#2
文章发表于：2007-07-20 09:36
原文：
ZigBee is emerging as a strong contender for low-bandwidth, low-power networks such as you'd find on the factory floor. Different network nodes will require different levels of processing power.
By Rick Gentile, Tom Lukasiak, and Glen Ouellette, Analog Devices -- EDN, 7/13/2007
In today's consumer market, two of the most prevalent local-area wireless technologies are Wi-Fi and Bluetooth. For connectivity between laptops, cell phones, and a variety of other handheld devices, these protocols make us wonder how we ever survived without them. However, ZigBee may be a better choice for low-power, low-bandwidth control-type applications, such as a sensor network for the factory floor. And with a powerful processor, ZigBee can effectively perform many complex tasks within its low-power, low-bandwidth constraints.
The ZigBee Alliance first published the ZigBee specification in 2004 to provide networking for industrial-control, automation, and monitoring applications. The ZigBee protocol specifies a software stack which sits on top of the IEEE 802.15.4 radio standard. The IEEE 802.15.4 specification, designed for low-bit-rate applications (up to 250 kbps), defines the physical layer for the radio. The ZigBee protocol serves as the logical network and application software. The ZigBee Alliance, which includes more than 200 member companies, governs the ZigBee standard and ensures compliance within the specification.
A few key aspects stand out when assessing ZigBee for an application. The first is the low processing requirements of the stack, which translates to low-cost processors. This directly supports the idea of remotely deployed devices that run on battery power. With a collection of remote devices, the next requirement is connectivity. ZigBee-based systems support a range of interconnects including tree, mesh, and hybrid configurations. This is a key advantage because it is makes a ZigBee network much more flexible and robust in a factory or building.
Power
ZigBee networks invariably have some number of nodes that are battery powered. Typical IEEE 802.15.4 nodes are efficient in terms of power consumption because they are designed for that purpose. These devices specify a battery life that is measured not in hours, but in years. To aid these demanding power requirements, the IEEE 802.15.4 protocol uses transmit times that are short in duration, and supports communication of both beacon-oriented and beacon-free modes.
In a beacon-enabled network, all nodes agree on a time-slice scheme that schedules when a specific device will transmit data or when it needs to be awake in order to confirm its membership in the network. A heartbeat type of connection that uses a beacon allows all nodes in the network to be powered off for the majority of the time, partially waking up to acknowledge the beacon interval. The beacon interval can be between 0.015 and 252 seconds. This allows all devices in the system, including the central coordinator, to remain idle until necessary.
In a beacon-free network, many battery-operated nodes can spend most of their time in an idle state. A beacon-free mode provides a way from some of the nodes to sleep for extended periods of time, provided they communicate with nodes that are always on. In practice, it is expected that many battery-powered nodes will run for many years before the battery needs to be replaced. However, this scenario dictates that one central node be awake at all times to receive messages from the battery-operated nodes.
The event-detection or pure-sensor type of node requires a processor, mostly to run the ZigBee stack itself. For this type of processing load, an 8-bit processor can handle the work. The 8-bit processor is also very power-efficient, so that battery life is maintained. Take for example the Atmel ATmega32L (8-bit AVR microcontroller). For this processor, the current draw for a 3.3V core is:
* Active: ~6 mA for 8 MHz clock, ~1.1 mA for 1 MHz clock
* Idle: ~3 mA for 8 MHz clock, ~0.35 mA for 1 MHz clock
* Power-down: < 1 µA
The good news is that as telemetry applications become more intelligent at the end node, developers are not forced to increase the power budget much beyond that of an 8-bit controller when adding a significant increase in performance. This is because today's embedded processors have the ability to work at a range of voltages and frequencies to optimize power consumption for each given application. That is, engineers can program processors with dynamic power management to match the exact processing requirement. The processors also have a variety of power-down modes that allow the bulk of the logic to remain powered off until detection of a specific event.
