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Using Cheap Asian Electronic Modules Part 19: by Jim Rowe Arduino FC Shield This low-cost NFC (Near-Field Communication) shield for Arduino uses the same technology as RFID and contactless payments (payWave/ PayPass). It allows just about any Arduino board to read data from NFC/RFID tags or cards or write data to certain devices. You can also exchange data with other NFC devices, including many smartphones. T his shield, plus an Arduino module, could be used as the basis for a number of useful devices, for example, to unlock a door using an access card, to monitor the passage of stock or other items on a conveyor belt, to transmit business card information to customers’ smartphones and so on. But before we explain how it works, we will explain how RFID and NFC work. NFC or Near-Field Communication is a set of protocols that enable two electronic devices to exchange data by bringing them within about 40mm of each other. Communication is by electromagnetic induction, ie, coupling signals between loop antennas in each device. In effect, when the two antennas are brought within 40mm of each other they form an air-cored RF transformer. NFC involves low-power RF signals with a carrier frequency of 13.56MHz, in one of the globally available and unlicensed ISM (industrial, scientific and medical) bands. One of the devices involved in NFC communication can be passive, ie, with no onboard power source. This is typically the case with RFID (radio-frequency identification) tags and smart cards. 86 Silicon Chip In this situation, the device that is powered powers the circuitry in the passive device via the energy of the 13.56MHz RF carrier. The passive device then “replies” by modulating the carrier, with the modulated signal picked up by the active device. RFID technology was developed in 1983 by Charles Walton. Sony and Philips agreed to establish a compatible specification and this was approved as an ISO/IEC standard in 2000 (ISO/IEC 14443). The passive tags and cards used for RFID typically store between 96 and 8192 bytes of data, which can be read (and in some cases, written) using the RFID protocol. Nokia, Philips and Sony established the NFC Forum in 2004. It is a nonprofit industry association which promotes the implementation and standardisation of NFC technology to ensure interoperability between devices and services. NFC provides the same functions as RFID but also allows for communications when both devices are powered. In this case, power is not transferred using the carrier and the two devices can exchange data in an ad-hoc peerto-peer fashion. The standard defining NFC is ISO/ IEC 18092. This technology is now Australia’s electronics magazine found in all manner of smartphones and other portable devices. Sony’s FeliCa RFID system includes dynamic encryption for increased security. It was considered as part of the ISO/IEC 14443 RFID standard but in the end, was not included. However, some of the principles used by FeliCa ended up being used as part of the later NFC standard. Three different data exchange rates are in current use by NFC devices: 106kb/s, 212kb/s and 424kb/s. If an active device transfers data at 106kb/s, it uses modified Miller coding with 100% modulation. Miller coding is a type of Modified Frequency Modulation known as “delay encoding”. This is similar to NRZ (non-return-to-zero) encoding but with less power radiated at lower frequencies. For the higher data rates, Manchester coding is used, with a modulation ratio of 10%. Manchester encoding is another method of turning a bitstream into a symmetrical AC signal and is also used for transmitting digital audio data (S/PDIF). Elecrow’s NFC shield The Elecrow ACS53201S NFC shield measures 69 x 53mm (including the loop antenna) and it plugs directly siliconchip.com.au Fig.1: simplified block diagram for the PN532 controller IC. The chip is based around four sections: an NXP 80C51 micro (an old design!), NFC communications block, serial block and power/clock/reset controller. into an Arduino Uno or Mega 2560, or one of the many compatible modules. It derives its power from the Arduino and even comes with a passive keyring NFC tag for testing (see lead photo). At the heart of the shield is the NXP/ Philips PN532 NFC controller IC. The internals of this IC are summarised in the simplified block diagram, Fig.1. It’s based around an NXP 80C51 microcontroller (upper right), which includes 1KB of RAM (working memory) and 40KB of ROM (for storing the firmware). The other main sections are the contactless interface unit or CIU (at lower right) which handles the actual NFC communication, and the host interface section at lower left which handles communication with the host computer or controller (in this case, the Atmel micro on the Arduino board). The host interface section can be configured to communicate via SPI (serial peripheral interface), I2C (inter-IC serial communication) or a high-speed UART (HSU; ie, serial) connection. But note that the PN532 chip used in the Elecrow NFC shield has been configured for SPI only. Fig.2 shows the full circuit of the Elecrow NFC shield, plus a block diagram of an Arduino host at upper left. siliconchip.com.au The PN532 device, IC1, connects to the NFC loop antenna at top right via the TX1, TX2 and RX pins and a network of passive components. These include inductors L1, L2 and various capacitors and a few resistors. These are used for impedance matching, to make the antenna resonant at the required frequency and to filter out unwanted signals. IC1 uses 27.12MHz crystal X1 to generate its