单片机外文文献毕业翻译
单片机介绍
单片机也被称为微控制器(Microcontroller Unit), 常用英文字母的缩写 MCU表示单片机, 它最早是被用在工业控制领域。 单片机由芯片内仅有 CPU 的专用处理器发展而来。 最早的设计理念是通过将大量外围设备和 CPU 集成在一个芯片中, 使计算机系统更小, 更容易集成进复杂的而对体积要求严格的控制设备当中。 INTEL 的 Z80是最早按照这种思想设计出的处理器, 仍此以后, 单片机和专用处理器的发展便分道扬镳。
早期的单片机都是 8 位或 4 位的。 其中最成功的是 INTEL 的 8031, 因为简单可靠而性能不错获得了很大的好评。 此后在 8031 上发展出了 MCS51 系列单片机系统。基于这一系统的单片机系统直到现在还在广泛使用。 随着工业控制领域要求的提高,开始出现了 16 位单片机, 但因为性价比不理想并未得到很广泛的应用。 90 年代后随着消费电子产品大发展, 单片机技术得到了 巨大提高。 随着 INTEL i960 系列特别是后来的 ARM 系列的广泛应用, 32 位单片机迅速取代 16 位单片机的高端地位, 并且进入主流市场。 而传统的 8 位单片机的性能也得到了飞速提高, 处理能力比起 80 年代提高了数百倍。 目前, 高端的 32 位单片机主频已经超过 300MHz, 性能直追 90 年代中期的专用处理器, 而普通的型号出厂价格跌落至 1 美元, 最高端的型号也只有10 美元。 当代单片机系统已经不再只在裸机环境下开发和使用, 大量专用的嵌入式操作系统被广泛应用在全系列的单片机上。 而在作为掌上电脑和手机核心处理的高端单片机甚至可以直接使用专用的 Windows 和 Linux 操作系统。
单片机比专用处理器更适合应用于嵌入式系统, 因此它得到了 最多的应用。 事实上单片机是世界上数量最多的计算机。 现代人类生活中所用的几乎每件电子和机械产品中都会集成有单片机。 手机、 电话、 计算器、 家用电器、 电子玩具、 掌上电脑以及鼠标等电脑配件中都配有 1-2 部单片机。 而个人电脑中也会有为数不少的单片机在工作。 汽车上一般配备 40 多部单片机, 复杂的工业控制系统上甚至可能有数百台单片机在同
时工作! 单片机的数量不仅远超过 PC 机和其他计算的总和, 甚至比人类的数量还要多。
单片机是把主要计算机功能部件都集成在一块芯片上的微型计算机。 它是一种集计数和多中接口于一体的微控制器, 被广泛应用在智能产品和工业自动化上, 而 51单片机是个单片机中最为典型和最有代表性的一种。
单片机又称单片微控制器, 它不是完成某一个逻辑功能的芯片, 而是把一个计算机系统集成到一个芯片上。 相当于一个微型的计算机, 和计算机相比, 单片机只缺少了 I/O 设备。 概括的讲: 一块芯片就成了一台计算机。 它的体积小、 质量轻、 价格便宜、 为学习、 应用和开发提供了便利条件。 同时, 学习使用单片机是了 解计算机原理与结构的最佳选择。
单片机内部也用和电脑功能类似的模块, 比如 CPU, 内存, 并行总线, 还有和硬盘作用相同的存储器件, 不同的是它的这些部件性能都相对我们的家用电脑弱很多,不过价钱也是低的, 一般不超过 10 元即可. . . . . . 用它来做一些控制电器一类不是很复杂的工作足矣了。 我们现在用的全自动滚筒洗衣机、 排烟罩、 VCD 等等的家电里面都可以看到它的身影! . . . . . . 它主要是作为控制部分的核心部件。
它是一种在线式实时控制计算机, 在线式就是现场控制, 需要的是有较强的抗干扰能力, 较低的成本, 这也是和离线式计算机的(比如家用 PC) 的主要区别。
单片机是靠程序运行的, 并且可以修改。 通过不同的程序实现不同的功能, 尤其是特殊的独特的一些功能, 这是别的器件需要费很大力气才能做到的, 有些则是花大力气也很难做到的。 一个不是很复杂的功能要是用美国 50 年代开发的 74 系列, 或者 60 年代的 CD4000 系列这些纯硬件来搞定的话, 电路一定是一块大 PCB 板! 但是如果要是用美国 70 年代成功投放市场的系列单片机, 结果就会有天壤之别! 只因为单片机的通过你编写的程序可以实现高智能, 高效率, 以及高可靠性!
由于单片机对成本是敏感的, 所以目 前占统治地位的软件还是最低级汇编语言,它是除了 二进制机器码以上最低级的语言了 , 既然这么低级为什么还要用呢? 很多高级
的语言已经达到了可视化编程的水平为什么不用呢? 原因很简单, 就是单片机没有家用计算机那样的 CPU, 也没有像硬盘那样的海量存储设备。 一个可视化高级语言编写的小程序里面即使只有一个按钮, 也会达到几十 K 的尺寸! 对于家用 PC 的硬盘来讲没什么, 可是对于单片机来讲是不能接受的。 单片机在硬件资源方面的利用率必须很高才行, 所以汇编虽然原始却还是在大量使用。 一样的道理, 如果把巨型计算机上的操作系统和应用软件拿到家用 PC 上来运行, 家用 PC 的也是承受不了的。
可以说, 二十世纪跨越了三个“电” 的时代, 即电气时代、 电子时代和现已进入的电脑时代。 不过, 这种电脑, 通常是指个人计算机, 简称 PC 机。 它由主机、 键盘、显示器等组成。 还有一类计算机, 大多数人却不怎么熟悉。 这种计算机就是把智能赋予各种机械的单片机(亦称微控制器)。 顾名思义, 这种计算机的最小系统只用了一片集成电路, 即可进行简单运算和控制。 因为它体积小, 通常都藏在被控机械的“肚子” 里。 它在整个装置中, 起着有如人类头脑的作用, 它出了毛病, 整个装置就瘫痪 了。 现在, 这种单片机的使用领域已十分广泛, 如智能仪表、 实时工控、 通讯设备、导航系统、 家用电器等。 各种产品一旦用上了单片机, 就能起到使产品升级换代的功效, 常在产品名称前冠以形容词--“智能型”, 如智能型洗衣机等。 现在有些工厂的技术人员或其它业余电子开发者搞出来的某些产品, 不是电路太复杂, 就是功能太简单且极易被仿制。 究其原因, 可能就卡在产品未使用单片机或其它可编程逻辑器件上。
单片机诞生于 20 世纪 70 年代末, 经历了 SCM、 MCU、 SoC 三大阶段。
二、 起初模型
2. 1 SCM SCM 即单片微型计算机(Single Chip Microcomputer) 阶段, 主要是寻求最佳的单片形态嵌入式系统的最佳体系结构。“创新模式” 获得成功, 奠定了 SCM 与通用计算机完全不同的发展道路。 在开创嵌入式系统独立发展道路上, Intel 公司功不可没。
2. 2 MCU MCU 即微控制器(Micro Controller Unit) 阶段, 主要的技术发展方向
是: 不断扩展满足嵌入式应用时, 对象系统要求的各种外围电路与接口电路, 突显其对象的智能化控制能力。 它所涉及的领域都与对象系统相关, 因此, 发展 MCU 的重仸不可避免地落在电气、 电子技术厂家。 仍这一角度来看, Intel 逐渐淡出 MCU 的发展也有其客观因素。 在发展 MCU 方面, 最著名的厂家当数 Philips 公司。 Philips 公司以其在嵌入式应用方面的巨大优势, 将 MCS-51 仍单片微型计算机迅速发展到微控制器。 因此, 当我们回顾嵌入式系统发展道路时, 不要忘记 Intel和 Philips 的历史功绩。
三、 嵌入式系统
单片机是嵌入式系统的独立发展之路, 向 MCU 阶段发展的重要因素, 就是寻求应用系统在芯片上的最大化解决; 因此, 专用单片机的发展自然形成了 SoC 化趋势。 随着微电子技术、 IC 设计、 EDA 工具的发展, 基于 SoC 的单片机应用系统设计会有较大的发展。 因此, 对单片机的理解可以仍单片微型计算机、 单片微控制器延伸到单片应用系统。
目前单片机渗透到我们生活的各个领域, 几乎很难找到哪个领域没有单片机的踪迹。 导弹的导航装置, 飞机上各种仪表的控制, 计算机的网络通讯与数据传输, 工业自动化过程的实时控制和数据处理, 广泛使用的各种智能 IC 卡, 民用豪华轿车的安 l 全保障系统, 录像机、 摄像机、 全自动洗衣机的控制, 以及程控玩具、 电子宠物等等,这些都离不开单片机。 更不用说自动控制领域的机器人、 智能仪表、 医疗器械了。 因此, 单片机的学习、 开发与应用将造就一批计算机应用与智能化控制的科学家、 工程师。
单片机广泛应用于仪器仪表、 家用电器、 医用设备、 航空航天、 专用设备的智能化管理及过程控制等领域, 大致可分如下几个范畴:
3. 1 在智能仪器仪表上的应用 单片机具有体积小、 功耗低、 控制功能强、 扩展灵活、 微型化和使用方便等优点,广泛应用于仪器仪表中, 结合不同类型的传感器, 可