An advanced telemetry application using a Blackfin processor, scaled to run in a low-power mode, interfacing to a ZigBee stack and performing some basic signal processing, consumes power on the order of 10s of milliwatts. When power-down sleep modes are used, the average current used is in the microamp region.
Again, looking at some numbers, Blackfin processors run at a range of voltage and frequencies. At the lowest voltage of 0.8V, the processor can run up to 250 MHz. This frequency supports any of the applications described below. The ZigBee stack takes up only a small portion of the MIPS (less than 10 MIPs). This leaves many additional MIPS to perform more advanced processing, such as JPEG encoding or analytic decision making. A typical device running at 50 MHz consumes about 20 mA. At 250 MHz, the consumption will increase to 50 mA.
The processor can also take advantage of its RTC (real-time clock) to wake up on a regular period. In this mode, the processor is off or in a low-power state the majority of the time. The RTC circuitry runs on a dedicated battery that consumes less than 30 µA of current and is programmed to awaken the processor after some time period. Alternately, the processor can be awakened from its power-down state by a variety of other external stimuli.
Network configurations
A ZigBee network can be as simple as two nodes communicating with each other. However, the strength of ZigBee comes from its ability to handle a collection of sensors, switches, and controllers spread across a large area. Various network topologies, such as tree and mesh, can be used to overcome the short range of 802.15.4 radios (typically up to 70m with line-of-sight) in a large system. A tree configuration relies on a fixed routing table to connect devices to each other. A mesh network optimally relays messages from the source node through intermediate nodes in order to reach the destination. This type of configuration provides network resilience; if one node fails, the network can reroute messages through other nodes.
Profiles and interoperability
The ZigBee protocol sits above the IEEE 802.15.4 MAC and PHY layers and encompasses the network and application-interface layers. In addition, the ZigBee Alliance also defines some higher-level application framework and application profiles.
These predefined application profiles allow developers the option of either using a public application profile or a custom private profile. Public profiles provide vendors with a means to share functions and control, thus allowing products from multiple vendors to interoperate.
The ZigBee Alliance has defined multiple basic public profiles for home automation, HVAC, and industrial sensors. As an example, a ZigBee-controlled light dimmer could use the home-automation profile. Because home automation is a public profile, a light dimmer from one vendor can interoperate with the light ballast of another vendor using that same home-automation profile.
Meanwhile, some developers may opt to use a private profile, because a custom profile can provide a way to create specific application interfaces that only interoperate with a limited number of trusted devices.
Security
As with Wi-Fi and Bluetooth, those who are new to ZigBee quickly raise questions with regards to security. Because of the types of applications where ZigBee networks find use, security is an important aspect of the protocol. In industrial applications, the networks may have hundreds or possibly thousands of nodes. Securing and managing such a network while retaining the simplicity and low cost of ZigBee becomes a formidable task.
To address the issue, chip and system designers have opted to take varying approaches. One approach is to offload the security functions to a dedicated hardware-accelerator block. This reduces the MIPs loading on the CPU, but may have future impacts should the security requirements change. Another approach is to perform the security in software with newer low-cost, low-power 16- or 32-bit embedded processors. These processors have the performance headroom to deliver the security requirements while retaining the advantages of software programmability.
Emerging ZigBee applications
ZigBee deployment traditionally encompasses control and automation of homes, buildings, factories, and entertainment systems. Most of these types of systems require only a simple microcontroller. Moving to a higher-performance embedded processor opens doors new kinds of ZigBee applications, which fall into the following three classes.
Class 1: Decision-maker
In the role of decision-maker, an embedded processor distills great amounts of data to find the essential information that is relevant to a given application domain. This small amount of information can then be sent to other nodes in a ZigBee network.
Figure 1: An embedded processor is required to run computationally-intensive algorithms to decide whether to grant door access based on a fingerprint scan. Security systems serve as good examples for this type of configuration. The use of biometric information has become an effective way to guard against access to sensitive resources. However, data from a fingerprint sensor or a retina scanner needs to be processed against a database of verified users. To ensure an acceptable user experience, the time to process the raw biometric information must not exceed a predetermined delay. Many manufacturers of biometric products aim for 500 msec as the upper time limit between the biometric scan and the match/no-match decision. In a real fingerprint-access system, a high-performance processor, such as one from the Blackfin family, can make a decision in less than 200 msec if running at 400 MHz. This provides time for the system to perform multiple scan and decision-making passes, thus increasing the overall accuracy (see Figure 1).
