【正文】 有余輸入至內部時鐘信號要通過一個二分頻 觸發(fā)器，因此對外部時鐘信號的脈寬無任何要求，但必須保證脈沖的高低電平要求的寬度。 振蕩器特性 :XTAL1和 XTAL2分別為反向放大器的輸入和輸出。注意加密方式 1時， /EA將內部鎖定為 RESET；當 /EA端保持高電平時，此間內部程序存儲器。 PSEN：外部程序存儲器的選通信號。如想禁止 ALE的輸出可在 SFR8EH地址上置 0。在 FLASH編程期間，此引腳用于輸入編程脈沖。隨著位設置， ALE為活躍，只有在執(zhí)行 MOVX或 MOVC指令。 進修 /編 地址鎖存使能鎖存在訪問外部存儲器地址的低字節(jié)輸出脈沖。當 P3口寫入“ 1”后，它們被內部上拉為高電平，并用作輸入。 P2口當用于外部程序存儲器或 16位地址外部數(shù)據(jù)存儲器進行存取時， P2口輸出地址的高八位。在 FLASH編 程和校驗時， P1口作為第八位地址接收。 P0能夠用于外部程序數(shù)據(jù)存儲器，它可以被定義為數(shù)據(jù) /地址的第八位。 引腳說明 Vcc：電源電壓。 功能特點 AT89S51內提供了以下標準特性： 4K字節(jié)閃存， 128字節(jié) RAM， 32個 I / O線，兩個 16位定時器 /計數(shù)器，一個五向量兩級中斷結構，一個全雙工串行口，片上振蕩器和時鐘電路。 說明 該 AT89C51是一種低功耗，高性能 CMOS 8位 4K的閃存可編程和可擦除只讀存儲器（ PEROM）字節(jié)的微型計算機?，F(xiàn)在盡管有許多其他類型的儀器在使用，這些填充式系統(tǒng)仍常用于直接的溫度測量。當量程已知，去獲取增益就非常簡單。 其標準范圍對電流來說是 4～ 20mA DC，對電壓信號來說是 1～ 5V DC。變送器安裝在靠近過程中的測量點上。這種系統(tǒng)常?？墒菤鈩踊蛘唠妱拥?。壓力變送器實現(xiàn)壓力測量并產生正比于所傳感壓力的氣動或電信號輸出。水，是生活的必需品，也是在壓力之下提供給我們 中的大多數(shù)人。實際上，計算機的所有工程和科學上的應用都能指定來進行這些種類的某一或某些工作。兩種情況下，序均能存于RAM，這讓程序能按應用情況變化時進行相應的變化，但是這使得總系統(tǒng)易受掉電影響而工作不正常，除非使用電池保證供電連續(xù)性。在過程控制應用中，由于這種設備以較少的數(shù)量生產，將整個系統(tǒng)安裝在單個芯片上所獲取的利益常比不上所涉及的高設計成本。當技術進一步發(fā)展，更強更強的處理器和更大更大數(shù)量的存儲器被包含形成單片微型計算機，結果使最終產品的裝配成本得以節(jié)省。大多數(shù)類型的 I/O裝置在某種程度下可編程，允許不同形式的操作 ，而有些則包含特殊用途微處理器的 I/O裝置不用主微處理器的直接干預，就可實施非常復雜的操作。被廣泛使用的可編程 ROM 可利用特殊紫外線燈察除，并被成為 E PROM，即可察除可編程只讀存儲器的縮寫。這種數(shù)據(jù)能以 任意一種 所希望的次序從存儲器中讀出， 而 不必 按照 寫入時的相同次序 讀出 ，所以有 “隨機 ”存取存儲器。外部的連接 部分 與工程系統(tǒng) 中 的其余部分（即非計算機部分）有關。由于系統(tǒng)幾乎完全由軟件所定義，所以對微處理器結構和其輔助電路這種看起來非常抽象的處理方法使其在應用時非常靈活。 在 以微處理器為基礎的 系統(tǒng)中，處理是由 以微處理器為基礎的 系統(tǒng) 自身 完成的。 一個系統(tǒng)例如 采用計數(shù)器的電子鐘之類的任一系統(tǒng)要使其存儲和處理能力遍布整個系統(tǒng)，因為每個計數(shù)器都能存儲和處理一些 數(shù)字。 information is stored in a set of numbered ?boxes? and it is referenced by the serial number of the ?box? in question. Microputers 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。1 Microputer Systems Electronic systems are used for handing information in the most general sense。 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 generalpurpose types while others are designed for controlling special hardware such as disc drives or controlling information transmission to other puters. Most types of I/O devices are programmable to some extent, allowing different modes of operation, while some actually contain specialpurpose microprocessors to permit quite plex 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 3 information to and from memory and input/output devices. Some input/output devices are generalpurpose types while others are designed for controlling special hardware such as disc drives or controlling information transmission to other puters. Most types of I/O devices are programmable to some extent, allowing different modes of operation, while some actually contain specialpurpose microprocessors to permit quite plex 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 lowcost 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 microputers 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 microputer whenever larger amounts of storage or input/output are required. Another major engineering a