here?).
    Current PCB size is 35x45mm.

    • Matt says:

      Mathieu Do you have a link to that PCB? I have one of your whistled and really like that and would be interested in the NFC board you have.

      • Hey Matt,
        It’ll still take a long while before we have anything ready (hopefully less than 2 months) as there’s a lot of firmware to be written. Do you have an xnt or an nfc ring? If so I may be able to hand solder one board if you’re willing to code with us ;).
        I’m glad to know you’re satisfied with the whistled :).

        • thematthewknot says:

          I actually have a xNT in my hand, and have three more friends local who also have one installed. I backed the ring project(haven’t arrived yet) but again 3 of my friends backed it too only one has gotten his in yet. The ring seams to be more particular than the xNT which was kinda surprising. Please do let me know if there’s anything i can do to help with your board i am super stoked to hear your working on one that small, currently the smallest NFC sensor I’ve been able to find is the PN532 on eBay.

        • 0xfred says:

          I’ve also got a xNT and will be interesting in seeing / helping with what you’re doing. I’ll drop you an email.

      • Przemo-c says:

        I have a xNT tag in my right hand and am experimenting with various readers .. also modifying s4 battery to read better. Im also very interested in your reader.

  2. chango says:

    ST has a similar series, 24LRXX, and a $31 (at Digikey) kit with 10 different form factor antenna reference designs: . There’s also a Discovery kit with one of these on it.

    I haven’t looked at the TI part, but what’s nice is that these appear on the I2C like their equivalent commodity 24LCxx parts, but with registers added to the I2C space for NFC configuration.

  3. ejonesss says:

    that’s nice but hearing all the horror stories about networked appliances being turned into spam bots and spying it may be a goos idea to check to make sure there is protection built in.

    • chango says:

      I could see where bad implementations of this could be used for evil.

      For setups where it’s communicating pairing data, failure to lock down writes from the NFC end could let an attacker write an NDEF with a malicious URL, so the next user would get sent to a site rather than receive pairing data.

      The worst case though would be if the EEPROM is used for storing config data for a router or some IoT thing. It would be easy for someone to do a walk-by and overwrite IP configuration on a router, or worse write a payload to abuse any exploits in the config parser.

      In any of these cases, physical access is needed.

  4. The ST M24SR are available in SO8 package – easier for soldering

  5. Petiepooo says:

    I’m interested in emerging NFC and RFID tech. I think this is the sort of thing that I would find useful for what I have in mind.

    I’m just having a little trouble parsing the Chinglish on their Indigogo page, though. Down towards the bottom of the page is a list of different perk levels and what they contain. What is the “Tag Board” item and how does it relate to the DNFC item?

  6. Joe says:

    “I picked one up, but it’s flexible funding, so buy it or don’t. I don’t care.”
    What a stupid, senseless thing to write.

    On the other hand, great project! Thanks.

  7. marcus says:

    why it is called “reprogrammable”? but I believe it is only “data updatable”?

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