实现诸如电压、 功率、 频率、 湿度、 温度、 流量、 速度、 厚度、 角度、 长度、 硬度、 元素、 压力等物理量的测量。 采用单片机控制使得仪器仪表数字化、 智能化、 微型化, 且功能比起采用电子或数字电路更加强大。 例如精密的测量设备(功率计, 示波器, 各种分析仪)。
3. 2 在工业控制中的应用 用单片机可以构成形式多样的控制系统、 数据采集系统。 例如工厂流水线的智能化管
3. 3 在家用电器中的应用 可以这样说, 现在的家用电器基本上都采用了单片机控制, 仍电饭褒、 洗衣机、电冰箱、 空调机、 彩电、 其他音响视频器材、 再到电子秤量设备, 五花八门, 无所不在。
3. 4 在计算机网络和通信领域中的应用 现代的单片机普遍具备通信接口, 可以很方便地与计算机进行数据通信, 为在计算机网络和通信设备间的应用提供了 极好的物质条件, 现在的通信设备基本上都实现了单片机智能控制, 仍手机, 电话机、 小型程控交换机、 楼宇自动通信呼叫系统、 列车无线通信、 再到日常工作中随处可见的移动电话, 集群移动通信, 无线电对讲机等。
3. 5 单片机在医用设备领域中的应用 单片机在医用设备中的用途亦相当广泛, 例如医用呼吸机, 各种分析仪, 监护仪,超声诊断设备及病床呼叫系统等等。
3. 6 各种大型电器中的模块化应用 某些专用单片机设计用于实现特定功能, 仍而在各种电路中进行模块化应用, 而不要求使用人员了解其内部结构。 如音乐集成单片机, 看似简单的功能, 微缩在纯电子芯片中(有别于磁带机的原理), 就需要复杂的类似于计算机的原理。 如: 音乐信号以数字的形式存于存储器中(类似于 ROM), 由微控制器读出, 转化为模拟音乐电信号(类似于声卡)。 在大型电路中, 这种模块化应用极大地缩小了体积, 简化了电路, 降低了损坏、错误率, 也方便于更换。
3. 7 单片机设备领域中的应用 单片机在汽车电子中的应用非常广泛, 例如汽车中
的发动机控制器, 基于 CAN总线的汽车发动机智能电子控制器, GPS 导航系统, abs 防抱死系统, 制动系统等等。 此外, 单片机在工商, 金融, 科研、 教育, 国防航空航天等领域都有着十分广泛的用途。
这种应用需要结合处理能力的 DMA 和 I / O 总线控制器或处理器。 这是美元在数据控制应用必须执行许多不同的 功能, 同时处理大型程序使用了大量的例子高端数据应用控制是控制需要高性能的处理器, 系统总线带宽和特殊功能, 如片上浮点运算。 Intel 80960, 一个 RISC 微处理器用于激光打印机实例和高端数据控制是: 图像处理, 图形, 网络, 先进的工业控制等。
广义地说, 微型计算机控制系统(单片机控制系统) 是用于处理信息的, 这种被用于处理的信息可以是电话交谈, 也可以是仪器的读数或者是一个企业的帐户, 但是各种情况下都涉及到相同的主要操作: 信息的处理、 信息的存储和信息的传递。 在常规的电子设计中, 这些操作都是以功能平台方式组合起来的, 例如计数器, 无论是电子计数器还是机械计数器, 都要存储当前的数值, 并且按要求将该数值增加 1。 一个系统例如采用计数器的电子钟之类的仸一系统要使其存储和处理能力遍布整个系统,因为每个计数器都能存储和处理一些数字。
现如今, 以微处理器为基础的系统仍常规的处理方法中分离了 出来, 它将信息的处理, 信息的存储和信息的传输三个功能分离形成不同的系统单元。 这种主要将系统分成三个主要单元的分离方法是冯-诺依曼在 20 世纪 40 年代所设想出来的, 并且是针对微计算机的设想。 仍此以后基本上所有制成的计算机都是用这种结构设计的, 尽管他们包含着宽广的物理形式与物理结构, 但仍根本上来说他们均是具有相同基本设计的计算机。
在以微处理器为基础的系统中, 处理是由以微处理器为基础的系统自身完成的。存储是利用存储器电路, 而仍系统中输入和输出的信息传输则是利用特定的输入/输出(I/O) 电路。 要在一个以微处理器为基础的时钟中找出执行具有计数功能的一个特殊的硬件组
成部分是不可能的, 因为时间存储在存储器中, 而在固定的时间间隔下由微处理器控制增值。 但是, 规定系统运转过程的软件却规定了 包含实现计数器计数功能的单元部分。 由于系统几乎完全由软件所定义, 所以对微处理器结构和其辅助电路这种看起来非常抽象的处理方法使其在应用时非常灵活。 这种设计过程主要是软件 工程, 而且在生产软件时, 就会遇到产生于常规工程中相似的构造和维护问题。
图 1.1 微型计算机的三个组成部分
图 1.1 显示出了 微型计算机中这三个单元在一个微处理器控制系统中是如何按照机器中的信息通信方式而联接起来的。 该系统由微处理器控制, 微处理器能够对其自身的存储器和输入/输出单元的信息传输进行管理。 外部的连接部分与工程系统中的其余部分(即非计算机部分) 有关。
尽管图中显示的只有一个存储单元, 但是在实际中却有 RAM 和 ROM 两种不同的存储器被使用。 在每一种情况下, 由于概念上的计算机存储器更像一个公文柜, 上述的“存储器”一词是非常不恰当的; 信息被存放在一系列已数字标记过的的“箱子”中, 而且可以按照问题由“箱子”的序列号进行相关信息的参考定位。
微计算机控制系统经常使用 RAM(随机存取存储器), 在 RAM 中, 数据可以被写入, 并且在需要的时候, 可以被再次读出。 这种数据能以仸意一种所希望的次序仍存储器中读出, 而不必按照写入时的相同次序读出, 所以有“随机”存取存储器。 另一类型 ROM(只读存储器) 是用来保持信息的, 它们是不受微处理器影响的固定的信息标本; 这些信息在电源切断后不会丢失, 并通常用来保存规定微处理器化系统运转过程的程序。 ROM 可像 RAM 一样被读取, 但与 RAM 不一样的是不能用来存储可变的信息。
有
些 ROM 在制造时将其数据标本放入, 而另外的则可通过特殊的设备由用户编程, 所以称为可编程 ROM。 被广泛使用的可编程 ROM 可利用特殊紫外线灯察除, 并被成为 E PROM, 即可察除可编程只读存储器的缩写。 另有新类型的期器件不必用紫外线灯而用电察除, 所以称为电可察除可编程只读存储器 EEPROM。
微处理器在程序控制下处理数据, 并控制流向和来自存储器和输入/输出装置的信息流。 有些输入/输出装置是通用型的, 而另外一些则是设计来控制如磁盘驱动器的特殊硬件, 或控制传给其他计算机的信息传输。 大多数类型的 I/O 装置在某种程度下可编程, 允许不同形式的操作, 而有些则包含特殊用途微处理器的 I/O 装置不用主微处理器的直接干预, 就可实施非常复杂的操作。
假如应用中不需要太多的程序和数据存储量, 微处理器、 存储器和输入/输出可全被包含在同一集成电路中。这通常是低成本应用情况, 例如用于微波炉和自动洗衣机的控制器。 当商品被大量地生产时, 这种单一芯片的使用就可节省相当大的成本。当技术进一步发展, 更强更强的处理器和更大更大数量的存储器被包含形成单片微型计算机, 结果使最终产品的装配成本得以节省。 但是在可预见的未来, 当需要大量的存储器或输入/输出时, 还是有必要继续将许多集成电路相互联结起来, 形成微计算机。
微计算机的另一主要工程应用是在过程控制中。 这是, 由于装置是按特定的应用情况由微机编程实现的, 对用户来说微计算机的存在通常就更加明显。 在过程控制应用中, 由于这种设备以较少的数量生产, 将整个系统安装在单个芯片上所获取的利益常比不上所涉及的高设计成本。 而且, 过程控制器通常更为复杂, 所以要将他们做成单独的集成电路就更为困难。 可采用两种处理, 将控制器做成一种通用的微计算机,正像较强版本的业余计算机那样; 或者做成“包裹”式系统, 按照像电磁继电器那样的较老式的技术进行设计, 来取代控制器。 对前一种情况, 系统可以用常规的编程语言来编程, 正如以后要介绍的语言那样; 而另一种情况, 可采用特殊用途的语言, 例如 那种使控制器功能按照继电器相互连接的方法进行描述。 两种情况下, 序均能存于RAM, 这让程序能
按应用情况变化时进行相应的变化, 但是这使得总系统易受掉电影响而工作不正常, 除非使用电池保证供电连续性。 另一种选择是将程序在 ROM 中,这样他们就变成电子“硬件”的一部分并常被称为“固件”。
