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Advancing RFID reader apps with convergent processors

Posted: 01 Jan 2007 ?? ?Print Version ?Bookmark and Share

Keywords:RFID tag? RFID? RFID apps? signal processor? ADC/DAC interface?

RFID stands poised to permeate many aspects of our lives. From inventory control to fast checkouts at the supermarket, the technology is transforming many existing applications and enabling new ones.

At the front-end, the "signal chain" starts with small tags on the units of interest, which then pass information to one or more RFID readers that detect when tags are present in a specific area. At the back-end a server-based system maintains and updates the tag database, perhaps generating alerts or kicking off other information-based processes within the enterprise.

Today, most RFID readers employ multiple processors to satisfy application requirements. One is typically a signal processor interfaced to an ADC and a DAC.

Another is a networked processor that communicates with a local or remote server for information storage and retrieval. In this article, we will describe how these seemingly disparate functionssignal conversion and network connectivitycan be managed by a single processor such as one from Analog Devices' Blackfin Processor family.

First, we will provide a brief overview of RFID technology, along with the present and future applications it enables. Then, focusing on RFID reader functionality, we'll explore the basic software components that need to run on the RFID reader as well as the server connection.

Emerging applications
RFID technology enables many new types of applications by allowing the monitoring of multiple items at a time, without requiring that a person "touch" each one (with a handheld barcode scanner, for example).

The application spaces that can take advantage of this automated identification include such diverse areas as inventory control, logistics management, surveillance, and toll collection.

Here are some more ways in which RFID systems are used today:

  • In supermarket food pallets and cases, giving asset visibility and allowing better management of the asset pool. With the ability to write to the tag, additional information (e.g., sell-by date) can be included. In addition, automatic re-ordering can be implemented to keep shelves properly stocked.

  • In libraries to automate the issue and return of materials. In the past, these items were identified using labels, each of which had to be read individually with a barcode scanner.

  • In clothing labels to identify the source. By using the tag's identification number, the item can be certified as authentic or deemed counterfeit.

  • In the pharmaceutical industry to safeguard against counterfeit supplies.

  • In sports competitions, to accurately track a runner's progress during a marathon.

RFID system overview

RFID is a system that uses RF transmissions to communicate with, identify, classify and/or track objects. Each object has its own RFID tag (also known as a transponder). The overall system utilizes a tag reader that receives RF energy from each tag.

The reader's embedded software manages the interrogation, decoding and processing of the received tag information, and it communicates with a storage system that houses a tag database and other relevant information. Figure 1 illustrates a typical RFID system.

Figure 1: Simplified representation of an RFID system.

RFID readers
The RFID reader provides the connectivity between individual tags and the end tracking/management system. It can come in a variety of form factors, but it is typically small enough to be mounted on a tripod or wall. Also, depending on the application and operating conditions, there may be multiple readers to fully cover a specific area.

For example, in a warehouse, there may be a network of coverage to ensure that as pallets pass from point A to point B, there is 100 percent chance of querying and logging all items that pass through.

Overall, the functionality of the reader consists of three main components. The first is the transmit/receive function to communicate with the tags and isolate individual ones. The second is the initial processing of received information. The third is the connection to the server that links the information into the Enterprise.

The reader in an RFID system has to deal with the fact that multiple tags simultaneously exist within the field of interest. This becomes very important in applications with many tags within a confined spatial area. The primary challenge in a multiple reader/tag scenario is that collisions will occur because of multiple readers sending out queries and multiple tags responding at the same time. There are many ways to avoid this problem.

The most common one is to use some form of time-division multiplexing algorithm. Readers can be set to interrogate at different times, while tags can be configured to respond after a random time interval. Having the ability to implement this function in embedded software provides additional flexibility.

RFID transponders (Tags)
An RFID tag consists of an IC holding unique information about the object to which the tag is affixed, an antenna (usually a printed-circuit antenna) for receiving RF energy from the reader and transmitting information, and some kind of housing that envelops the tag's components.

The term "object" applies to any number of different things, from factory goods to animals to people. The distance from the tag to the reader is an important system variable, and it is directly influenced by the tag technology.

Let's take a look at the various tag technologies in common use today:

Passive
The simplest type of tag is the passive tag. It's powered exclusively by RF energy sent from the reader, so it doesn't have the extra size and cost of an integrated battery. Because of this, it's inexpensive, mechanically robust, and can be quite small (around the size of a thumbnail, for instance).

However, because a passive tag's received power is proportional to its physical proximity to the RFID reader, the downside is that such tags have a limited reader-to-tag range.

Speaking of range, the RF frequency chosen has a lot to do with the actual range of the link. Low-frequency (LF) tags commonly utilize the 125-135kHz portion of the spectrum, and since their range is constricted, their main use is for access control and animal tagging.

High-frequency (HF) tags, mostly operating in the 13.56MHz band, allow a range of around a foot or two. Their primary uses are for simple one-on-one object reads such as access control, toll collection, and tracking items like library books.

UHF tags, on the other hand, operate around the 850-950MHz range and have a considerably longer range of 10 feet or more. Moreover, a reader can interrogate lots of these tags at a time, as opposed to the one-on-one HF tag reading process.

This trait also helps offset the need for multiple readers in a given zone. Because of this feature, UHF tags are very popular in industrial applications, for inventory tracking and control. However, one major disadvantage of UHF tags is their inability to penetrate liquids efficiently.

