Design of universal card reader circuit based on NFC

In many current RFID applications, equipment manufacturers may not necessarily decide which transceivers, especially transceiver chips, will be used by customers. Therefore, to maximize their chances of winning a bid for a particular project, equipment manufacturers must offer readers that either support as many transceiver chips on the market as possible, or that are themselves at least relatively easy to Customized. In addition to requiring it to support a range of protocols, standards and transceivers, customers may have other functional requirements for card readers, such as high performance, anti-collision, long/near sensing distance, mobility and power consumption. But it is difficult to meet so many requirements simultaneously in a single card reader. To meet all these requirements, manufacturers may need to offer a range of card readers that meet different requirements.

EM4094 is an integrated transceiver chip that can be used to build an analog front-end module for RFID readers. The chip's data transmission and reception links allow transmission and decoding of any communication protocol, so the EM4094 supports all EM's 13.56MHz transceiver chips, ISO15693, ISO14443 A&B, and Sony Felica protocols. With appropriate settings, the EM4094 can even communicate with NFC devices. This article will explain through a series of steps how a hardware engineer should integrate and utilize the EM4094 RFID reader circuit.

Typical application circuit configuration.

Card reader structure

Basically, the analog front-end module is one of the components of the RFID reader. It is responsible for encoding/decoding data and driving the RFID reader antenna with appropriate power. The analog front-end module itself is driven by a microcontroller. The microcontroller is responsible for managing the frame decoding tasks of different protocols, as well as the communication interface (serial interface, USB interface or Ethernet interface) with a PC or other background control device. Some IC vendors offer chips that integrate an analog front end and a microcontroller. In many cases, the microcontroller or integrated memory space is either more than needed or not sufficient, and standalone modules allow manufacturers to select the microcontroller and memory capacity that best suits their design requirements. Of course, the manufacturer will depend on the desired functionality. Make your decision based on factors such as level, read range, and power requirements. Since there are already chips like the EM4094 that provide complete flexibility, below we will discuss how to integrate this card reader chip in an actual card reader. Figure 1 shows a typical application circuit configuration.

Power supply design

First, we briefly discuss the power supply design of the EM4094. The chip has three different power supply pins, among which VDDA1 and VDDA2 are used to power the internal antenna drivers ANT1 and ANT2. Each drive can be independently powered. Since these two drivers may generate or synchronize large currents, it is recommended to connect a 3.3μF capacitor between the VDDA1 and VDDA2 pins to provide sufficient energy to the antenna. In addition, we also recommend connecting two capacitors with capacitances of 1nF and 100nF in parallel next to the capacitor to decouple and filter the power supply.

The third power supply pin, VDD, is used to power all other internal modules. On this power line, the engineer will have to plug in two filter capacitors with capacitance values ​​of 1nF and 100nF for the same reasons mentioned above. They are preferably ceramic capacitors made of dielectric materials such as COG and X7R. , so this type of capacitor has small tolerance and high temperature stability. The important thing here is that the same voltage (3.3V or 5V) should be applied to these three power lines. These power lines should also be connected to analog ground.

Antenna driver output

ANT1 and ANT2 are the two outputs of the antenna driver. They can be driven in phase or in opposite phase. This makes it possible to connect the reader antenna in different ways and to generate antennas with four different power levels depending on the structure chosen. The EM4094 can also be used with a remote antenna, in which case the output impedance of the EM4094 (see Figure 3) must match the communication line impedance.

Impedance matching circuit.

If coaxial cable is used, then when only one antenna driver is used, the output impedance of EM4094 will have to be adjusted between 10 ohms (ANT1 optional) and 50 ohms; when two antennas are used in parallel, the output impedance of EM4094 The impedance will have to be adjusted between 5 ohms (ANT1 is adjustable) to 50 ohms. To achieve a good impedance match, developers can use a Smith chart to select an LC PI network and select appropriate component parameter values.

If the card reader antenna can be integrated on the same PCB as the EM4094, you can use the direct antenna connection method (see Figure 2). In this case, the antenna and series capacitor form an LC series loop. The resonant frequency of this loop is the frequency of the card reader. The series resistor is used to suppress the quality factor and set the antenna current below the rating of the EM4094. When the antenna is operating at its resonant frequency, higher power can be obtained by directly connecting the antenna. Please refer to the EM4094 application guide for different connection methods of the IC antenna.

direct antenna connection

transceiver signal reception

RFIN1 and RFIN2 are the two input pins on the IC receiving chain. They are used by EM4094 to demodulate the data stream sent by the transceiver. The voltage on their pins must be set between GND and VDD. These two The demodulation inputs must have the same performance and exhibit the same sensitivity. Together with an external matching impedance circuit, these two inputs can be used to demodulate the input phase or amplitude modulated signal. Unused input pins should be connected to analog ground through a 10nF capacitor. The high sensitivity of the input pin allows the reader to have a long reading distance even at the minimum power level of the electronic tag.

security function

Many current RFID applications use encryption algorithms to encrypt data or prove identity. Some encryption algorithms are public and some are not. Some vendors provide their unique encryption algorithms in source code or object code, or other independent secure elements (such as SIM cards). Of course, the many options available allow for great flexibility in design. You can implement one or more algorithms in a traditional microcontroller or a secure controller. Another option is to build an interface with a separate component responsible for secure operation. It is of course possible to mix the two options. EM provides its proprietary encryption algorithm via SIM card.