Because the decision output from the processor requests a small amount of information (such as. "passed fingerprint authentication" or "failed fingerprint authentication"), the bandwidth requirement on the network is low. The main aspect of the network in security systems like fingerprint access is the latency. A rule of thumb is that every hop that a message goes through adds between 10 and 100 msec of latency, depending on the load of the network. This latency, along with the time needed to verify a fingerprint, must not exceed the acceptable user delay defined by the end manufacturer.
An embedded processor, coupled with the flexibility of a ZigBee wireless network, allows for a cost-effective security system solution.
This type of application would be wall-powered, but it would still be considered a low-power system. At 400 MHz, a Blackfin processor can consume less than 100mA at this frequency.
Main requirements: Decision must be made quickly; network latency should be low
Examples: Fingerprint lock; security camera
Class 2: Media compressor/decompressor
While ZigBee is not a high-data-rate protocol, it is possible to judiciously transfer audio and video media across a ZigBee infrastructure. This can be achieved by using one processor to compress media at the source ZigBee node and another processor at the destination ZigBee node to decompress the media.
Figure 2a: High-performance media processors can enable image/video transmission over ZigBee through computationally-intensive but very bandwidth-efficient compression algorithms. Applications such as video doorbells can then be easily deployed in apartment buildings where ZigBee is already used for more traditional applications.One example application of this type is a video doorbell. The setup of such a doorbell system is actually quite simple with a ZigBee network. A video camera at the front door, connected to an efficient media processor and a ZigBee transceiver, serves as the broadcast point. Multiple video displays, utilizing media processors connected to ZigBee transceivers, can be placed throughout the building to pick up images or low-bit-rate video of guests (see Figure 2a). As ZigBee-enabled households and apartment buildings become more prolific, primarily for control of appliances, climate, and entertainment devices, it may be convenient to leverage that infrastructure when adding a video-doorbell system.
Figure 2b: A demo system used to prototype a video-doorbell application has been created using the Analog Devices ADSP-BF533 embedded media processor and the Ember EM260 ZigBee Co-processor. More details are available at the Ember Developer Forum. Because of the real-time aspect of the data, the media processor must perform the encode and decode algorithms within acceptable user-defined time constraints. A prototype video-doorbell system has been implemented using an Analog Devices ADSP-BF533 embedded media processor and an Ember EM260 ZigBee coprocessor (see Figure 2b). A JPEG encoder/decoder implemented on the 32-bit Blackfin ADSP-BF533 media processor consumes ~50 cycles/pixel for a 12:1 compression ratio. For one frame with 640×480 pixels of resolution, this is 15.4 million cycles. Using this example, it is easy to see that CPU utilization of media algorithms can easily overpower 8-bit microprocessors.
While standard low-bit-rate video encoding is an easy task for today's media processors, it does pose challenges to low-bit-rate ZigBee networks. The most bandwidth-efficient media codecs are the most computationally intensive. The nominal rate of the underlying IEEE 802.15.4 standard is 250 kbps. Because of the CSMA/CA architecture, the realistic maximum is on the order of half the nominal rate. In practice, one can expect to achieve upwards of 60 kbps with large packet sizes in a low-frills configuration without security. With network bandwidth of this order, a good-quality VGA-sized JPEG image can be transmitted every second. One way to increase the frame-rate when transmitting images over a low-bit-rate link like ZigBee is to implement a more advanced codec that yields a higher compression ratio than JPEG.
Advanced media codec algorithms, running on modern processors enable the possibility of media transmission over ZigBee networks.
This application is also most likely to be a wall-powered system.
