MSP430与TRF79xxA：超低功耗NFC和RFID卡存在检测应用报告

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本篇应用报告(SLOA184)探讨了在2013年的NFC（近场通信）和RFID（射频识别）读卡器中实现超低功耗卡片存在检测的方法。报告由Alexander Kozitsky和Josh Wyatt撰写，关注于MCU（微控制器）在安全和安全应用中的角色，特别是使用MSP430系列微控制器和TRF79xxA系列芯片。 NFC和RFID读卡器在电池供电的应用中，能源消耗的限制和成本效率是设计的关键考量因素。为了满足这些要求，报告提出了一种创新策略，即通过在现有设计中添加一个简单的电路和小型固件控制逻辑环路，显著改进了之前卡片检测方案的性能。这一改进不仅提高了能源管理，还降低了整体产品成本，对于推动行业的低功耗设计标准具有重要意义。报告首先概述了已知和常见的电池供电RFID和NFC读卡器实施方法的优缺点，帮助读者理解市场现状和挑战。然后，深入介绍了由德州仪器（TI）为顾客开发的创新技术，它结合了MSP430微控制器的高效性和TRF79xxA系列芯片的优势，旨在提供一种既节能又经济的卡片存在检测解决方案。在技术创新部分，报告详细解释了如何通过优化电源管理、信号处理算法以及与硬件的协同工作，确保在极低的功耗下准确地检测卡片的存在。此外，它还可能涉及了如何利用休眠模式、唤醒机制以及数据传输优化来延长电池寿命。总结来说，这份应用报告不仅提供了实用的设计指南，还展示了如何通过巧妙的技术整合来解决NFC和RFID读卡器中的能源效率问题，对于那些寻求在资源受限环境中提高设备效能的工程师和技术人员来说，是一份极具价值的参考资料。

= 13.56MHz

TRF79xxA

13.56MHz

MCU

Z Matching /

Filtering

SPI or Parallel

OUT

RX_IN

DD_X

DD_I/O

SUPPLY

(2.7VDC to 5.5VDC)

SUPPLY

OSCILLATOR

DC OUTPUT

AIR

current

sensor

capacitive

plate

capacitive

plate

(effective)

in target

www.ti.com

Commonly Implemented NFC/RFID Card Presence Detection

Capacitive Proximity Sensor – The NFC/RFID reader antenna also comprises areas used for a

capacitive sensor. Capacitive proximity sensors detect "target" objects due to their ability to be electrically

charged. Because even non-conductors can hold charges, this means just about any object can be

detected with this type of sensor. The reader would send out activation commands for the protocols and

types of cards expected each time the sensor detects any change.

Pro: Flexible firmware solution, good detection range can be achieved, can be used with all NFC/RFID

card types.

Con: High power consumption, detector is measuring E-Field, NFC/RFID card is using H-Field, which can

lead to false triggers and wake-ups, and many additional components are required for implementing.

Figure 4. Capacitive Proximity Sensor Approach

TRF7960A or TRF7970A Received Signal Strength Indicator (RSSI) – The TRF79xxA family of readers

employs dual receivers, each with an RSSI detector, inside the IC. The reader system would normally be

in an RSSI detection loop, issuing activation commands (by protocol), and then reading out the latched

RSSI value (in the RSSI register). Based on the RSSI value returned, the firmware logic would then either

issue additional commands (for that protocol) or remain in RSSI detection loop.

Pro: No additional components needed, flexible firmware solution, good detection range can be achieved,

can be used with all NFC/RFID card types.

Con: Higher power consumption and peak current, medium detection resolution (both only as compared to

other solutions).

Figure 5. TRF79xxA Based RSSI Approach

SLOA184–March 2013 NFC and RFID Reader Ultra-Low-Power Card-Presence Detection Using

MSP430 and TRF79xxA

Submit Documentation Feedback

Ultra-Low Power (ULP) NFC/RFID Card Presence Detection

www.ti.com

4 Ultra-Low Power (ULP) NFC/RFID Card Presence Detection

4.1 Introduction

The proposed solution combines some of the older ideas with new approaches. Carefully chosen values

of the analog circuit components and some MSP430 firmware, which leverages the low power modes and

port settings available in the MSP430 itself, with logical loop control of the TRF79xxA power modes are

the main differences. Compared to the implementations listed in earlier sections, the technique outlined in

this section dramatically lowers the power consumption.

The basic idea of this improvement is to combine part of the resonator concept with the existing

NFC/RFID reader system based on the TRF79xxA + MSP430. The existing NFC/RFID Reader system

used in this example is the TRF7970AEVM, which has the TRF7970A and the MSP430F2370 or the

MSP430G2xx parts that have the MSP430 Comparator A+ module integrated and the ability to accept HF

clock in (for synchronization purposes).

4.2 Technical Summary

This card presence detection system senses the card by measuring the decay time of a transmitter signal

after it has been turned off. When a card is in the field of the transmitter, power transfer occurs, and with

more current the voltage on the transmitter output increases. The closer the card is to the reader the

higher the voltage will be from the not present state. Measuring the decay of this signal by a comparator

creates a simple A/D, since effectively the voltage is what is being measured. Longer times, until the

output signal crosses a lower threshold, indicate higher voltages and shorter times indicate lower voltages.

This system is created by adding and changing a few components from the original TRF7970A EVM and

using an internal comparator on the MSP430F2370 that was not used before.

The power savings are quite substantial from always running the transmitter. In a system where there are

three polls per second, the system is only active approximately 1% of the time. In the sleep state the

TRF79xxA consumes almost no power, while the MSP430 is consuming a negligible amount of current

(approximately 0.8 µA). In the active state, which lasts several milliseconds, the TRF7970A is quickly

turned on, initialized, and a transmitter burst is performed. This turns on the transmitter for approximately

20 µs. Before it is turned off, the comparator is initialized and a timer is started to measure the time. The

timer runs until the comparator issues an interrupt indicating that the threshold voltage has been crossed.

The timer time at this point is the decay time of the signal. As mentioned earlier, longer times indicate

power coupling, which means that a card may have been in the field.

To determine if a certain time measurement indicates that a card is in the field, it is run through what is

referred as "automatic calibration algorithm". To explain this need, let's examine what would happen

without it. Without it, a specific time value, that once exceeded, would indicate that a card is in the field.

However, due to power supply or temperature drifts, this threshold time would be naturally crossed

periodically, causing false positives. False positives are very undesirable in this system as they cause a

reading process to happen, which uses a lot of power.

The "automatic calibration algorithm" takes the highest sample that it finds in its history memory buffer and

adds THRESHOLD_OFFSET to it. This new value becomes the threshold in determining if the current

sample needs a read to be performed.

NFC and RFID Reader Ultra-Low-Power Card-Presence Detection Using SLOA184–March 2013

MSP430 and TRF79xxA

Submit Documentation Feedback

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