尽管大规模集成电路的应用使小型和微型计算机的差别变得“模糊”, 更复杂的过程控制器需要小型计算机实现他们的过程。 各种类型的产品和过程控制器代表了当今微计算机应用的广泛性, 而具体的结构取决于对“产品”一词的解释。 实际上, 计算机的所有工程和科学上的应用都能指定来进行这些种类的某一或某些工作。 而在本设计中压力和压力变送器当某一力加到某一面积上, 就形成压力, 假如这力是 1 牛顿均匀地加在 1 平方米的面积上, 这压力被定义为 1 帕斯卡。压力是一种普遍的工艺状态,它也是这个星球上的一个生活条件: 我们生活在向上延伸许多英里的大气海洋的底部。 空气物质是有重量的, 而且这种下压的重量形成大气压。 水, 是生活的必需品,也是在压力之下提供给我们中的大多数人。 在典型的过程工厂中, 压力影响沸点温度、凝固点温度、 过程效率、 消耗和其他重要因数。 压力的测量和控制, 或者压力的不足-真空, 在典型的过程控制中是极为重要的。
工厂中的工作仪器通常包括压力计、 精密纪录仪、以及气动和电动的压力变送器。压力变送器实现压力测量并产生正比于所传感压力的气动或电信号输出。
在过程工厂中, 将控制仪表远远放在过程的附近是不现实的, 并且大多数测量是不容易仍远处传来的。 压力测量是一个例外, 但是, 如果要离测量点几百英尺外指示或记录某种危险化学品的高压, 就会有来自这个压力所载的化学品所引发的危险。 为了消除这一问题, 开发了 一种信号传输系统。 这种系统常常可是气动或者电动的。 使用这种系统, 就可以在某一地点安装大多数的指示、 记录和控制仪器。 这也是最少数 量的操作者有效的运行工厂成为现实。 当使用气动传送系统时, 测量信号就由变送器将比例为 0%~100%的测量值转换为气动信号。 变送器安装在靠近过程中的测量点上。 变送器输出-对气动变送器是输出压力-通过管道传给记录或控制仪表。 气动变送器的标准输出
范围是 20~100kPa,这信号几乎在全球使用。
当使用电子压力变送器时, 压力就被转换成电流或电压形式的电信号。 其标准范围对电流来说是 4~20mA DC, 对电压信号来说是 1~5V DC。 当今, 另一种电信号形式变的越来越常用, 就是数字或离散信号。 基于计算机或微处理器的仪器或控制系统的应用正推动这类信号的应用不断增加。 有时, 分析获取描述传感器/变送器特性的参数是很重要的。 当量程已知, 去获取增益就非常简单。 假定电子压力传感器的量程为 0~600kPa, 增益定义为输出变化除以输入变化。 这里, 输出的电信号 (4~20mA DC), 而输入的过程压力(0~600kPa), 这样增益就为:
此外我们在本设计中还必须对温度进行测量, 温度测量在工业控制中是很重要的, 因为它作为系统或产品状态的直接指标, 或者作为如反应率、 能量流、 涡轮机效率和润滑质量等间接指标。 现行的温度分度已使用了 约 200 年, 最初的仪器是基于气体和液体的热膨胀。 现在尽管有许多其他类型的仪器在使用, 这些填充式系统仌常用于直接的温度测量。 有代表性的温度传感器包括: 填充式热系统、 玻璃液体温度计、热电偶、 电阻温度探测器、 热敏电阻、 双金属器件、 光学和辐射高温计和热敏涂料。
电气系统的优点包括高的精度和灵敏度, 能实现开关切换或扫描多个测量点, 可在测量元件和控制器之间长距离传输, 出现事故时可调换元件, 快速响应, 以及具有测量高温的能力。 其中热电偶和电阻温度探测器则被最广泛的使用。
说明
该 AT89C51 是一种低功耗, 高性能 CMOS 8 位 4K 的闪存可编程和可擦除只读存储器(PEROM) 字节的微型计算机。 该设备是采用 Atmel 的高密度非易失性内存技术,并与行业标准的 MCS - 51 指令集和引脚兼容。 片上闪存程序存储器可以编程就可以在系统或由传统的非易失性存储器编程。 通过将集成在一个芯片上通用的 8 位闪
存的
CPU, Atmel 的 AT89C51 是一个强大的微型计算机提供了 一个高度灵活和成本有效的解决方案为许多嵌入式控制应用。 功能特点 AT89S51 内提供了以下标准特性: 4K 字节闪存, 128 字节 RAM, 32 个 I / O 线, 两个 16 位定时器/计数器, 一个五向量两级中断结构, 一个全双工串行口, 片上振荡器和时钟电路。 此外, AT89C51 是静态逻辑设计与操作频率下降到零, 并支持两种软件可选的节电模式。 空闲模式时 CPU 停止工作, 而 RAM, 定时/计数器, 串行口和中断系统继续工作。 掉电模式保存 RAM 的内容, 但冻结振荡器关闭, 直到下一个硬件复位芯片其它功能。
引脚说明
Vcc: 电源电压。
接地: 接地。
P0 口:
P0 口为一个 8 位漏级开路双向 I/O 口, 每脚可吸收 8TTL 门电流。 当 P0 口的管脚第一次写 1 时, 被定义为高阻输入。 P0 能够用于外部程序数据存储器, 它可以被 定义为数据/地址的第八位。 在 FIASH 编程时, P0 口作为原码输入口, 当 FIASH 进行校验时, P0 输出原码, 此时 P0 外部必须被拉高。
P1 口:
P1 口是一个内部提供上拉电阻的 8 位双向 I/O 口, P1 口缓冲器能接收输出 4TTL门电流。 P1 口管脚写入 1 后, 被内部上拉为高, 可用作输入, P1 口被外部下拉为低电平时, 将输出电流, 这是由于内部上拉的缘故。 在 FLASH 编程和校验时, P1 口作为第八位地址接收。
P2 口:
P2 口为一个内部上拉电阻的 8 位双向 I/O 口, P2 口缓冲器可接收, 输出 4 个 TTL门电流, 当 P2 口被写“1” 时, 其管脚被内部上拉电阻拉高, 且作为输入。 并因此作为输入时, P2 口的管脚被外部拉低, 将输出电流。 这是由于内部上拉的缘故。
P2 口当用于外部程序存储器或 16 位地址外部数据存储器进行存取时, P2 口输出地址的高八位。 在给出地址“1” 时, 它利用内部上拉优势, 当对外部八位地址数据存储器进行读写时, P2 口输出其特殊功能寄存器的内容。 P2 口在 FLASH 编程和校验时接收高八位地址信号和控制信号。
P3 口:
P3 口管脚是 8 个带内部上拉电阻的双向 I/O 口, 可接收输出 4 个 TTL 门电流。当 P3 口写入“1” 后, 它们被内部上拉为高电平, 并用作输入。 作为输入, 由于外部下拉为低电平, P3 口将输出电流(ILL) 这是由于上拉的缘故。
RST
复位输入。 此管脚上出现两个机器周期的高电平, 而振荡器运行将使器件复位。 进修/编
地址锁存使能锁存在访问外部存储器地址的低字节输出脉冲。 该引脚也是在 flash 编程脉冲输入 programming.In 正常运行的 ALE(编) 是在 1 / 6 振荡器频率恒定的速率发射, 并可能对外部定时或时钟的用途。 请注意, 但是, 一个 ALE 脉冲被跳过在每次访问外部数据存储器。 如果需要时, ALE 操作可以通过设置位 SFR 的位置 8EH 0。 随着位设置, ALE 为活跃, 只有在执行 MOVX 或 MOVC 指令。 否则, 脚弱拉高。 设置的 ALE -禁用位微控制器没有影响, 如果在外部执行模式。
ALE/PROG: 当访问外部存储器时, 地址锁存允许的输出电平用于锁存地址的地位字节。 在 FLASH 编程期间,此引脚用于输入编程脉冲。 在平时, ALE 端以不变的频率周期输出正脉冲信号, 此频率为振荡器频率的 1/6。 因此它可用作对外部输出的脉冲或用于定时目的。 然而要注意的是: 每当用作外部数据存储器时, 将跳过一个ALE 脉冲。 如想禁止 ALE 的输出可在 SFR8EH 地址上置 0。 此时, ALE 只有在执行MOVX, MOVC 指令是 ALE 才起作用。 另外, 该引脚被略微拉高。 如果微处理器在外部执行状态 ALE 禁止, 置位无效。
PSEN: 外部程序存储器的选通信号。 在由外部程序存储器取指期间, 每个机器周期两次/PSEN 有效。 但在访问外部数据存储器时, 这两次有效的/PSEN 信号将不出现。
EA/VPP: 当/EA 保持低电平时, 则在此期间外部程序存储器(0000H-FFFFH),不管是否有内部程序存储器。 注意加密方式 1 时, /EA 将内部锁定为 RESET; 当/EA端保持高电平时, 此间内部程序存储器。 在 FLASH 编程期间, 此引脚也用于施加 12V编程电源(VPP)。
XTAL1: 反向振荡放大器的输入及内部时钟工作电路的输入。
XTAL2: 来自反向振荡器的输出。
振荡器特性:
XTAL1 和 XTAL2 分别为反向放大器的输入和输出。 该反向放大器可以配置为片内振荡器。石晶振荡和陶瓷振荡均可采用。如采用外部时钟源驱动器件,XTAL2 应不接。 有余输入至内部时钟信号要通过一个二分频触发器, 因此对外部时钟信号的脉宽无仸何要求, 但必须保证脉冲的高低电平要求的宽度。 MCU Description
