Even though at the time we sorta thought this would be a niche product, it turns out it’s quite popular thanks to the straight-forward simplicity
Basically, this chip is a ‘smart’ RFID/NFC tag. Normally, when you think of an NFC tag, you’d picture one of these RFID cards or stickers. These are small chips that have built in memory that can be read or written when self-powered by an RFID field. One of the neat things is that they are electronic devices that do not get powered by a battery. Don’t get us wrong, we really like batteries but they are real pain to manage – you have to manage sleep stages, recharging, under/over voltage, chemical leaks, etc. Low power management is really hard.
The nifty thing about RF-powered electronics is how many problems they ‘solve’: by default they draw no power, so you don’t have to worry about sleep modes. There’s no battery, so no recharging or replacement needed. They only turn on when physically close to the reader, and auto-turn-off when the reader is away – the UX is intuitive and does not require any buttons.
Most tags and cards address a static chunk of memory, maybe a few K in size. You touch to write/read or authenticate. They’re super cheap, easy to embed in every credit card. However, they often don’t do anything other than the read/write/auth behaviors – it isn’t possible to expand the capabilities since there are only a couple different chips. That’s where the ST25DV series comes in!
The ST25 series looks just like an ISO 15693 tag to a reader, but there’s also an I2C interface that can be used to easily change the memory contents. This means it’s a fast and mobile-friendly way to transmit data back and forth without a massive amount of development to handle the data encapsulation. Both Android and iOS have built in NFC support for this tag format and you can easily query/write data via the NFC API. ISO 15693 has a specification of up to 1 meter (when used with a large-antenna RFID reader), a few inches from a phone.
For the antenna design, ST has a nice eDesignSuite website that will do all the math for you, to create an antenna tuned to the size you have available.
Even if your product does have a battery, say for a datalogging system, using NFC for data transfer will use a lot less power than BLE or WiFi – and without the mess of differentiating many nearby devices, BLE pairing, or wireless/cellular credentials & deployment.
And usage is super simple – we were grateful to see an Arduino library written by ST with an instant gratification example. Thanks to I2C’s universality, the code can be ported to any microcontroller.
If your microcontroller is a low-power device, you may be able to use the energy harvesting output from the ST25 chip to power your circuitry. Depending on the EM field strength, you can get up to about 3V at 1mA or 7mA at 2.3V.
You can order these in a few different SMT packages and sizes from Digi-Key – the SOIC and TSSOP packages are available now with 4Kbit,
16Kbit and 64Kbit memory size.
The new KC series seems to add the ability to change the I2C address, but if you don’t need that, the plain K series is pin compatible as well. We recommend starting with the SOIC version of the ST25DV16KC for an easy-to-solder starter chip. It’s in stock and ready to ship immediately from Digi-Key – order today and you can be NFC’ing by tomorrow afternoon!
See at Digi-Key at https://www.digikey.com/short/jt4h3rrh
See the ST video below: