An Introduction to NFC
I used to work in a research lab that worked primarily with RFID (Radio Frequency IDentification) and NFC (Near Field Communication) technology, particularly for the agriculture industry and cattle identification. These were very specialized fields; however, the lab also worked on projects which involved retail and various other applications for NFC. It’s an amazing technology that you might be using every day without thinking about it - building access to your mobile phone payments, for instance.
NFC, while similar in many respects to RFID and has its history based on RFID, it is a separate concept. Where RFID has passive and active tags, with active tags being able to be read from relatively long distances, NFC, as its name implies, works in the near field region of the electromagnetic field.
RFID is still around and will be for the foreseeable future. NFC is a direct evolution of RFID, which you could perhaps consider a parallel branch. On the most basic level, typically, NFC is two inductively coupled devices, with the communication performed by modulating the power absorbed by the passive device. Passive RFID absorbs RF power, and then uses that to transmit data back to the reader - active RFID may use its own power source to transmit data back to the reader. As always seems to be the case with NFC, there’s an exception to the rule - with a type 5 NFC tag working longer distances (up to 1 metre), but we’ll get into that later.
In the most typical implementation of NFC, one device is Active, acting as a master in the communication and creating the modulated RF near-field that will power the Passive slave device. The active device often takes the name of the reader, while the passive one is called Tag. Common examples of Tags include stickers and embedded systems; the most common reader you will likely see for NFC in day to day life is a smartphone or payment terminal.
Near Field Communication
In typical RF communication, the transmitting antenna emits RF signals into free space, requiring antennas of at least λ/4 (quarter wavelength) to be effective. Two RF devices can typically communicate with each other when separated by over 2λ (two wavelengths), for example, about 245mm (10 inches) for 2.4GHz signals.
NFC instead communicates within the near-field region of space under λ/2 (less than half a wavelength). Two near field devices act as coupled inductors or two coils of a transformer wound through a common core.
Thanks to the adoption of NFC by both Apple and Google as a physical layer for payments, the number of smartphones shipped with the technology and the increasing familiarity of users with it has skyrocketed in recent years. This allows a huge range of interesting possibilities for applications and projects based around NFC.
NFC Technical Parameters
Because NFC tags operate in the near field, the technical capabilities and specifications are radically different from more conventional far field-based wireless technologies you might be more familiar with. Let us take a look at some of the interesting technical data associated with NFC and how it might compare to a far-field wireless technology.
A typical maximum outdoor range for 2.4GHz WiFi devices can be estimated at around 50 to 75 meters, although specialised point-to-point equipment can reach over 100Km while respecting the standard. Bluetooth devices, while running at the same 2.4GHz of WiFi, compromise on the bandwidth to improve power consumption and reach over 300 meters for the 5.0 revision. LoRa devices can surpass 10Km in range with a much lower power consumption than both technologies and exceedingly limited bandwidth.
NFC is limited by design to a maximum of 10cm.
Near field magnetic induction communication systems such as NFC have a very restrictive power density. The power density attenuates at a rate proportional to the inverse of the range to the sixth power. This is significantly greater than far field communications, so much so that at the end of the near field region for 13.56MHz (the most common frequency for NFC), energy levels are 10,000 to 1,000,000 times (-40dB to -60dB) lower than an equivalent intentional far field transmitter.
Like any good rule, this comes with an exception. NFC Type 5 Tags that use the ISO-15693 protocol can be read up to one-meter distance using specialised hardware; however, most if not all smartphones only adhere to the 10cm limit in practice. Some high-performance NFC antennas allow reading tags up to 15cm away, while on the opposite side of the spectrum, some the smaller stickers limit the distance to about 2 centimetres.
The reduced distance introduces the need for an explicit physical action by the user for the protocol to be functional. NFC is the only widespread wireless communications protocol that requires a user to take conscious action to use, in contrast to the always-on nature of both WiFi and Bluetooth.
The MHz is a unit of measure no one would expect to find in a modern smartphone’s wireless specifications. The iPhone 11 features communication frequencies of up to 8GHz, and 60GHz WiGig devices are getting traction in the market. NFC takes a different approach, lowering power consumption, range, price, and frequency.