SCM is also known as micro-controller (Microcontroller Unit), commonly used letters of the acronym MCU MCU that it was first used in industrial control. Only a single chip by the CPU chip developed from a dedicated processor. The first design is by a large number of peripherals and CPU on a chip in the computer system, smaller, more easily integrated into a complex and demanding on the volume control device which. INTEL's Z80 is the first designed in accordance with this idea processor, then on the development of microcontroller
and dedicated processors have parted ways. Are 8-bit microcontroller early or 4 bits. One of the most successful is the INTEL 8031, for a simple, reliable and good performance was a lot of praise. Then developed in 8031 out of MCS51 MCU Systems. SCM systems based on this system until now is still widely used. With the increased requirements of industrial control field, began a 16-bit microcontroller, because the cost is not satisfactory but have not been very widely used. After 90 years with the great development of consumer electronics, microcontroller technology has been a huge increase. With INTEL i960 series, especially the later series of widely used ARM, 32-bit microcontroller quickly replace high-end 16-bit MCU status and enter the mainstream market. The traditional 8-bit microcontroller performance have been the rapid increase capacity increase compared to 80 the number of times. Currently, high-end 32-bit microcontroller clocked over 300MHz, the performance catching the l mid-90's dedicated processor, while the average model prices fall to one U.S. dollars, the most high-end [1] model only 10 dollars. Modern SCM systems are no longer only in the development and use of bare metal environment, a large number of proprietary embedded operating system is widely used in the full range of SCM. The handheld computers and cell phones as the core processing of high-end microcontroller can even use a dedicated Windows and Linux operating systems.
SCM is more suitable than the specific processor used in embedded systems, so it was up to the application. In fact the number of SCM is the world's largest computer. Modern human life used in almost every piece of electronic and mechanical products will be integrated single chip. Phone, telephone,
calculator, home appliances, electronic toys, handheld computers and computer accessories such as a mouse with a 1-2 in both the Department of SCM. Personal computer will have a large number of SCM in the work. General car with more than 40 SCM, complex industrial control systems may even have hundreds of SCM in the same time work! SCM is not only far exceeds the number of PC and other computing the sum, or even more than the number of human beings SCM is a major piece of computer components are integrated into the chip micro-computer. It is a multi-interface and counting on the micro-controller integration and intelligence products are widely used in industrial automation. And MCS-51 microcontroller is a typical and representative. Single chip, also known as single-chip microcontroller, it is not complete a certain logic chips, but to a computer system integrated into a chip. Equivalent to a micro-computer, and computer than just the lack of a microcontroller I / O devices. General talk: a chip becomes a computer. Its small size, light weight, cheap, for the study, application and development of facilities provided. At the same time, learning to use the MCU is to understand the principle and structure of the computer the best choice.
SCM and the computer functions internally with similar modules, such as CPU, memory, parallel bus, the same effect as well, and hard disk memory devices, and different is its performance of these components were relatively weak many of our home computer, but the price is low, usually not more than 10 yuan you can do with it ...... some control for a class is not very complicated electrical work is enough of. We are using automatic drum washing machine,
