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一線式A/D轉換器DS2450的原理與應用

摘要： DS2450是一線式4通道逐次逼近式A/D轉換器，其輸入電壓范圍、轉換精度位數、報警門限電壓可編程；每個通道有各自的存儲器以存儲電壓范圍設置、轉換結果、門限電壓等參數；普通方式下串行通信速率達16.3kbps，超速工作時速率達142kbps，片內16位循環冗余校驗碼生成器可用于檢測通信的正確性；DS2450采用8引腳SOIC小體積封裝形式，既可用單5V電源供電，也可采用寄生電源方式供電。

關鍵詞： 數據轉換 A/D轉換器 A/D 控制字上電復位

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前言

DS2450是一線式4通道逐次逼近式A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" 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title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器" title="A/D轉換器">A/D轉換器，其輸入電壓范圍、轉換精度位數、報警門限電壓可編程；每個通道有各自的存儲器以存儲電壓范圍設置、轉換結果、門限電壓等參數；普通方式下串行通信速率達16.3kbps，超速工作時速率達142kbps，片內16位循環冗余校驗碼生成器可用于檢測通信的正確性；DS2450采用8引腳SOIC小體積封裝形式，既可用單5V電源供電，也可采用寄生電源方式供電。電路正常工作時僅消耗2.5mW功率，不工作時消耗25μW。多個DS2450或其他功能的具有MicroLAN接口的一線式芯片可以并聯，CPU只需一根端口線就能與諸多一線式芯片通信，占用微處理器的端口較少，可節省大量的引線和邏輯電路。

2 DS2450的引腳排列和內部結構

DS2450為8引腳SOIC型封裝，其管腳功能如下：

1腳（VDD）：工作電源接入端
2腳（N.C）：空引腳
3腳（DATA）：串行數據輸入/輸出端
4腳（GND）：接地端
5腳（AIN－A）：A路模擬電壓輸入端
6腳（AIN－B）：B路模擬電壓輸入端
7腳（AIN－C）：C路模擬電壓輸入端
8腳（AIN－D）：D路模擬電壓輸入端

圖1 DS2450的內部結構

DS2450的內部結構如圖1所示。光刻ROM中的64位序列號是出廠前被光刻好的，它可以看作是該DS2450的地址序列碼；64位光刻ROM的排列是：開始8位（20H）是產品類型標號，接著的48位是該DS2450自身的序列號，最后8位是前面56位的循環冗余校驗碼（CRC=X8＋X5＋X4＋1）。光刻ROM的作用是使每一個一線式器件的地址都各不相同，以便實現一根總線上掛接多個一線式電路。

對于一線端口，在ROM功能建立之前，其它功能是無法實現的。總線控制器必須首先通過DATA引腳對DS2450提供7個ROM功能控制命令（8位）之一：（1）讀ROM，命令字[33H]；（2）匹配ROM[55H]；（3）搜索ROM[F0H]；（4）跳過ROM[CCH]；（5）條件搜索ROM[ECH]；（6）超速跳過ROM[3CH]；（7）超速匹配ROM[69H]。其中，超速跳過ROM或超速匹配ROM命令執行后，串行通信速率可高達142kbps。如果多個器件連接在一線上，這些命令對每個器件的64位ROM部分進行操作，并挑選出一個特定的器件。對選中的DS2450，執行下一步的A/D轉換控制命令以及讀寫存儲器的命令，所有命令或數據的讀/寫均從最低位開始。

3 存儲器組織方式

DS2450內部有24個地址毗連的8位存儲器，可將其分成3頁，每頁8字節。DS2450的存儲器組織方式及各位含義如表1所示（高位地址為00H）。

第0頁為A/D轉換結果存儲器，每個通道占2個字節16位，芯片上電復位時該頁清0；其中00H、01H存儲A通道轉換結果，LSBA為最低位，MSBA為最高位；02H、03H存儲B通道轉換結果;04H、05H存儲C通道轉換結果;06H、07H存儲D通道轉換結果，對應位含義與A通道相同。

第1頁為A/D轉換控制與狀態存儲器，08H、09H對應于A通道，其余通道依次類推，各位含義相同。RC3、RC2、RC1、RC0的組合控制A/D轉換的精度位數，0000為16位、0001為1位、……、1111為15位；特別值得注意的是，若控制轉換精度不足16位，則在轉換結果的“低位”補0，湊足16位，因此讀出結果的16位值中,哪些是有效位與RC3、RC2、RC1、RC0的組合有關。對于用作模擬量輸入的通道，輸出使能位OE必須為0，否則模擬輸入不被接受，轉換結果始終全為0，而此時輸出控制位OC可以不必關心。不用作模擬量輸入的通道可以作為漏極開路的數字輸出端，外接上拉電阻器和工作電源，在輸出使能位OE=1時，若輸出控制位OC寫入1，則輸出高電平，OC寫入0，則輸出低電平。IR控制輸入電壓范圍，IR=1時模擬輸入高限為5.10V，IR=0時輸入高限為2.55V。AEH、AEL分別為高、低門限電壓報警允許控制端；AFH、AFL分別為模擬輸入是否超過規定的高門限、低于規定的低門限的狀態指示位，若輸入超限，相應位自動置1。上電復位標志位POR與通道無關，上電復位時4個POR自動置1，說明控制字和門限值等未準備好，該位可用軟件清0。表中為0的位無效，讀出時始終為0且不能寫入1。DS2450上電復位時，默認的控制/狀態數據的低位為08H、高位為8CH，即4個通道均作為模擬輸入通道、8位轉換精度、輸入高限2.55V、允許高低限報警。

第2頁為各通道輸入高/低限報警值存儲器，10H存放A通道低門限8位報警值、11H存放A通道高門限8位報警值，其余通道依次類推，各位含義相同。在判斷是否超限時，只將存儲的門限值與轉換結果的高8位進行比較，然后自動改變AFH、AFL的狀態。上電復位時，高限值自動設置為FFH，低限值自動設置為00H。

4 轉換與讀/寫控制

4．1轉換控制

通過DS2450的DATA端串行送出轉換命令字[3CH]，隨后送出通道選擇字和預置控制字，啟動A/D轉換器進行轉換。DS2450的通道選擇字和預置控制字的各位含義如表2所示。

在通道選擇字中，對應位為1表示該通道參與轉換。同時選擇多個通道時，其轉換順序為A→B→C→D，未選中的通道被跳過。其A/D轉換的時間可用下式近似計算：轉換時間=通道數×轉換精度位數×80μs＋160μs。當所有通道轉換完畢，發讀存儲器命令可以獲得轉換結果和對應的狀態。