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1、基于DS2432型自动加密电路的USB口1Wire总线适配器的设计 袁春慧1，钱建立2 （1、空军第一通信修理所，北京 100078；2、空军工程大学 电讯工程学院 陕西 西安 710077） 1 引言 随着计算机硬件、软件技术及集成电路技术的迅速发展，基于现场总线技术的通信网络逐步取代分散式工业控制系统，成为计算机技术及电子技术的应用领域中最具活力的一种边缘分支并取得巨大进步，美国美信公司设计的1Wire总线是众多现场总线中极具竞争力的一种，具备能与计算机进行数字通信、总线负载量大、布线简练，精度高、性能稳定，价格便宜等诸多特点，特别是非常适合中短距离通信，是工业系统设计的高级境界。 1Wr
2、ie1测量网络的实现需要与PC主机相结合，所以在1Wire总线组成的测量网络中，网络适配器是必不可少的，它担负着主机监控单元与网络节点之间数据传输的重要任务，USB接口总线具有高速传输、支持热插拔、即插即用、开发成本低等优点。DS2432提供了一种紧凑的密钥存储方案。有效地降低了硬件仿制的可行性。据此，本文提出了一种基于DS2432的1Wire总线USB口适配器的设计方法。 2 DS2432的内部结构和加密原理 2.1 DS2432的内部结构 DS24322是倒装芯片，有8个引脚，将控制、寻址、数据和电源集于一个数据引脚，可以在4085，2.8V5.25V的宽电压范围内进行读、写操作，内置多点
3、控制，其引脚排列如图1所示。 DS2432的内部原理框图如图2所示。该电路主要由64位掩模ROM、64位暂存器、4个32字节的EEPROM，64位寄存器、64位密钥存储器及512位SHA1（安全散列算法）引擎6个主数据部件组成。 2.2 DS2432的加密原理 DS2432有四个存储区：数据存储器、密钥存储器、含有特定功能和用户字节的寄存器页和暂存器。数据存储器每页32个字节。密钥、寄存器页和暂存器均为8字节。向数据存储器写数据、装载初始密钥，或者向寄存器页写入数据时，暂存器作为缓存器使用。数据存储器、密钥存储器和寄存器页位于线性地址空间中，数据存储器和寄存器页对读访问没有限制，但向数据存储器
4、和寄存器页写数据时则需要知道密钥，密钥的安装有二种方法，一是把数据从暂存器复制到密钥存储器，二是当前密钥和暂存器内容经过运算后生成新的密钥，密钥不能直接读取，只有SHA引擎能够访问它，计算信息鉴定码MAC。 地址0088H008FH为寄存器页，含有特定功能寄存器、通用用户字节以及1个工厂字节。一旦编程为AAH或55H。这些字节中的大多数将被写保护而不能再更改，其他所有代码既不能写保护，也不能激活与这个特定字节相关的特殊功能，特殊功能包括仅写保护密钥，同时写保护4个数据存储器页，仅激活数据存储器页1的EPROM模式，仅激活数据存储器页的0的EPROM模式，一旦EPROM模式被激活，数据存储器未加
5、写保护的情况下，地址0020H003FH中的位只能从逻辑1改为逻辑0。若读取到的地址设为55H，表明地址008E和008F是可读/写的用户字节，没有任何特定功能和锁定机制。代码AAH表明这二个字节被编程为16位的ID，并且被加写保护，主机与1Wire设备进行通信时，用户要提供16位的ID识别码，用来协助应用软件识别包含DS2432的1Wire网络适配器。3 适配器的设计 3.1 硬件电路设计 适配器需2个接口电路分别与计算机主机系统和1Wire网络进行通信，与主机系统进行通信主要通过USB协议实现，称为USB接口电路；与1Wire网络之间的通信通过USB到1Wire总线的桥接电路DS2490S
6、实现，但需1个RJ11插座，称为RJ11桥接电路，本设备采用USB总线的供电方式。 3.1.1 USB接口电路设计 USB接口电路的原理如图3所示。 R3是上拉电阻器，它可使USB口的D端上拉到DS2490S的VB端，表示USB主机系统是高速设备，同时这个上拉电阻器告诉主机有USB设备插入。该上拉电阻器的设置对适配器的影响很大，它的负载值和1Wire网络的总厂决定1Wire总线电压上升到5V的速度，经过实验测试选择R3的阻值为27欧姆10。R1、R2为USB数据线保护电阻器。L1、L2具有禁止高频干扰并且减弱EMI辐射的功能，LF33CV为3.3V电压稳压器，与周围元件C1、C2组成强上拉部分
7、，给EEPROM或温度传感器等器件提供额外的电源。 3.1.2 RJ11桥接电路的设计 RJ11桥接电路的原理如图4所示。 DS2490S3是USB口与1Wire器件的桥接电路，可直接连至USB主机系统。它可产生严格定时和受电压摆率控制的1Wire波形，并且可满足USB接口通信协议的要求，保证USB口与1Wire器件正常，安全地通信。 晶体振荡器电路主要由X1构成，在整个系统中非常重要，可保证DS2490S稳定工作。为了产生12Mb/s的USB总线速率，系统使用12MHz的晶体振荡器，电容器的接入是为了使晶体振荡器更容易起振。肖特基二极管D1，D2可消除来自于ESD采样数的尖峰信号或通过分别导
8、通D1，D2至GND，交叉耦合其他的邻近电缆，防止1Wire总线过电压，电阻器R5可限制ESD的电流并保护D1和D2，R4、R5和D3组成谐振电路，其特有的静电晶体振荡器放电功能可对1Wire总线起到保护作用。 建议在1Wire网络应外中使用阻抗约100欧姆非屏蔽双绞电缆，在本电路设计中考虑到这方面的原因，将R4、R5的阻值定位1k欧姆，满足了适配器数据通信的要求，整体电路原理如图5所示。 3.2 适配器软件设计 软件设计4主要包括计算机端USB接口驱动程序模块设计和DS2490S桥接模块与读取DS2432模块的固件设计。在设计中将软件程序模块化，可方便调试和完善，本适配器软件的设计是在Win
9、dows2000系统环境下进行的，设计工具是VisualC和DriverStudio。Maxim公司为软件开发者提供了的大量的驱动程序和测试代码。使用时仅需针对具体情况进行修改。这里不对源代码进行赘述，仅给出加密软件流程。如图6所示。 4 适配器通信信号性能测试 对于远程1Wire测量网络的通信，在核实信号时，尤其是在电缆末端，必须使用差分探测器和探测器的地浮动参考，另外，探测器的地连接将设旁路1Wire参考地并改变系统的拓扑。如果没有采用差分探测器，必须从总体地通过变压器或使用电池供电的示波器隔离示波器的地，二者选其一来分离示波器电源供电电缆的完全地，当从远端反射信号时，在图形下降沿末尾的中