The limited frequency in comparison to widespread high-speed connections, coupled with the short-range, makes NFC antennas relatively stress-free to implement. A wise designer may want to adopt a module with integrated antenna for Bluetooth or WiFi, for reasons of simplicity or inexperience. In the NFC realm, if you follow the recommendations for your IC, your design is likely to perform well regardless of minor manufacturing variances or nearby objects such as is the case at microwave frequencies.
The NFC standard supports a maximum bandwidth of 424Kbit/s, about eight times the speed of a legacy dial-up 56K connection. The limitation makes the standard comparable in performance to Bluetooth, at approximately half the data rate of the 4.0 revision. Unfortunately, the standard has a lot of overhead, and most devices often operate under 50Kbit/s. Even with such a limited data rate over the connection, with a bit of ingenuity and some creativity, applications are endless. With relatively limited memory on most tags, there is little need for higher data rates.
Most NFC tags include a memory between 100bytes and 1Kbyte, although models are available with a memory capacity of up to 64 Kbytes. These larger memory capacities are often adopted for smart card uses.
Whilst this amount of memory sounds somewhat limiting, it allows a significant number of 8-16 bit (1-2 byte) sensor readings or data about what the Tag is attached to.
Many NFC ICs offer an energy harvesting output able to supply around 5mA in optimal conditions. Here are suggestions in no particular order of what to do with less than 5mA:
- Power external sensors and analog circuitry. In the case of temperature sensing, NFC ICs with embedded temperature sensors are already widespread to track perishable goods.
- Power an external memory bigger than what your NFC IC provides.
- Update an e-ink display.
- Recharge a small lithium battery.
- Recharge a supercapacitor (electrolytic double layer capacitor)
NFC stickers, tags and basic ICs are widely used for asset tracking, and as such need to be extremely affordable. If every item in a supermarket has an NFC tag on it, or every garment in a clothing store, even 50c per tag would quickly become prohibitive. Luckily the simplicity of NFC allows easy production of stickers and basic tags, with costs ranging from under 10c to 50c per tag depending on the volume.
When reading the NFC standards, you get the idea that they have been built to support a multitude of existing standards and applications. Luckily for our mental sanity, almost no electronic engineer working on mainstream NFC implementations will need an understanding of the depths of NFC Tag Types; All modern smartphones have to support every single Tag Type to be NFC-compliant. The capabilities and features of each and every NFC IC, be it active or passive, Tag or Reader, Sticker, or an entire SoC, will be clearly stated in the datasheet.
The vast majority of NFC cards, stickers, and asset tags implement NFC Forum Tag Type 2. Rudimentary storage and retrieval of information, either through the NFC communication or an I2C interface for connection to a microcontroller, can be satisfied by Tag Type 2.
The capability of performing computations on top of the storage and retrieval, and advanced security features, are supported primarily by Tag Type 4. Finally, if you need long-range reading coupled with allowing users to interact with the Tag through a smartphone, you’re going to need a Tag Type 5.
If you are building a reader for a custom application, the tag type you use is most likely going to be determined by the application and what tags you can purchase rather than the selection of a specific tag type. If you are building a custom tag, your selection will likewise usually be made for you, as the NFC IC you are using for your tag implementation will determine the tag type.
NFC Forum Tag Type 1
The NFC Type 1 Tag, as the number may indicate, is the simplest and cheapest among all NFC Tag types. Typical Type 1 Tags support the read-only or write-once operation. However, R/W capable models are available, have memories commonly of around a hundred bytes (2 Kbyte max), and comparatively slow 106kbit/s bandwidth. Typical applications include stickers, marketing, product tracking, and as such, these Tags are of limited interest to the average electronic engineer.
The standard adopted by NFC Type 1 tags is ISO-14443A.
NFC Forum Tag Type 2
Similar to Type 1 tags, NFC Type 2 Tags supports only the ISO-14443A in both read-only and read-write applications. Many Type 2 ICs offer energy harvesting functionality and I2C connection to an external microcontroller; however, Type 2 tags are prevalent as stickers, cards, and tickets.