Main requirements: Media demands a fair amount of memory; media must be compressed and decompressed quickly to maintain streaming
Examples: Video doorbell
Class 3: Gateway/administrator
The Gateway/administrator class of ZigBee nodes is used to bridge a large ZigBee network with a more prolific technology like Ethernet, Wi-Fi, or USB. An embedded processor with higher levels of peripheral integration can decrease the bill of materials.
Figure 3: One possible ZigBee administration model is a central embedded server. This device should connect a number of ZigBee nodes to a PC via an ubiquitous presentation model like a web server. An example application is a central administrator ZigBee node that collects data from hundreds of nodes in an industrial environment (see Figure 3). The data may need to be processed or filtered before being presented to a PC user through any standard interface like Wi-Fi. The presentation of data can most easily be done with a Web server running on the administrator node.
Depending on the number of slave ZigBee nodes the administrator serves, the embedded processor can be taxed in terms of data bandwidth. There is also a possibility that the administrator needs to store a lot of data locally. For this reason, it is advisable to choose a processor with an on-chip flash, a NAND-flash port, or a hard-drive controller.
Main requirements: Rich portfolio of peripherals including USB, Wi-Fi, Ethernet, and storage (flash or hard drive)
Examples: Ethernet/ZigBee gateway; ZigBee USB dongle
Summary
As the ZigBee community continues to evolve, so do ZigBee-enabled applications. Low power, configurable architectures can be deployed quickly. The nature of ZigBee system facilitates security. And once networks are built, developers can easily extend them in a variety of topology configurations. The end nodes are no longer relegated to the realm of microcontrollers. Embedded processors can provide an application with added capability at the end node—without exceeding the power budget. The extended capabilities include analytics, media compression, and gateway technology.
#3
文章发表于：2007-07-20 09:37
32位处理器在低功耗、低带宽ZigBee设计中的应用
方案--嵌入式
简介：如同我们在某些厂房中看到的一样，ZigBee已经成为低带宽、低功耗网络的强有力的竞争者。不同的网络节点需要不同等级的处理能力。
