RFID is an advanced technique for attaching information to objects of all kinds, and ICs such as the ST25DV series can help you to implement this technology.

Those of us who have no experience with manufacturing environments or shipping warehouses probably cannot imagine the immense effort required to “keep track of everything,” for lack of a more sophisticated description. I’m assuming that bar codes and other techniques are still widely used, but RFID (radio-frequency identification) offers significant advantages.

So what exactly is RFID? It’s the same general idea as a bar code: We want to attach information to a physical object. Bar codes accomplish this using black-and-white patterns that are optically scanned and interpreted as data. RFID accomplishes this via tags that are attached to or embedded within an object; the tag contains digital data that can be accessed via electromagnetic radiation.

 

Those of you who enjoy a trip to the library have perhaps seen RFID tags similar to this one, which happens to be adorning the inside back cover of a delightful Monet collection.

 

Ongoing semiconductor miniaturization allows RFID tags to store relatively large amounts of data, but the primary advantage is the ease of accessing this data. Optical scanning requires a clear line of sight, whereas RFID comes with the typical benefits of RF communication: spatial flexibility, long range, and the ability to transcend the limits of opacity.

 

Passive vs. Active

An active RFID tag has its own power source. This is presumably the preferred approach when long range is an important design goal. In general, though, I’m more impressed with passive tags, which are powered (wirelessly) by the RFID reader device. The ST25DV series from STMicroelectronics are highly integrated RFID ICs that allow for both active and passive operation.

 

Diagram taken from the ST25DV datasheet.

 

Actually, the ST25DV devices are described as “NFC/RFID” tags. NFC stands for near-field communication and refers to a specific short-range RFID protocol that is commonly incorporated into mobile devices. Thus, if you want your device to offer convenient wireless communication with a smartphone, an ST25DV can help you get there.

 

What Exactly Does It Do?

The datasheet describes an ST25DV device as a “mailbox between RF . . . and an I2C host” (usually the host would be a microcontroller). I’m not sure if I find this description helpful; in any event, the ST25DV is not a single-chip solution. It’s designed to work in conjunction with a microcontroller.

The ST25DV communicates with the RFID reader via a protocol described on page 98 of the datasheet. Here’s the general idea:

  1. Communication is initiated by the reader, aka the VCD (vicinity coupling device).
  2. The ST25DV must detect and correctly interpret the message from the VCD before it attempts to transmit.
  3. Communication proceeds as an exchange: first a request is sent by the VCD, then the ST25DV responds. The request and response messages are separated by delays (labeled t1 and t2 in the diagram below).

 

Diagram taken from the ST25DV datasheet.

 

Harvesting Energy?

As I mentioned above, the ST25DV can be used as a passive RFID device. The datasheet describes this as an energy-harvesting feature, which seems a bit odd to me, as I usually think of energy harvesting as powering an electronic device using ambient energy that was not generated with the intention of powering an electronic device. Semantics aside, I appreciate the extensive power-supply functionality that is incorporated into the ST25DV series.

The chip can be powered by a battery via the internal 1.8 V regulator. For VCD-powered applications, the antenna connected to pins AC0 and AC1 serves as both a communication interface and a power supply.

It turns out, though, that the ST25DV can also power additional devices from the received field. This is done through the V_EH pin, which serves as a power source for external circuitry. If the ST25DV does not have enough power for itself and external devices, the V_EH pin is automatically set to high-impedance. If you’re interested in more details, you can find a painfully complicated state diagram on page 61.

 

Applications

Above I mentioned manufacturing and shipping, but RFID really has a wide variety of possible applications. It’s worth considering whenever you need to associate easily accessible digital information with a physical object (or an animal, or even a person, but at that point you’re wading into murky ethical waters).

One thing I like about the ST25DV is that the package options are actually experimentation-friendly:

 

Diagram taken from the ST25DV datasheet.

 

It’s not every day that you can buy a new, highly sophisticated IC in an SO8 package. And of course there’s also the wafer option . . . but you might want to hold off on that until you’ve spent a day or two with your prototype.

 


 

Have you ever designed an RFID device? If you have any tips, feel free to mention them in the comments.

 

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