【正文】 8位中央處理器和 Flash 存儲單元， AT89C52單片機(jī)在電子行業(yè)中有著廣泛的應(yīng)用。 復(fù)位電路通常分為兩種：上電復(fù)位（圖 3）和手動復(fù)位（圖 4）。若晶振為 12MHZ，則定時器計數(shù)頻率為1MHZ，計數(shù)的脈沖周期為 1us。外部事件計數(shù)時最高計數(shù)頻率為晶振頻率的二十四分之一，外部輸入脈沖高電平和低電平時間必須在一個機(jī)器周期以上。 8 輸入的 220V 電壓太大，對元件要求大，決定先降壓為 9V，再經(jīng)過整流，濾波，穩(wěn)壓后得到 5V 穩(wěn)定電壓。 整流電路 單相橋式整流電路使用的整流器件較多，但其實(shí)現(xiàn)了全波整流電路，它與半波整流電路相比，在相同的變壓器副邊電壓下，對二極管的參數(shù)要求一樣，還具有輸出電壓高、變壓器利用率高、脈動小等優(yōu)點(diǎn)。 利用電容元件儲能的特性，將整流后輸出的電壓的能量儲存起來，然后緩慢的釋放給負(fù)載。電容濾波電路及輸出波形如圖 7，圖 8所示： 圖 9 電容濾波電路 圖 10 電容濾波輸出波形 穩(wěn)壓電路 使輸出的直流電壓穩(wěn)定，不隨交流電網(wǎng)電壓和負(fù)載的變化而變化。在整形之前由于不清楚被測信號的強(qiáng)弱的情況。 分頻電路用于擴(kuò)展單片機(jī)頻率測量范圍，并實(shí)現(xiàn)單片機(jī)頻率測量使用統(tǒng)一信號，可使單片機(jī)測頻更易于實(shí)現(xiàn)， 而且也降低了系統(tǒng)的測頻誤差。 分頻器 74LS161 芯片 74LS161 是常用的四位二進(jìn)制可預(yù)置的同步加法計數(shù)器。 74LS161還有一個進(jìn)位輸出端 CO，其邏輯關(guān)系是 CO= Q0 CET。只有在選通端 STROBE 為低電平時才可選擇數(shù)據(jù)。頻率值顯示電路采用四位共陽極數(shù)碼管動態(tài)顯示頻率計被測數(shù)值，量程轉(zhuǎn)換指示電路由紅、黃、綠三個 LED 分別指示 Hz、 KHz 及 MHz頻率單位，使讀數(shù)簡單可觀。 圖 14 兩種數(shù)碼管內(nèi)部原理圖 因?yàn)檫@里用的是共陽極數(shù)碼管，它的公共端為高電平，因此 要讓其中各段（ DP,A,B,C,D,E,F,G)的發(fā)光二極管發(fā)光，只要使它另一端置為低電平，即置 0。 程序入口， main()函數(shù)框圖 : 圖 18 主函數(shù)框圖 初始化模塊 初始化模塊初始化分頻器、量程檔位、 LED 顯示、工作寄存器、中斷控制和定時／計數(shù)器的工作方式等。由于所有 4 位數(shù)碼管的8 根段選線由單片機(jī)的 P2口控制 ,因此 ,在每一瞬間 4位數(shù)碼管會顯示相同的字符 ,要想每位顯示不同的字符就必須采用掃描方法輪流點(diǎn)亮各位數(shù)碼管 ,即在每一瞬間只點(diǎn)亮某一位顯示字符 ,由 ,在此瞬間 ,段選控制口 P2輸出相應(yīng)字符。 顯示模塊框圖： 圖 22 顯示子程序流程圖 延時模塊 延時模塊框圖： 圖 23 延時程序流程圖 18 五、數(shù)字頻率計仿真 電源模塊仿真 此模塊用 仿真軟件 MULTISIM 仿真 圖 24 電源模塊仿真圖 其中 9/220=，因此變壓器 耦合系數(shù) 設(shè)置為 。 點(diǎn)擊按鈕 后，編譯連接，若代碼無問題，之后會生成 文件，如下圖： 圖 32 （ .hex）文件圖 使用軟件 Proteus 仿真頻率計 因?yàn)殡娫茨K和放大整形模塊都用 仿真軟件 MULTISIM 仿真過，所以這里不再對其進(jìn)行仿真，信號直接輸入，穩(wěn)壓直流電源也直接輸入。在運(yùn)行之前，要設(shè)置信號源和大小， 因?yàn)樵O(shè)計要求測量范圍 10HZ— 2MHZ，所以將信號源的大小設(shè)置在 10HZ— 2MHZ 的范圍內(nèi)。 的信號波 圖 42 運(yùn)行結(jié)果如下： 圖 43 頻率計測量 數(shù)碼管顯示 ，綠色 LED 亮，表示數(shù)碼管顯示值的單位是 MHZ，誤差為 因此讀數(shù)為 。t agree with the disciplinary action your employer has taken against you your employer dismisses you and you think that you have been dismissed unfairly. For more informu, take advice from one of the anisations listed under Further help. Employment tribunals are less formal than some other courts, but it is still a legal process and you will need to give evidence under an oath or affirmation. Most people find making a claim to an employment tribunal challenging. If you are thinking about making a claim to an employment tribunal, you should get help straight away from one of the anisations listed under Further help. ation about dismissal and unfair dismissal, see Dismissal. You can make a claim to an employment tribunal, even if you haven39。