10、心会产生一些波动（俗称鞍），这是传输线的自然动作，电缆短于100m时看不到该鞍，由于条件所限，本适配器的通信信号性能测试在实验室内进行，将适配器接入主机USB接口，再将适配器接入主机，来扫描适配器传输的信号，图7所示为示波器扫描时序，由图中可以看出输入/输出信号的时序基本满足1Wire器件的信号时序，未出现尖峰或混沌现象，适配器的工作基本稳定。5 结束语 本设计在适配器硬件、用户软件及身份认真方面做了一些探索性工作。1Wire为总线设备是Maxim公司力推的产品，虽然价格较高，上市时间不长，在业界还没有得到广泛的应用，但具有众多优点，将会有很好的应用前景。Editors note: Judso
11、n Jones is a meteorologist, journalist and photographer. He has freelanced with CNN for four years, covering severe weather from tornadoes to typhoons. Follow him on Twitter: jnjonesjr (CNN) - I will always wonder what it was like to huddle around a shortwave radio and through the crackling static f
12、rom space hear the faint beeps of the worlds first satellite - Sputnik. I also missed watching Neil Armstrong step foot on the moon and the first space shuttle take off for the stars. Those events were way before my time.As a kid, I was fascinated with what goes on in the sky, and when NASA pulled t
13、he plug on the shuttle program I was heartbroken. Yet the privatized space race has renewed my childhood dreams to reach for the stars.As a meteorologist, Ive still seen many important weather and space events, but right now, if you were sitting next to me, youd hear my foot tapping rapidly under my
14、 desk. Im anxious for the next one: a space capsule hanging from a crane in the New Mexico desert.Its like the set for a George Lucas movie floating to the edge of space.You and I will have the chance to watch a man take a leap into an unimaginable free fall from the edge of space - live.The (lack o
15、f) air up there Watch man jump from 96,000 feet Tuesday, I sat at work glued to the live stream of the Red Bull Stratos Mission. I watched the balloons positioned at different altitudes in the sky to test the winds, knowing that if they would just line up in a vertical straight line we would be go f
16、or launch.I feel this mission was created for me because I am also a journalist and a photographer, but above all I live for taking a leap of faith - the feeling of pushing the envelope into uncharted territory.The guy who is going to do this, Felix Baumgartner, must have that same feeling, at a lev
17、el I will never reach. However, it did not stop me from feeling his pain when a gust of swirling wind kicked up and twisted the partially filled balloon that would take him to the upper end of our atmosphere. As soon as the 40-acre balloon, with skin no thicker than a dry cleaning bag, scraped the g
18、round I knew it was over.How claustrophobia almost grounded supersonic skydiverWith each twist, you could see the wrinkles of disappointment on the face of the current record holder and capcom (capsule communications), Col. Joe Kittinger. He hung his head low in mission control as he told Baumgartne
