In the Near Field Communication (NFC) design, developers have always face various challenges for optimizing radio frequency performance, hardware design and software. However, now, single-chip NFC solutions and all-round software support greatly change the essence of integrated NFC function in household electronic devices, wearable devices and Internet access (IoT) equipment.
Therefore, developers can join many application functions, but hardly affect design packages, power consumption or project plans.
The two-way communication capabilities of NFC are unique, providing simple and low-power in nature. Wireless connection. Communication can only be communicated only when the two devices are close to each other, so there is no message interception, and the potential network attack path is minimized. Moreover, only one device is energized during communication, so the average power consumption can be maintained at a relatively low level.
In fact, NFC can bring huge benefits to various smart home and Internet of Things applications. Users only need to enable NFC's smartphones to enable NFC-enabled products, you can complete Bluetooth or Wi-Fi pairs. NFC can be used as a personalized underlying technology, and simplifies smartphone tasks such as configuration settings, transfer data or registration products.
Embedded NFC
NFC is a subset of radio frequency identification (RFID), working at 13.56 MHz, which can be performed with many of the same features as traditional RFID tags and non-contact smart cards. At the same time, NFC also has greater flexibility, working in three communication modes: card simulation, point-to-point and read / write.
In the card simulation mode, the NFC device is used as a non-contact smart card, which can be used in a variety of existing applications, including ticketing, access control systems, transportation, toll stations, and non-contact payments. Point-to-point mode allows two devices that enable NFC to connect and exchange information. For example, the user can use a smartphone that enables NFC to set the Bluetooth or Wi-Fi setting parameters of other devices, or debug it in a trusted network. In read / write mode, an NFC device can read data from another NFC device. For example, enabling an NFC smartphone to read URLs or other data, such as selling coupons embedded on a retail store promotional sign.
After the embedded NFC device for the label is connected to the host processor within the product, its work is similar to the dual port memory. One of the memory ports can be accessed wirelessly through the NFC interface. Another port can be accessed by an I2C interface of an embedded system. Therefore, external data sources such as smartphones can be passed to embedded systems. Conversely, the host processor can update the data stored in the NFC device, even if the product is powered down, it can provide this data to the external device enabled NFC.
This approach can be used for applications that need to transmit data between embedded systems and external systems (such as smart phones that enable NFC). In fact, the developer can use this method to update the data, even firmware of the embedded system in this way for temporary storage, using the NFC device wireless communication link.
Single-chip NFC controller
In the past, designers wished to add NFC capabilities based on MCU, but they face two major challenges of hardware and software. Hardware engineers using traditional NFC devices need to ensure design to meet key timings between NFC devices and hosts, maintain low power requirements, and minimize design packages and materials list (BOM). However, perhaps the biggest impact is that engineers usually have to write their own code to handle many low-level transactions that form a single application level NFC operation.
Advanced NFC devices, such as NXP PN7150, aimed at simplifying NFC functions in the Internet of Things or any embedded systems. The PN7150 combines RF front and low power ARM® Cortex®-M0 cores, memory, and I / O peripherals (Figure 1).
By ensuring the optimum timing between the host and the radio frequency front end, the device greatly eliminates the traditional hardware integration problem while supporting higher RF output power. In addition, integrated I2C interfaces are compatible with NXP NTAG I2C Plus, suitable for sensors, luminaires, and other devices related to smart home networks. At the same time, the device helps to reduce power consumption: PN7150 can automatically switch to low power mode while keeping the host sleep until radio frequency communication is required.
In addition to simplifying hardware design, PN7150 is also significant in software. NXP is pre-installed embedded data and code memory, and can expand the NFC controller interface (NCI). NFC Forum Manage NCI Technical Specifications, defines logical interfaces between NFC Controllers (NFCCs) and device hosts (DHs) that run advanced operating systems such as Android, Linux or Windows IoT.
The embedded NCI firmware of PN7150 reduces some host interactions, and provides a higher abstract level for NFC application developers, thereby reducing software development burden. By moving the low-level code to the firmware, the PN7150 also reduces the application code base of the host end.