internal master clock and divides this by a factor of two to pro- duce the NFC carrier frequency of 13.56MHz. Although IC1 can operate from supply voltages of 2.7-5.5V, in the Elecrow shield it is powered from a 3.3V regulated supply. This is derived from the Arduino’s 5V supply via REG1, an MIC5205-3.3 LDO (low dropout) regulator. This is separate from the 3.3V rail from the Arduino since IC1 can draw up to 200mA when transmitting, which could overload the regulator on the Arduino. This means that level translation is necessary on the SPI signal lines between IC1 and the Arduino. That’s provided by IC2, a 74LV4T125 quad buffer translator. Three of IC2’s buffers are used to translate the logic levels of the MOSI, SCK and MISO signal lines, with the fourth buffer unused. IC2 is also powered from the +3.3V supply from REG1. The only other components on the board are a number of bypass and filter capacitors for IC1, IC2 and REG1, a pull-up resistor on pin 38 of IC1 to enable it and a capacitor and resistor connected to the Vmid pin of IC1 (pin 9), which is used to DC bias its RX pin (pin 10) to half supply. Note that Jaycar sell a similar NFC Shield, made by “linksprite” (Cat XC4542). While not identical to the Elecrow shield, it is compatible and we have tested it successfully with the same software. Arduino software Luckily Elecrow has made the software side of things quite easy by making available an Arduino library written specifically for communicating with the PN532 via the SPI port. The library can be downloaded from the Elecrow website The NFC shield easily slots into an Arduino Mega or Uno, as shown in Fig.2. Australia’s electronics magazine September 2018  87 Fig.2: complete circuit diagram for the Elecrow ACS53201S near-field communication (NFC) shield, and wiring diagram for the shield with an Arduino, or similar, board. Screen 1: using the readMiFareMemory.ino sketch reads and then prints the data from the RFID card and keytag. 88 Silicon Chip Screen 2: the readAllMemoryBlocks.ino sketch reads the card’s memory after writing 16 bytes into it (to block 8). Australia’s electronics magazine siliconchip.com.au Fig.3: wiring diagram for a Micromite to the Elecrow NFC shield. We’ve converted the Arduino NFC library into a BASIC library so that you can use it with the Micromite. The library can be downloaded from the Silicon Chip website. S53201S NFC shield would be to modify one of the example sketches. Or if you’re doing something fairly complex (eg, which involves both reading and writing data), you may need to incorporate bits and pieces of the example sketches into your own sketch. If you want to fully understand how to use the PN532_ SPI library functions, it is simply a matter of studying the example sketches to see how they operate. Using it with a Micromite in a zip file called PN532_SPI.zip (see Links panel). This can be made available for use in the Arduino IDE by clicking on the Sketch menu and then on the Include Library → Add .ZIP Library menu option. Select the ZIP file that you have downloaded and it will be added to the IDE’s library list. The library comes with six example sketches, named: writeMifareMemory readMifareTargetID readMifareMemory readAllMemoryBlocks PtoPTarget PtoPInitiator siliconchip.com.au “Mifare” is another way of referring to passive NFC tags and cards. The “PtoP” part of the last two sketch titles is short for “peer to peer” and indicates that these sketches are used for NFC communication between two active devices. I tried opening and running the first four of these example sketches with the Elecrow NFC shield connected to both an Arduino Uno and a Mega 2560. In each case, I tested it with both the keyring tag that came with the shield and also with an NFC card that came with another NFC/RFID reader. In each case, it worked exactly as expected and I was able to read data from and write data to both passive devices with no problems. You can get an idea of how the example sketches work from the adjacent screen grabs. Both grabs are of the Arduino IDE’s Serial Monitor. Screen 1 shows the output when reading the card first and then the keyring tab, using the readMifareMemory sketch. Screen 2 shows the results when using the readAllMemoryBlocks sketch to interrogate the card after using the writeMifareMemory sketch to write 16 bytes into the card’s memory (to block 8). The easiest way to build your own application using the Elecrow ACAustralia’s electronics magazine What if you want to use the shield with another MCU, like a Micromite? Since it has a standard SPI port, it’s quite easy to make the required connections, as shown in Fig.3. The software is a bit more tricky though since there was no Micromite library available to interface with the PN532 IC. So, we have translated the Arduino library into a Micromite BASIC file and have made this available for download from our website (free for subscribers). We have also translated some of the example programs. The download package includes several BASIC files which all start with an identical set of library functions but have different sample code snippets at the bottom. Having wired up the shield as shown in Fig.3, it’s simply necessary to upload one of these programs to the Micromite and run it. You should see similar output on the Micromite serial console as shown in the screen grabs above. Extra links NFC Forum: https://nfc-forum.org/ PN532 data sheet: siliconchip.com. au/link/aakl PN532 user manual: siliconchip. com.au/link/aakm How to use the PN532: siliconchip. com.au/link/aakn Elecrow shield library: siliconchip. SC com.au/link/aako September 2018  89