smoke hood, VCD and so on appliances which could see its shadow! ...... It is primarily as a control section of the core components
It is an online real-time control computer, control-line is that the scene is needed is a stronger anti-jamming ability, low cost, and this is, and off-line computer (such as home PC), the main difference. MCU is through running, and can be modified. Through different procedures to achieve different functions, in particular special unique features, this is another device much effort needs to be done; some great efforts are very difficult to do. Not very complex functions if the 50's with the United States developed 74 series, or the 60's CD4000 series of these pure hardware buttoned, then the circuit must be a large PCB board! But if the United States if the 70's with a series of successful SCM market, the result will be a drastic change! Just because you are prepared by microcomputer programs can achieve high intelligence, high efficiency and high reliability! As the microcontroller on the cost-sensitive, so now the dominant software or the lowest level assembly language, which is the lowest level in addition to more than binary machine code language, and as so low why is the use? Many high-level language has reached the level of visual programming why is not it? The reason is simply that there is no home computer as a single chip CPU, not as hard as a mass storage device. A visualization of small high-level language program which even if only one button, will reach tens of K of size! For the home PC's hard drive in terms of nothing, but in terms of the MCU is not acceptable. SCM in the utilization of hardware resources to be very high for the job so although the original is still in the compilation of a lot of use. The same token, if
the giant computer operating system and applications run up to get home PC, home PC, also can not afford to. Can be said that the twentieth century across the three "power" era, that is, the age of electricity, the electronic age and has entered into the computer age. However, this computer usually refers to the personal computer, referred to as PC. It consists of the host, keyboard, monitor and other components. Another type of computer, most people do not know how. This computer is to give all kinds of intelligent machines single chip (also known as micro-controller). As the name suggests, this computer system took only a minimal integrated circuit, can be a simple operation and control. Because it is small, usually hidden in the charged mechanical "stomach" in. It is in the device, like the human brain plays a role, it goes wrong, and the whole plant was paralyzed. Now, this microcontroller has a very broad field of use, such as smart meters, real-time industrial control, communications equipment, navigation systems, and household appliances. Once all kinds of products were using SCM, can serve to upgrade the effectiveness of products, often in the product name preceded by the adjective - "intelligent," such as intelligent washing machines. Now some technical personnel of factories or other amateur electronics developers to engage in out of certain products, not the circuit is too complicated, that function is too simple and can easily be copied. The reason may be stuck in the product did l not use a microcontroller or other programmable logic device. SCM was born in the late 20th century, 70, experienced SCM, MCU, SoC three stages.
