Near field communication (NFC) is a wireless communications technology that allows two devices to communicate when they are in close proximity, typically just a few inches from each other. Today, it is mostly recognised by consumers as a way to make contactless purchases with smartphones. However, it also contributes to connecting objects to the Internet of Things, which changes the way consumers interact with those connected things.
Although NFC capability is standard in many new smartphones and tablets, the time is coming for NFC connectivity in many other kinds of electrical or electronic devices.
The key benefit of embedding NFC capability into almost any kind of equipment or device is that the device can then be designed to offer all the features that would be possible if they had keyboards, graphic displays and Internet connections, without actually having to incorporate these expensive and space-consuming features into the equipment itself – the user’s smartphone already has them and can ‘lend’ them.
One of the potential benefits enabled by STMicroelectronics’ M24SR family is convenient Bluetooth pairing of a smartphone and an audio device. Instead of having to open the phone’s settings, turn Bluetooth on, select the audio device code name, and sometimes enter a pass code, users can simply wave their smartphone by the NFC-enabled audio device and automatically activate Bluetooth pairing between the two devices as settings have been transmitted over NFC.
Other examples include resetting the clock on any appliance after a power blackout or a change in daylight saving hours or rapid diagnosis of faults in malfunctioning equipment. Instead of having to look for the instruction manual or call the manufacturer’s customer service, the user could simply run the ‘Reset my appliance clock’ or ‘Diagnose Problem’ app on their smartphone, touch the equipment with the smartphone, and the clock would be automatically reset or the problem remotely diagnosed through the connection to the manufacturer’s website.
NFC in detail
NFC is a flavour of RFID (radio-frequency identification), but it additionally has a specific set of standards ensuring interoperability of NFC-enabled equipment. NFC standards determine the operating environment and data formats, transfer rates, modulation and so on.
NFC uses inductive coupling between two NFC devices and operates with electromagnetic field at 13,56 MHz – a licence-free allocation in the HF portion of the radio spectrum. An NFC device can draw power from the field generated by another NFC device. This enables some NFC devices to be exempt of power supply and to take the form of tiny objects such as tags, stickers, key fobs or cards.
Because the transmission range is so short, NFC-enabled transactions are inherently more secure than transactions in some other wireless technologies. With little energy required to cover the interaction zone with RF electromagnetic field, the NFC technology consumes very little power and is ideal for battery powered devices such as smartphones.
NFC defines two types of NFC devices. These are known as initiator and target. As the names imply, the initiator is the device that initiates the communication. It also controls the data exchanges. The target device is the one that responds to the request from the initiator and accepts the communication with the initiator to happen.
An NFC initiator can be, for example, an RFID reader or a smartphone. In proximity of another NFC device, it initiates a communication, then collects information from it or runs an action according to that information. Identification of a commercial article bearing an NFC tag is a good example of collecting information.
NFC also recognises two modes of operation: passive mode and active mode. In the passive mode of operation, only one NFC device generates an RF field. In that sense, it is active and always plays the role of NFC initiator. The other device is passive, and it always plays the role of NFC target.
In the active mode of operation, both NFC devices generate RF electromagnetic fields. The radio transmissions are half-duplex, as the same radio channel is used for both transmit and receive. To prevent collisions, the devices operate what is termed a ‘listen-before-talk’ protocol. With active mode, as compared to passive mode, larger operating distances (depending on the protocol) can be reached.
Further, there are three modes of communication: read/write mode, card emulation mode and peer-to-peer mode. An NFC device communicating in read / write mode reads data from or writes data to an NFC object. In card emulation mode, the NFC device behaves as a standard contactless smartcard.
The NFC device emulating a smartcard usually operates in passive NFC mode. In peer-to-peer (P2P) mode, the NFC-enabled devices operate in active mode. One of the devices initiates a communication link. Once the link is established, the devices talk to one another alternatively, applying the ‘listen-before-talk’ rule.
Architecture example
As a basic element to serve as an example, consider the M24LR04E-R dynamic NFC/RFID tag. With its unique combination of industry-standard serial bus (I2C) and contactless RF interfaces (ISO15693), ST’s M24LR EEPROM memory has the ability to communicate with the host system ‘over-the-wire’ or ‘over-the-air.’ Furthermore, its RF interface can convert ambient radio waves emitted by RFID reader-writers and NFC phones or tablets into energy to power its circuits and enable complete battery-free operation. The M24LR dynamic NFC/RFID is a typical target device that communicates always in passive mode and acts as an NFC card (card emulation mode).
Integrating the M24LR in an application is simple: on the I²C side, there is no specific design requirement as the device interfaces exactly as any serial I²C EEPROM device. On the RF side, the M24LR needs to be connected to an external antenna to operate.
Antenna design
The design principle of the M24LR antenna is very simple: the external antenna inductance (Lantenna) that needs to be designed on board the PCB should match the M24LR internal tuning capacitance (Ctuning) in order to create a circuit resonating at 13,56 MHz. The basic equation of the tuning frequency is:
A 13,56 MHz antenna can be designed with different shapes, depending on the application requirements. If different antenna dimensions need to be tested quickly, it is recommended to test some of the existing ST reference antenna designs directly in the application. The next step can be the design of a proprietary antenna with the aid of online or offline tools.
The following antenna parameters are critical for planar rectangular coil inductance:
• The number of turns.
• The number of segments.
• The conductor width in millimetres.
• The conductor spacing in millimetres.
• The conductor thickness in micrometres.
• Outer coil dimensions: length and width in millimetres.
Depending on the PCB technology parameters and the application dimensional constraints, an inductive antenna simulation can be started using eDesignSuite.
Energy harvesting
When the M24LR operates in the RF mode, it is powered by the RFID reader. No battery is then required to access it whether in write or read mode. With its external inductive antenna, the M24LR draws all of its operating power from the reader’s electromagnetic field.
The RFID reader plays the same role as the primary of a voltage transformer that powers the secondary (in this case, the M24LR and its inductive antenna). The energy transfer ratio from the reader to the M24LRxx is similar to the coupling factor of a voltage transformer.
Upon application of the RF field to the antenna, the IC transforms the induced energy into electrical current to supply the tag IC, the microcontroller and, possibly, other components such as a sensor. The components can then operate as long as the RF field is present and strong enough to supply them.
The energy harvesting function brings multiple benefits: enabling battery-free NFC products, waterproofing (no need of connectors or battery compartment), battery life saving on battery-powered devices, automatic wakeup when an NFC device comes in proximity, and current supply to other components.
Software system integration
Integrating the M24LR into the software part of the user application is quite simple. As mentioned earlier, on the I²C side, there is no need for a specific software device driver because the reading and writing interface is the same as any serial I²C EEPROM device. The designer would only need to spend minor effort to implement additional chip features that are not present on traditional EEPROM memories. These are mainly energy harvesting configuration and usage, memory content access password protections or an interrupt line handling. Typically, all these extra features are covered by embedded software libraries and drivers provided directly by the chip manufacturer.
On the RF interface side, the M24LR chip encapsulates all the necessary functionality of the ISO 15693 standard, so a developer does not need to study communication protocol, commands, coding or timing specificities of the ISO 15693 standard. Instead, they can program their NFC-capable smartphone
application. In order to implement an Android application, for example, the developer does not need to study the low level principles of the NFC technology, but can rely on NFC
Android API (application programming interface), which is easy to understand and straightforward to use. There are many articles and online guides explaining how to develop an Android application and several examples of NFC API usage.
For more information contact Gyula Wendler, Arrow Altech Distribution, +27 (0)11 923 9600, [email protected], www.arrow.altech.co.za
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