当前的消费类市场上，最流行的两个本地无线技术无疑是Wi-Fi和蓝牙。我们已经无法想像如果笔记本、手机以及其他一些手持设备间没有了这些互联协议该怎么办。但是，对于低功耗、低带宽的控制类应用，比如厂房中的传感器网络，ZigBee或许是个更好的选择。配备一个功能强大的处理器后，ZigBee可以在低功耗、低带宽的限制下高效完成很多复杂的应用。
ZigBee联盟在2004年首次推出ZigBee规范，该网络主要用于工控、自动化以及监控应用。ZigBee协议是在IEEE802.15.4无线标准基础上定义了一个软件协议栈，适用于低比特（最高到250kbps)应用的IEEE802.15.4标准则定义了无线的物理层，ZigBee协议相当于逻辑网络和应用软件。由200多家成员公司组成的ZigBee联盟负责ZigBee标准并确保符合规范。
ZigBee在实际应用中有两点很突出：一是对协议栈低的处理需求，这意味着可以采用低成本的处理器；二是多样的互联方式使得组网方式灵活而稳健。
功率：
ZigBee网络中无可避免的有些节点采用电池供电。典型的IEEE 802.15.4节点在功耗方面很高效，因为它正是为此目的而设计。这些设备对电池寿命的衡量单位已经不是“小时”而是“年”。
在一个信标使能（beacon-enabled）的网络中，当某个特定设备想要发送数据或者为了确定其在网络中的成员位置而需要唤醒时，就会确定一个所有节点都遵从的时间片方案。一个采用信标的心跳(heartbeat)型连接可以容许网络中的所有节点在大部分的时间内处于关断状态，只是在响应信标间隔时才会部分唤醒。信标间隔可以在0.015到252秒之间，这就使得包括中央协调器在内的所有系统设备在无需响应时保持空闲。
在无信标的网络中，许多电池供电的节点可以处于空闲状态。
事件检测或者纯传感器类型的节点需要一个处理器，主要用来运行ZigBee协议本身。对于这样的处理负荷，一个8位的处理器就可以应付。8位处理器功耗也很小，这样就可以保证电池的使用寿命，比如Atmel的ATmega32L 8位AVR处理器。
遥测应用在终端变得越来越智能的情况下，有个好消息是开发者在性能上的显著提升并不需要比8位处理器增加更多的功耗开销。这是因为现在的嵌入式处理器有能力在一定的电压和频率范围内优化功耗。
一个采用Blackfin处理器的高级遥测应用运行在低功耗模式下时，与ZigBee协议栈接口并执行一些基本的信号处理，功率消耗在几十个毫瓦的数量级。
再来看一些数据，Blackfin处理器可以运行在一系列的电压和频率上。在最低的0.8V可以跑到250MHz。这个频率可以支持下面的任何一个应用。ZigBee协议栈只会占用很少的MIPS（少于10MIPS）。这就给其他高级处理留下了很多的MIPS，比如JPEG编码或者统计决定。
处理器还可以利用它的RTC实时时钟来定期唤醒。在这个模式下，处理器在大部分时间里处于关断或者低功耗状态。RTC电路在专用电池供应下仅消耗不到30 µA的电流，在一段时间后可以被编程唤醒。
网络配置: ZigBee网络可以简单到是两个节点的互通。但是，ZigBee的强项在于它可以在很大范围内处理许多四处发布的传感器、交换点以及控制器。诸如树形和Mesh等众多网络拓扑就克服了在一个大系统中802.15.4无线短距的问题（典型最高到视距70米）。
分布与互操作性：ZigBee协议基于IEEE802.15.4 MAC和PHY层，并包含有网络层和应用接口层。另外，ZigBee联盟还定义了一些高层应用框架和应用分布。这些预先定义的应用分布容许开发者在公共应用分布和定制私有分布间进行选择。公共应用分布使得供应商可以共享功能和控制，因此来自不同厂家的产品可以互操作。
安全性：ZigBee网络应用的场合中，协议的安全性是个很重要的问题。为了解决这个难题，芯片和系统设计者采用了很多不同方案。其中之一是将安全功能交给特定的硬件加速模块，另一个方法是采用更新的低成本、低功耗的16位或32位嵌入式处理器来软件处理。
ZigBee新应用：
传统上ZigBee适用于家庭、建筑、工厂以及娱乐系统的控制和自动化，大部分这样的应用只需要一个简单的微控制器。采用高性能的嵌入式处理器后可以实现很多新的ZigBee应用，大体上可以分为以下三类：
第一类：决策者：作为决策者，嵌入式处理器针对指定应用从大量数据中提取相关的基本信息，并把这小部分信息送给网络中的其他节点。
主要需求：迅速决断，低的网络延时
举例：指纹识别门禁；安全照相机
第二类：媒体压缩/解压缩：虽然ZigBee并不是一个高数据率的协议，但是仍然有可能在ZigBee中很好的传输音视频媒体内容。通过在源端的ZigBee节点上用一个处理器压缩媒体然后在目的ZigBee节点上用另一个处理器解压缩就可以实现这一过程。
主要需求：媒体需要一定的存储空间，为了保证通畅媒体必须快速压缩和解压缩
举例：视频门禁
第三类： 网关/管理器
ZigBee节点在网关/监控类应用中主要用来把ZigBee网络与其他诸如以太、WiFi或者USB等大量存在的技术进行桥接。具有更多外设集成的嵌入式处理器可以减少物料单。
主要需求：丰富的外设资源包括USB、WiFi、以太以及存储（Flash或硬盘）
举例：以太/ZigBee网关；ZigBee USB dongle
总结：随着ZigBee的继续发展，基于ZigBee的应用也越来越多。ZigBee系统具有可以快速应用的低功耗可配置的结构，而其特性也促进了安全性。嵌入式处理器在功耗预算范围内可以给终端的应用增加很多功能，这些扩展的功能包括：分析、媒体压缩和网关功能。