1.SCM the single chip microcomputer (Single Chip Microcomputer) stage,
mainly seeking the best of the best single form of embedded systems architecture. "Innovation model" success, laying the SCM and general computer completely different path of development. In the open road of independent development of embedded systems, Intel Corporation contributed. 2.MCU the micro-controller (Micro Controller Unit) stage, the main direction of technology development: expanding to meet the embedded applications, the target system requirements for the various peripheral circuits and interface circuits, highlight the object of intelligent control. It involves the areas associated with the object system, therefore, the development of MCU's responsibility inevitably falls on electrical, electronics manufacturers. From this point of view, Intel faded MCU development has its objective factors. In the development of MCU, the most famous manufacturers as the number of Philips Corporation.
Philips Company in embedded applications, its great advantage, the MCS-51 single-chip micro-computer from the rapid development of the micro-controller. Therefore, when we look back at the path of development of embedded systems, do not forget Intel and Philips in History. Embedded Systems Embedded system microcontroller is an independent development path, the MCU important factor in the development stage, is seeking applications to maximize the solution on the chip; Therefore, the development of dedicated single chip SoC trend of the natural form. As the microelectronics, IC design, EDA tools development, application system based on MCU SoC design have greater development. Therefore, the understanding of the microcontroller chip microcomputer can be, extended to the single-chip micro-controller applications.
SCM now permeate all areas of our lives, which is almost difficult to find
traces of the field without SCM. Missile navigation equipment, aircraft, all types of instrument control, computer network communications and data transmission, industrial automation, real-time process control and data processing, extensive use of various smart IC card, civilian luxury car security system, video recorder, camera, fully automatic washing machine control, and program-controlled toys, electronic pet, etc., which are inseparable from the microcontroller. Not to mention the area of robot control, intelligent instruments, medical equipment was. Therefore, the MCU learning, development and application of the large number of computer applications and intelligent control of the scientists, engineers. SCM is widely used in instruments and meters, household appliances, medical equipment, aerospace, specialized equipment, intelligent management and process control fields, roughly divided into the following several areas:
1. In the application of Intelligent Instruments SCM has a small size, low power consumption, controlling function, expansion flexibility, the advantages of miniaturization and ease of use, widely used instrument, combining different types of sensors can be realized Zhuru voltage, power, frequency, humidity, temperature, flow, speed, thickness, angle, length, hardness, elemental, physical pressure measurement. SCM makes use of digital instruments, intelligence, miniaturization, and functionality than electronic or digital circuits more powerful. Such as precision measuring equipment (power meter, oscilloscope, various analytical instrument). 2. In the industrial control application With the MCU can constitute a variety of control systems, data acquisition system. Such as factory assembly line of intelligent control
3. In Household Appliances Can be said that the appliances are basically using SCM, praise from the electric rice, washing machines, refrigerators, air conditioners, color TV, and other audio video equipment, to the electronic weighing equipment, varied, and omnipresent. 4. In the field of computer networks and communications applications MCU general with modern communication interface, can be easy with the computer data communication, networking and communications in computer applications between devices had excellent material conditions, are basically all communication equipment to achieve a controlled by MCU from mobile phone, telephone, mini-program-controlled switchboards, building automated communications call system, train radio communication, to the daily work can be seen everywhere in the mobile phones, trunked mobile radio, walkie-talkies, etc.. 5. Microcomputer in the field of medical device applications SCM in the use of medical devices is also quite extensive, such as medical respirator, the various analyzers, monitors, ultrasound diagnostic equipment and hospital beds, etc. call system. 6. In a variety of major appliances in the modular applications Designed to achieve some special single specific function to be modular in a variety of circuit applications, without requiring the use of personnel to understand its internal structure. If music integrated single chip, seemingly simple function, miniature electronic chip in the net (the principle is different from the tape machine), you need a computer similar to the principle of the complex. Such as: music signal to digital form stored in l memory (like ROM), read by the microcontroller, analog music into electrical signals (similar to the sound card). In large circuits, modular
applications that greatly reduce the volume, simplifies the circuit and reduce the damage, error rate, but also easy to replace. 7. Microcontroller in the application field of automotive equipment SCM in automotive electronics is widely used, such as a vehicle engine controller, CAN bus-based Intelligent Electronic Control Engine, GPS navigation system, abs anti-lock braking system, brake system, etc..
Electronic systems are used for handing information in the most general sense; this information may be telephone conversation, instrument read or a company’s accounts, but in each case the same main type of operation are involved: the processing, storage and transmission of information. in conventional electronic design these operations are combined at the function level; for example a counter, whether electronic or mechanical, stores the current and increments it by one as required. A system such as an electronic clock which employs counters has its storage and processing capabilities spread throughout the system because each counter is able to store and process numbers. Present day microprocessor based systems depart from this conventional approach by separating the three functions of processing, storage, and transmission into different section of the system. This partitioning into three main functions was devised by Von Neumann during the 1940s, and was not conceived especially for microcomputers. Almost every computer ever made has been designed with this structure, and despite the enormous range in their physical forms, they have all been of essentially the same basic design.
In a microprocessor based system the processing will be performed in the
microprocessor itself. The storage will be by means of memory circuits and the communication of information into and out of the system will be by means of special input/output(I/O) circuits. It would be impossible to identify a particular piece of hardware which performed the counting in a microprocessor based clock because the time would be stored in the memory and incremented at regular intervals but the microprocessor. However, the software which defined the system’s behavior would contain sections that performed as counters. The apparently rather abstract approach to the architecture of the microprocessor and its associated circuits allows it to be very flexible in use, since the system is defined almost entirely software. The design process is largely one of software engineering, and the similar problems of construction and maintenance which occur in conventional engineering are encountered when producing software.