Direct replacement solution
PN7150 has integrated hardware and software, dedicated to direct replacement NFC solutions, applicable to developers working in Android, Linux, or Windows environments (Figure 2). In fact, developers who are unfamiliar with NFC can utilize Arduino (NXP OM578 / PN7150ARDM), BeagleBone Black (NXP OM578 / PN7150BBBM) and Raspbery Pi (NXP OM578 / PN7150RPIM) existing PN7150 presentation kit. Each kit contains a PN7150 NFC control board, a dedicated interface board and an NFC sample card.
Designers need less components to create a complete NFC subsystem for existing MCU-based design. In fact, in some cases, engineers can further reduce BOM (Fig. 3) by eliminating or combining some passive components in an antenna matching circuit.
In a typical antenna circuit design, the RQ damping resistor on the antenna lead is required to reduce the high antenna quality factor that has adversely affects the shaping of the resulting signal. In the design of the nominal antenna quality factor, the designer can remove these RQ damper resistors from the antenna. In a matching circuit, when a particular design antenna lead has a very low maximum peak voltage, the designer can replace the pair of parallel capacitors (and eliminate the connection of the EMC filter) with a single capacitor. In a typical application, the small antenna is connected to the PN7150, and the peak voltage generated on the antenna will be relatively low. Therefore, the designer can also remove the decoupling CRX capacitor and connect the RRX resistor directly to the antenna to simplify the RX path.
Simplified software
From a software perspective, PN7150 provides a simple implementation model that can further accelerate product development (Figure 4). The device host architecture combines the transport layer driver, the NCI driver, and an NFC execution environment (NFCEE) middleware including read / write, point-to-point or card simulation library. For NFC operations, the host only needs to send advanced NCI to PN7150 via the I2C interface. In turn, the PN7150 firmware can perform detailed transactions required in the NFC protocol.
In fact, from the perspective of developers, the NFC application development is available at a high level of the full-scale software platform. For Internet applications that enable NFC, common operations include swap NFC data exchange format (NDEF) data. NDEF is managed by NFC Forum, which is a standardized data format that can be used to exchange URI or plain text between any compatible NFC device and another NFC device or tag.
The NXP Linux_libnfc-NCI library provides a simple Application Programming Interface (API) to abstract the low-level transaction to be more advanced for applications. For example, developers can write a label by simply calling the WritetAg routine (list 1). This library utilizes a series of low-level routines to decompose this application layer to the desired series of steps to verify, format, and transfer data.
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The device host interacts with the NFC controller using NCI control messages. A particularly important NCI directive sequence provides a mechanism to find other cards, card readers, or peer devices. This instruction sequence called RF Discovery allows the Equivalent NFC device (such as PN7150) to alternate between other transmitting devices and transmitting (polling phases) to find remote cards or tags.
As with any radio frequency technique, transmission requires higher power than radio reception (Figure 5). In fact, in the polling phase, the PN7150 power consumption is approximately 30 mA, depending on the antenna characteristics. In the listening phase, the PN7150 will wait outside the generated radio frequency carrier, and when the standby mode is enabled, the current consumption will fall to about 20 μA.
Typically, the polling phase will last for about 20 milliseconds, while the listening phase is 300 milliseconds to 500 milliseconds. For the 500 millisecond listening phase, the average power consumption is:
(30 x 20 + 0.02 x 500) / 520 = 1.17 mA.
In order to reduce the power requirements of RF Discovery, the NXP N7150 provides a proprietary mechanism called a low power card detector (LPCD) mode. In LPCD mode, PN7150 looks for changes in antenna impedance caused by magnetic coupling generated when another antenna is approaching. If the impedance changes are higher than the predefined threshold, the PN7150 automatically enters the standard NFC Forum RF Discovery sequence. Therefore, this "event-driven" method can significantly reduce the length of the RF Discovery phase, thereby reducing the average power consumption (Figure 6).
Conclusion
NFC provides secure low-power connections, which can significantly enhance interconnected home electronic devices, wearable devices, and other Internet of-connected equipment. Simply enable NFC's smartphone, users can debug products, load access information and retrieve information stored in the product. However, in the past, the NFC was implemented in the MCU-based system, which brought design problems for hardware and software integration. In contrast, such as NXP PN7150 and other NFC integrated devices, the NFC design provides a nearly direct replacement solution to simplify hardware and software development in enabling NFC applications.
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