As mentioned above, a Type 2 tag is the most commonly found on the market, in a huge range of shapes and sizes for stickers to cable ties.
NFC Forum Tag Type 3
The Type 3 tag, based on the Sony FeliCa protocol, is adopted mostly in Japan and Asia. This Tag Type is utilised frequently for electronic money, identification, transit tickets, and similar applications in the relevant Japanese market. The standard has seen limited adoption for international electronic goods, and its primary uses include supporting legacy applications.
NFC Forum Tag Type 4
The Type 4 Tag, compatible with ISO-14443A and ISO-14443B protocols, adds support for the ISO-7815 standard for smart card identification. These tags can modify the data contained in their memory and are customarily employed for security, identification, and payment applications.
NFC Forum Tag Type 5
The Type 5 Tag is the latest specification to be released. The underlying physical layer is different from all other NFC Tag types as it is based on RFID technology (ISO-15693) instead of ISO-14443A, allowing for an increased reading distance of up to 1.5m. Customer NFC devices such as smartphones are, however, limited to 10cm as any other NFC tag type, and interacting from further away require specialised readers.
Types of NFC Interaction
NFC supports three main communication modes: Read/Write, Peer-to-Peer, and Card emulation.
The Read-Write mode is the simplest and most common operating mode of the NFC standard. The NFC tag implements a memory whose content usually consists of NDEF (NFC Data Exchange Format) formatted data. The reader can read or write the content of such memory.
The definition is rather simple, but like most simple concepts in electronics, it’s exceptionally versatile.
The memory of the NFC tag can often be accessed not only by the reader but also by the Tag itself when it has the capabilities of a full SoC (System on Chip) or connects with an external microcontroller. In this scenario, the Tag acts as a dual-port memory, like a database accessed by two computers. Furthermore, the Tag itself can be powered both by the NFC Reader and/or by the secondary MCU. Thus, the Tag is always powered wherever it is accessed, just like a database hosted somewhere on the internet.
Peer to Peer Mode
In Peer-to-Peer mode, two active devices communicate with each other through one of the following two approaches.
One device could simulate being a Tag, while the other could simulate being a Reader. This scenario is clearly a master-slave solution. It’s well suited for when you have a “smart” and a “dumb” part of your system, such as a master microcontroller communicating with a secondary smaller MCU through a physical barrier. This mode also allows the system to retain compatibility with smartphones since the smartphone itself can act as the reader and the embedded device as the Tag.
The more abstract approach, but also the more flexible one, of implementing Peer-to-Peer Is through a protocol called LLCP (Logical Link Control Protocol), which supports binary communication both with (TCP style) and without (UDP style) an underlying connection. The LLCP protocol is meant to allow arbitrary and flexible communication between two active devices that resembles BSD sockets and is familiar to many embedded and Unix programmers. Uniquely, the LLCP protocol allows the quick porting of existing protocols such as Modbus, RS485, CAN, LIN, or UART over an NFC connection, allowing novel ways of communicating.
The disadvantage of exchanging data through the LLCP protocol is it’s lack of support on smartphones, relegating it to embedded-to-embedded communication or embedded-to-pc. If you’re building a custom NFC application or NFC could be used to meet your project requirements without any need for smartphone integration, LLCP might be what you’re looking for if you need a close-range wireless connection.
Card Emulation Mode
Card emulation mode allows NFC enabled devices to simulate NFC Tags, similar to the way iPhones allow payments to Apple Pay® or Android devices through Google Pay®. This mode is likely to be of minimal relevance for most electronic engineering applications.
NFC is a versatile protocol which adds significant new capabilities over RFID for short-range tag reading and communication. It can be used to build access control/authentication to devices, sensor data sharing, and a myriad of other applications.
If you require a secure short-range wireless connection, NFC offers significant advantages over a typical far-field wireless protocol due to the inherently limited range of communication and rapid dropoff of field strength that makes intercepting communications extremely difficult. This can add an additional layer of security on top of encrypted protocols.
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