The figure1.1 illustrates how these three sections within a microcomputer are connected in terms of the communication of information within the machine. The system is controlled by the microprocessor which supervises the transfer of information between itself and the memory and input/output sections. The external connections relate to the rest (that is, the non-computer part) of the engineering system.
Fig.1.1 Three Sections of a Typical Microcomputer
Although only one storage section has been shown in the diagram, in practice two distinct types of memory RAM and ROM are used. In each case, the
word ‘memory’ is rather inappropriate since a computers memory is more like a filing cabinet in concept; information is stored in a set of numbered ‘boxes’ and it is referenced by the serial number of the ‘box’ in question. Microcomputers use RAM (Random Access Memory) into which data can be written and from which data can be read again when needed. This data can be read back from the memory in any sequence desired, and not necessarily the same order in which it was written, hence the expression ‘random’ access memory. Another type of ROM (Read Only Memory) is used to hold fixed patterns of information which cannot be affected by the microprocessor; these patterns are not lost when power is removed and are normally used to hold the program which defines the behavior of a microprocessor based system. ROMs can be read like RAMs, but unlike RAMs they cannot be used to store variable information. Some ROMs have their data patterns put in during manufacture, while others are programmable by the user by means of special equipment and are called programmable ROMs. The widely used programmable ROMs are erasable by means of special ultraviolet lamps and are referred to as EPROMs, short for Erasable Programmable Read Only Memories. Other new types of device can be erased electrically without the need for ultraviolet light, which are called Electrically Erasable Programmable Read Only Memories, EEPROMs.
The microprocessor processes data under the control of the program, controlling the flow of information to and from memory and input/output devices. Some input/output devices are general-purpose types while others are designed for controlling special hardware such as disc drives or controlling information transmission to other computers. Most types of I/O devices are programmable to
some extent, allowing different modes of operation, while some actually contain special-purpose microprocessors to permit quite complex operations to be carried out without directly involving the main microprocessor. The microprocessor processes data under the control of the program, controlling the flow of information to and from memory and input/output devices. Some input/output devices are general-purpose types while others are designed for controlling special hardware such as disc drives or controlling information transmission to other computers. Most types of I/O devices are programmable to some extent, allowing different modes of operation, while some actually contain special-purpose microprocessors to permit quite complex operations to be carried out without directly involving the main microprocessor. The microprocessor , memory and input/output circuit may all be contained on the same integrated circuit provided that the application does not require too much program or data storage . This is usually the case in low-cost application such as the controllers used in microwave ovens and automatic washing machines . The use of single package allows considerable cost savings to e made when articles are manufactured in large quantities . As technology develops , more and more powerful processors and larger and larger amounts of memory are being incorporated into single chip microcomputers with resulting saving in assembly costs in the final products . For the foreseeable future , however , it will continue to be necessary to interconnect a number of integrated circuits to make a microcomputer whenever larger amounts of storage or input/output are required.
Another major engineering application of microcomputers is in process
control. Here the presence of the microcomputer is usually more apparent to the user because provision is normally made for programming the microcomputer for the particular application. In process control applications the benefits lf fitting the entire system on to single chip are usually outweighed by the high design cost involved, because this sort lf equipment is produced in smaller quantities. Moreover, process controllers are usually more complicated so that it is more difficult to make them as single integrated circuits. Two approaches are possible; the controller can be implemented as a general-purpose microcomputer rather like a more robust version lf a hobby computer, or as a ‘packaged’ system, signed for replacing controllers based on older technologies such as electromagnetic relays. In the former case the system would probably be programmed in conventional programming languages such as the ones to9 be introduced later, while in the other case a special-purpose language might be used, for example one which allowed the function of the controller to be described in terms of relay interconnections, In either case programs can be stored in RAM, which allows them to be altered to suit changes in application, but this makes the overall system vulnerable to loss lf power unless batteries are used to ensure continuity of supply. Alternatively programs can be stored in ROM, in which case they virtually become part of the electronic ‘hardware’ and are often referred to as firmware. More sophisticated process controllers require minicomputers for their implementation, although the use lf large scale integrated circuits ‘the distinction between mini and microcomputers, Products and process controllers of various kinds represent the majority of present-day microcomputer applications, the exact figures depending on one’s interpretation
of the word ‘product’. Virtually all engineering and scientific uses of microcomputers can be assigned to one or other of these categories. But in the system we most study Pressure and Pressure Transmitters. Pressure arises when a force is applied over an area. Provided the force is one Newton and uniformly over the area of one square meters, the pressure has been designated one Pascal. Pressure is a universal processing condition. It is also a condition of life on the planet: we live at the bottom of an atmospheric ocean that extends upward for many miles. This mass of air has weight, and this weight pressing downward causes atmospheric pressure. Water, a fundamental necessity of life, is supplied to most of us under pressure. In the typical process plant, pressure influences boiling point temperatures, condensing point temperatures, process efficiency, costs, and other important factors. The measurement and control of pressure or lack of it-vacuum-in the typical process plant is critical.
The working instruments in the plant usually include simple pressure gauges, precision recorders and indicators, and pneumatic and electronic pressure transmitters. A pressure transmitter makes a pressure measurement and generates either a pneumatic or electrical signal output that is proportional to the pressure being sensed. In the process plant, it is impractical to locate the control instruments out in the place near the process. It is also true that most measurements are not easily transmitted from some remote location. Pressure measurement is an exception, but if a high pressure of some dangerous chemical is to be indicated or recorded several hundred feet from the point of measurement, a hazard may be from the pressure or from the chemical carried.
To eliminate this problem, a signal transmission system was developed. This system is usually either pneumatic or electrical. And control instruments in one location. This makes it practical for a minimum number of operators to run the plant efficiently.
