【 NXP 】PCF7991 驱动天线

I. Foreword

       This article mainly introduces the configuration of NXP PCF7991 drive antenna, and introduces the MCU configuration process of PCF7991 drive antenna. Then, the antenna drive current is detected by the current clamp to verify that the MCU configures the PCF7991 to drive the antenna successfully.

 

2. Basic introduction

2.1 Overview

       PCF7991AT is a fully integrated advanced base station integrated circuit ABIC (Advanced Basestation IC), designed for automobile engine anti-theft systems, providing read and write access to the ID transponders. The device is designed to use transponders that operate at 125KHz and use ASK to write and AM/PM to read. The amplifier gain, filter cut-off frequency, etc. can be optimized according to the needs of the system and the transponders.

       The PCF7991AT is designed for easy integration into the engine's internal read-write or read-only system, characterized by a high degree of integration and a very small number of external components. The device integrates a powerful antenna driver/modulator, a low-noise adaptive sampling time demodulator, a programmable filter/amplifier, and a digital converter, which is suitable for base stations that need to be designed with high performance. PCF7991AT can communicate in both directions with the transponders. By connecting DIN and DOUT, the three-wire interface can be configured to operate as a two-wire interface.

2.2 Minimum application circuit

      
       The figure shows the minimum application circuit of PCF7991AT. The antenna coil La and the capacitor Ca form a series resonant LC circuit (f= 125KHz). The antenna voltage is attenuated by the input impedance of the RV and RX pins.

2.3 Antenna drive

       The antenna driver is a full-bridge antenna circuit with the ability to provide a square wave voltage to the serial resonance antenna, which is connected between TX1 and TX2. The full-bridge driver is characterized by low output impedance, it has a large driving voltage for the resonant antenna circuit, and the standard carrier frequency of the antenna is 125KHz.

3. PCF7991 antenna detection

3.1 PCF7991AT antenna detection Overview

       In order to detect the short circuit or open circuit of the antenna, the chip monitors the input voltage on the RX-pin of the receiver, and the antenna failure is reported by the status bit ANTFAIL.

       If the input voltage of the receiver does not exceed the diagnostic threshold level VDTH, the status bit ANTFAIL is set, and vice versa, it is cleared to 0. This status bit is updated once per antenna carrier cycle. And can be read by the GET_CONFIG_PAGE2 or GET_CONFIG_PAGE3 commands. In power-down or suspend mode, at oscillator startup time, and when the antenna driver is disabled, this status bit is an indeterminate ANTFAIL.

3.2 PCF7991AT detects antenna fault status

       Read the status bit ANTFAIL to detect the open antenna fault by sending the GET_CONFIG_PAGE3 command, and use a logic analyzer or oscilloscope to analyze the PCF7991 SCLK, DOUT, and DIN pin data.

3.2.1 Read the status bit ANTFAIL when disconnecting the antenna connection

       Send the command SET_CONFIG_PAGE3, and grab the data of the PCF7991 SCLK, DOUT, and DIN pins through the logic analyzer. You can see that the status bit ANTFAIL read when the antenna is disconnected is set.

3.2.2 Read the status bit ANTFAIL when connecting to the antenna

       Send the command SET_CONFIG_PAGE3, and grab the data of the PCF7991 SCLK, DOUT, and DIN pins through the logic analyzer. It can be seen that after the antenna is connected, the setting of the status bit ANTFAIL is cleared when the antenna is disconnected.

Fourth, PCF7991 configuration drive antenna process

4.1 Typical power-up sequence

       The typical power-up sequence of a PCF7991 ABIC base station system, the time required to complete the power-up sequence depends on the parameters of the system components such as the power supply, MCU, base station antenna, and transponders.

4.2 General Setting Procedure (General Setting Procedure)

       After the clock stabilizes after power-on or power-off, µC should be executed in the following order.

             SET_CONFIG_PAGE 2, 1011b;           THRESET=1, FREEZE1=1, FREEZE0=1

             The delay is 5 ms, waiting for the acceleration filter and demodulator configuration.

             SET_CONFIG_PAGE 2, 1000b;           THRESET=1, FREEZE1=0. FREEZE0=0

             The delay is 1 ms, waiting for the ordinary filter and demodulator to operate in one millisecond.

             SET_CONFIG_PAGE 2, 0000;            THRESET=0, FREEZE1=0 FREEZE0=0)

             READ_TAG( );

             Waiting for data and waiting for thresholds, filters, and demodulators to work normally. The given time is the safe basis for the design of the startup system.

4.3 WRITE_TAG_N mode

       After the typical power-up sequence configuration, the WRITE_TAG_N command is sent to enter the WRITE_TAG_N mode. The high level on DIN is equivalent to the antenna drive is turned off, and the low level is equivalent to the antenna drive is turned on. If N3-N0 is set to 0, the signal on DIN will switch to the driver similarly transparently.

       The picture below shows the current flowing through the current clamp acquisition antenna. The antenna has not been changed by the antenna hardware (tuning, adjusting the Q value), and it can be seen that there is an obvious problem with the antenna current.

5. PCF7991 adjusts the resonant capacitance

5.1 Oscilloscope measures ① point voltage

       The waveform should be as follows. When these 5V mutation edges are at the maximum and minimum values of the sine wave, the system is tuned. If this is not the case, the system can be adjusted by changing the resonance capacitance. Measure the peak voltage of the antenna, and the amplitude of the antenna voltage Vant (peak) is half of the measured voltage.

       The quality factor Q of the antenna is calculated by the measured voltage, and the value of Q is preferably between 5 and 15.

5.2 Tuning overview

       The picture below shows the equivalent circuit of the driver and the antenna. You can use a vector network analyzer to analyze the impedance of the antenna. The purpose of tuning is to match the impedance of the antenna and the driver. The performance is to eliminate the imaginary part of the impedance by adjusting the capacitance value of the tuning capacitor. The capacitance stability of different materials is also different. Try to choose ones that are not susceptible to temperature interference.Capacitors ensure the stability of the circuit.


       The instrument shown in the figure below is a vector network analyzer

5.3 Comparison before and after tuning

       By changing the resonance capacitance adjustment system, the figure below shows the voltage and antenna current at the first point before tuning, and the mutation edge of 5V is on the right side of the peak.

 

通过改变谐振电容调整系统后 ， ① 点电压和天线电流 ，5V 的突变边沿处在峰值 。

六、参考资料

【DataSheet PCF7991AT. pdf】
https://wenku.baidu.com/view/7ed6c243a8956bec0975e334.html  

【PEPS 天线应用笔记分享 LF】
https://www.wpgdadatong.com/blog/detail?BID=B2644  

【差分天线匹配调试方法分享 NFC NXP】
https://www.wpgdadatong.com/blog/detail?BID=B0754