s antiracism taskforce, is in London for the Football Association39。s 21 win. Michel Platini, president of European football39。 May be back in the past, to oneself the paranoid weird belief disillusionment, these days, my mind has been very messy, in my mind constantly. Always feel oneself should go to do something, or write something. Twenty years of life trajectory deeply shallow, suddenly feel something, 28 do it. 一字開頭的年齡已經(jīng)到了尾聲。 The end of our life, and can meet many things really do? 而窮盡我們的一生，又能遇到多少事情是真正地非做不可？ During my childhood, think lucky money and new clothes are necessary for New Year, but as the advance of the age, will be more and more found that those things are optional。t want to give vent to out your inner voice can be in the high school children of the feelings in a period, but was eventually infarction when graduation party in the throat, later again stood on the pitch he has sweat profusely, looked at his thrown a basketball hoops, suddenly found himself has already can39。s time, your ideas are always special to clear. Want, want, line is clear, as if nothing could shake his. Also once seemed to be determined to do something, but more often is he backed out at last. Dislike his cowardice, finally found that there are a lot of love, there are a lot of miss, like shadow really have been doomed. Those who do, just green years oneself give oneself an arm injection, or is a selfrighteous spiritual. 一個人的時候，自己的想法總是特別地清晰。那些曾經(jīng)所謂的非做不可，只是青蔥年華里自己給自己注射的一支強(qiáng)心劑，或者說，是自以為是的精神寄托罷了。 Have declared the end of the encounter that haven39。 instead of letting the world soften us, we let it drive us deeper into ourselves. We try to deflect the hurt and pain by pretending it doesn’t exist, but although we can try this all we want, in the end, we can’t hide from ourselves. We need to learn to open our hearts to the potentials of life and let the world soften us. 生活發(fā)生不幸時，我們常常會關(guān)上心門；世界不僅沒能慰藉我們，反倒使我們更加消沉。 Whenever a painful situation arises in your life, try to embrace it instead of running away or trying to mask the hurt. When the sadness strikes, take a deep breath and lean into it. When we run away from sadness that’s unfolding in our lives, it gets stronger and more real. We take an emotion that’s fleeting and make it a solid event, instead of something that passes through us. 當(dāng)生活中出現(xiàn)痛苦的事情時，別再逃跑或隱藏痛苦，試著擁抱它吧；當(dāng)悲傷來襲時，試著深呼吸，然后直面它。其實(shí)，我們有能力面對這些痛苦的感受，從中領(lǐng)悟到出路。與其一味顧慮重重，不如從局外人的角度看待決策之事。t. Baumgartner still will free fall 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). cause there