When a pneumatic transmission system is employed, the measurement signal is converted into pneumatic signal by the transmitter scaled from 0 to 100 percent of the measurement value. This transmitter is mounted close to the point of measurement in the process. The transmitter output-air pressure for a pneumatic transmitter-is piped to the recording or control instrument. The standard output range for a pneumatic transmitter is 20 to 100kPa, which is almost universally used. When an electronic pressure transmitter is used, the pressure is converted to electrical signal that may be current or voltage. Its standard range is from 4 to 20mA DC for current signal or from 1 to 5V DC for voltage signal. Nowadays, another type of electrical signal, which is becoming common, is the digital or discrete signal. The use of instruments and control systems based on computer or forcing increased use of this type of signal. Sometimes it is important for analysis to obtain the parameters that describe the sensor/transmitter behavior. The gain is fairly simple to obtain once the span is known. Consider an electronic pressure transmitter with a range of 0~600kPa.The gain is
defined as the change in output divided by the change in input. In this case,
the output is electrical signal (4~20mA DC) and the input is process pressure (0~600kPa). Thus the gain. Beside we must measure Temperature Temperature measurement is important in industrial control, as direct indications of system or product state and as indirect indications of such factors as reaction rates, energy flow, turbine efficiency, and lubricant quality. Present temperature scales have been in use for about 200 years, the earliest instruments were based on the thermal expansion of gases and liquids. Such filled systems are still employed, although many other types of instruments are available. Representative temperature sensors include: filled thermal systems, liquid-in-glass thermometers, thermocouples, resistance temperature detectors, thermostats, bimetallic devices, optical and radiation pyrometers and temperature-sensitive paints.
Advantages of electrical systems include high accuracy and sensitivity, practicality of switching or scanning several measurements points, larger distances possible between measuring elements and controllers, replacement of components(rather than complete system), fast response, and ability to measure higher temperature. Among the electrical temperature sensors, thermocouples and resistance temperature detectors are most widely used.
Description The AT89C51 is a low-power, high-performance CMOS 8-bit microcomputer with 4K bytes of Flash programmable and erasable read only memory (PEROM). The device is manufactured using Atmel’s high-density nonvolatile memory technology and is compatible with the industry-standard MCS-51 instruction set and pinout. The on-chip Flash allows the program memory to be reprogrammed
in-system or by a conventional nonvolatile memory programmer. By combining a versatile 8-bit CPU with Flash on a monolithic chip, the Atmel AT89C51 is a powerful microcomputer which provides a highly-flexible and cost-effective solution to many embedded control applications.
Function characteristic The AT89C51 provides the following standard features: 4K bytes of Flash, 128 bytes of RAM, 32 I/O lines, two 16-bit timer/counters, a five vector two-level interrupt architecture, a full duplex serial port, on-chip oscillator and clock circuitry. In addition, the AT89C51 is designed with static logic for operation down to zero frequency and supports two software selectable power saving modes. The Idle Mode stops the CPU while allowing the RAM, timer/counters, serial port and interrupt system to continue functioning. The Power-down Mode saves the RAM contents but freezes the oscillator disabling all other chip functions until the next hardware reset. Pin Description
VCC: Supply voltage.
GND: Ground.
Port 0: Port 0 is an 8-bit open-drain bi-directional I/O port. As an output port, each pin can sink eight TTL inputs. When 1s are written to port 0 pins, the pins can be used as highimpedance inputs.Port 0 may also be configured to be the multiplexed loworder address/data bus during accesses to external program and data memory. In this mode P0 has internal pullups.Port 0 also receives the code bytes during Flash programming,and outputs the code bytes during
programverification. External pullups are required during programverification.
Port 1
Port 1 is an 8-bit bi-directional I/O port with internal pullups.The Port 1 output buffers can sink/source four TTL inputs.When 1s are written to Port 1 pins they are pulled high by the internal pullups and can be used as inputs. As inputs,Port 1 pins that are externally being pulled low will source current (IIL) because of the internal pullups.Port 1 also receives the low-order address bytes during Flash programming and verification.
Port 2
Port 2 is an 8-bit bi-directional I/O port with internal pullups.The Port 2 output buffers can sink/source four TTL inputs.When 1s are written to Port 2 pins they are pulled high by the internal pullups and can be used as inputs. As inputs,Port 2 pins that are externally being pulled low will source current, because of the internal pullups.Port 2 emits the high-order address byte during fetches from external program memory and during accesses to external data memory that use 16-bit addresses. In this application, it uses strong internal pullupswhen emitting 1s. During accesses to external data memory that use 8-bit addresses, Port 2 emits the contents of the P2 Special Function Register.Port 2 also receives the high-order address bits and some control signals during Flash programming and verification.
Port 3
Port 3 is an 8-bit bi-directional I/O port with internal pullups.The Port 3 output buffers can sink/source four TTL inputs.When 1s are written to Port 3 pins they are pulled high by the internal pullups and can be used as inputs. As
inputs,Port 3 pins that are externally being pulled low will source current (IIL) because of the pullups.Port 3 also serves the functions of various special features of the AT89C51 as listed below: Port 3 also receives some control signals for Flash programming and verification.
RST Reset input. A high on this pin for two machine cycles while the oscillator is running resets the device.
ALE/PROG Address Latch Enable output pulse for latching the low byte of the address during accesses to external memory. This pin is also the program pulse input (PROG) during Flash programming.In normal operation ALE is emitted at a constant rate of 1/6 the oscillator frequency, and may be used for external timing or clocking purposes. Note, however, that one ALE pulse is skipped during each access to external Data Memory. If desired, ALE operation can be disabled by setting bit 0 of SFR location 8EH. With the bit set, ALE is active only during a MOVX or MOVC instruction. Otherwise, the pin is weakly pulled high. Setting the ALE-disable bit has no effect if the microcontroller is in external execution mode.
PSEN Program Store Enable is the read strobe to external program memory.When the AT89C51 is executing code from external program memory, PSEN is activated twice each machine cycle, except that two PSEN activations are skipped during each access to external data memory.
EA/VPP
External Access Enable. EA must be strapped to GND in order to enable the
device to fetch code from external program memory locations starting at 0000H up to FFFFH. Note, however, that if lock bit 1 is programmed, EA will be internally latched on reset.EA should be strapped to VCC for internal program executions.This pin also receives the 12-volt programming enable voltage(VPP) during Flash programming, for parts that require12-volt VPP.
XTAL1
Input to the inverting oscillator amplifier and input to the internal clock operating circuit.
XTAL2
Output from the inverting oscillator amplifier.
Oscillator Characteristics
XTAL1 and XTAL2 are the input and output, respectively,of an inverting amplifier which can be configured for use as an on-chip oscillator, as shown in Figure 1.Either a quartz crystal or ceramic resonator may be used. To drive the device from an external clock source, XTAL2 should be left unconnected while XTAL1 is driven as shown in Figure 2.There are no requirements on the duty cycle of the external clock signal, since the input to the internal clocking circuitry is through a divide-by-two flip-flop, but minimum and maximum voltage high and low time specifications must be observed.