This makes them less useful for liquid-filled objects like beverages and humans. For tracking these items, HF tags are often used instead.

Semi-active
Like a passive tag (and unlike an active tag), a semi-active tag reflects RF energy back to the tag reader to transmit identification information. However, it also contains a battery that powers the IC portion of the tag.

This allows for some interesting applications, like putting a sensor in each tag. This way, each transponder can transmit not only static identification data, but also real-time attributes like temperature, humidity, and time/date.

By using the battery only to power the IC and sensor, the semi-active tag achieves a compromise between cost, size and range.

Active
The active tag goes one step further beyond the semi-active tag by powering both the tag IC (along with any sensors) and the RF transmitter with an integrated battery. Because of this, it can operate over a much larger reader-to-tag range (100m+).

This also translates into allowing goods to move past the reader at a much higher speed than in the case of a passive or semi-active tag system. Additionally, an active tag can carry much more product information than just a simple product ID code.

Software architecture of the RFID reader
Having described the basic functionality of an RFID reader, we now turn to how to implement the reader with a convergent processor such as the Blackfin. The three elements of the RFID reader software architecture are: the back-end server interface, the middleware, and the front-end tag reader algorithms.

It is worth pointing out that while the elements of the software architecture are distinct, they can all run together on a single Blackfin processor.

Back-end server and connectivity
Often, the RFID reader contains a networking element that connects single RFID read events to a central server. This backend networking interface might be wired Ethernet (IEEE 802.3), wireless Ethernet (IEEE 802.11 a/b/g), or ZigBee (IEEE 802.15.4), for instance.

The central server runs a database application, with functions that include matching, tracking and storage. In many applications, an "alert" function is also present. For supply chain and inventory management systems, this may be the re-order trigger; for security applications, this could be an alert to a guard. Building a reader around a high-performance embedded processor that runs uClinux is a terrific advantage when communicating with a back-end server. Critical items such as the robustness of the TCP/IP stack and the availability of SQL database engines reduce what is otherwise a huge development and integration burden in the development process.

Middleware
In RFID terms, middleware is the software translation layer between the front-end RFID reader and the back-end Enterprise system. The middleware filters the data from the reader and ensures it is free of multiple reads or bad data. In early RFID systems, the middleware ran on the server.

But increasingly, the filtering of RFID data is being performed on the reader, prior to sending it through the Enterprise's network. This increased functionality is another advantage embedded processors bring to this application space.

Front-end reader
The signal processing occurs in the front-end of the reader system. This signal processing is typically filter- and transform-intensive, which is why a processor with strong signal processing performance, such as the Blackfin, is necessary.

Now that we have a general sense of an RFID system's components, let's briefly look at connectivity from the RFID reader's viewpoint. For communicating with a tag, the ADC/DAC is the interface of concern.

ADC/DAC
Analog Devices offers several IF Subsystem MxFE ICs. These devices are general-purpose IF subsystems that digitize narrowband IF input signals. They also include low-noise amplifiers, mixers, analog-to-digital converters, AGC circuitry, and programmable filters.

Output streams of I & Q data connect directly to processor parallel ports. These MxFE ICs offer the highest performance narrowband receiver available, well-suited to RFID applications and others.

Blackfin processor and its applicability
In this instance, the Blackfin Processor provides connectivity to both wired and wireless networking. Some processors, such as the ADSP-BF536/7, have a 10/100BaseT Ethernet MAC on chip.

On the wireless side, all Blackfin processors connect directly to both 802.15.4 ZigBee and IEEE 802.11 chipsets via the SPI and SPORT peripherals. Line speed transfers can be attained without consuming the entire processor bandwidth.

In addition, Blackfin Processors include a Parallel Peripheral Interface (PPI), which can connect directly to ADC/DACs like the ones discussed above. Some Blackfin Processors include two PPIs, which can expand system functionality even furtherby connecting a camera to an RFID reader, for instance.

As an aside, these Blackfin features make it especially attractive for 1-D and 2-D bar code applications, because of Blackfin's ability to perform system control, networking and image processing on the same device.

From a Blackfin software standpoint, the RFID reader offering includes the drivers necessary to interface to the mixed-signal front-end.

In addition, there is a DMA driver that is also very useful in moving data through a system. The uClinux-based network stack, as well as SQL database engines, are also available.

From a system perspective, additional features such as 802.11 WiFi cards, USB thumb drives, and CompactFlash card interfaces can very quickly be integrated with Blackfin devices.

Conclusion
As we have seen, RFID applications no longer require both a dedicated signal processor for ADC/DAC interfacing and a microcontroller for networking. A convergent processor from the Blackfin family can handle the networking and control with plenty of performance to spare for converter interfacing and pattern matching algorithms.

In turn, this leads to BOM cost reduction and faster time-to-market for the next wave of RFID applications.

About the authors
David Katz
and Rick Gentileare Senior Applications Engineers at Analog Devices Inc. They are the authors of the book Embedded Media Processing, which had its second printing in 2005. They can be reached at david.katz@analog.com and richard.gentile@analog.com.

Glen Ouellette is a Senior Applications Engineer at Analog Devices. He can be reached at glen.ouellette@analog.com. Giuseppe Olivadoti is a Field Applications Engineer for Analog Devices. He can be reached at giuseppe.olivadoti.




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