19、r the disappointing news: Mission aborted.The supersonic descent could happen as early as Sunday.The weather plays an important role in this mission. Starting at the ground, conditions have to be very calm - winds less than 2 mph, with no precipitation or humidity and limited cloud cover. The balloo
20、n, with capsule attached, will move through the lower level of the atmosphere (the troposphere) where our day-to-day weather lives. It will climb higher than the tip of Mount Everest (5.5 miles/8.85 kilometers), drifting even higher than the cruising altitude of commercial airliners (5.6 miles/9.17
21、kilometers) and into the stratosphere. As he crosses the boundary layer (called the tropopause), he can expect a lot of turbulence.The balloon will slowly drift to the edge of space at 120,000 feet (22.7 miles/36.53 kilometers). Here, Fearless Felix will unclip. He will roll back the door.Then, I wo
22、uld assume, he will slowly step out onto something resembling an Olympic diving platform.Below, the Earth becomes the concrete bottom of a swimming pool that he wants to land on, but not too hard. Still, hell be traveling fast, so despite the distance, it will not be like diving into the deep end of
23、 a pool. It will be like he is diving into the shallow end.Skydiver preps for the big jumpWhen he jumps, he is expected to reach the speed of sound - 690 mph (1,110 kph) - in less than 40 seconds. Like hitting the top of the water, he will begin to slow as he approaches the more dense air closer to
24、Earth. But this will not be enough to stop him completely.If he goes too fast or spins out of control, he has a stabilization parachute that can be deployed to slow him down. His team hopes its not needed. Instead, he plans to deploy his 270-square-foot (25-square-meter) main chute at an altitude of
25、 around 5,000 feet (1,524 meters).In order to deploy this chute successfully, he will have to slow to 172 mph (277 kph). He will have a reserve parachute that will open automatically if he loses consciousness at mach speeds.Even if everything goes as planned, it wont. Baumgartner still will free fal
26、l at a speed that would cause you and me to pass out, and no parachute is guaranteed to work higher than 25,000 feet (7,620 meters).It might not be the moon, but Kittinger free fell from 102,800 feet in 1960 - at the dawn of an infamous space race that captured the hearts of many. Baumgartner will attempt to break that record, a feat that boggles the mind. This is one of those monumental moments I will always remember, because there is no way Id miss this.
