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Improve power consumption in card detection system

Posted: 03 Jun 2013 ?? ?Print Version ?Bookmark and Share

Keywords:battery life? NFC? RFID? card detection? reader?

Battery-powered near field communication (NFC) and radio frequency identification (RFID) reader applications must have a defined and limited energy consumption budget and be low cost for viable product realisation. Techniques and strategies for card presence detection have emerged over the years that attempt to address both concerns. This article will contribute to the previously identified card detection techniques and strategies by presenting an alternative solution including the addition of a simple circuit and a small firmware control logic loop to an existing design, which offers a dramatic improvement in current savings and results in longer battery life.

Additionally, a brief overview of the benefits and disadvantages of the most common implementations of battery-powered RFID and NFC readers will be given, and in-depth details provided on the innovative technique and approach, developed by leveraging both ultra-low power 16bit MCUs and highly integrated NFC/RFID reader/writer integrated circuits (ICs) in battery-powered NFC and HF RFID reader applications or designs.

Criteria for system design decisions
Designers and developers of low-power NFC or RFID systems usually have a list of key requirements that are driven directly by their target markets or focused end equipment. These requirements might include access control (building access), digital door locks, smart utility meters (for prepayment, technician access, or firmware upgrades), portable speakers, handheld inventory control, handheld data logging collection or medical diagnostic equipment, and mobile/handheld ticketing or payment terminals. All of the end equipment examples have similar key "care-abouts" such as:

???Total system cost: Cost must be optimised.
???Electrical design: Platform or modular designs are becoming more prevalent, with emphasis on being deployable worldwide. For example, a design that can be used in any country like a 13.56MHz NFC/RFID system.
???Mechanical design: It must be robust, safe, and provide various levels of protection against vandalism.
???User friendly and intuitive: Users should never need much training to interact with them, so the design must always lend itself to an easy user experience.
???Low power consumption: A power budget is more important than ever, and when implemented as described later in this article, can be a major differentiator and add competitive advantage to designs.

Figure 1: Examples of mechanical and optical card detection systems.

Previous approaches/implementations
An overview of other approaches to card detection in battery-powered NFC and RFID reader applications are presented for review in order to provide the reader with a basic understanding of what has been done in the past, and then be able to fully appreciate the advanced technique.

Mechanical or optical: This approach comprises a card slot equipped with a mechanical switch or break beam detector that triggers a read cycle of the NFC/RFID card when activated by the card. This implementation is a carry-over from magnetic card swipe applications (figure 1).

The advantages of this approach include very low power consumption; and the fact that many consumers are familiar with this interaction. Unfortunately, this implementation is challenged by limited card form factors and a lack of robustness that allows the system to be easily broken or rendered unusable.

Resonator implementation: This method includes an NFC/RFID reader equipped with an additional resonator, oscillator, or crystal and resonant coil. The MCU enables the signal to be generated as a short burst (20 to 50?s) and detects the antenna dampening (figure 2).

Figure 2: Diagram of a typical resonator implementation, which includes an additional resonator, oscillator, or crystal and resonant coil.

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