找出微處理器ADC電壓的十六進(jìn)制編碼值

這種設(shè)計方案針對低檔八管腳flash存儲的8位微處理器，例如Freescale的MC68HC908QT4A，但是它也同樣適用于任何一款擁有ADC模塊的8位微處理器。在芯片內(nèi)，ADC轉(zhuǎn)換輸入的模擬電壓成數(shù)字信號格式。數(shù)字信號格式為8位的十六進(jìn)制編碼值，如$00。微處理器從ADC端口采集輸入模擬信號范圍從VSS對應(yīng)的 $00到VDD對應(yīng)的$FF?；谶@些十六進(jìn)制編碼，在VSS和VDD之間的輸入電壓呈現(xiàn)出總共256個電平的線性變換。換句話說，輸入電壓越高，十六進(jìn)制編碼值就越大。

　　最困難的地方在于寫匯編代碼實(shí)現(xiàn)算法的程序員必須知道不同輸入模擬電平（例如1.6V）對應(yīng)什么十六進(jìn)制編碼值。涉及到微處理器規(guī)格，甚至聯(lián)系生產(chǎn)廠商也不能給出滿意的答案。

　　然而，本文提出解決這個問題的方案。假定微處理器工作電源電壓為VDD，應(yīng)用下面這個簡單公式得到十六進(jìn)制編碼值：VIN/(VDD/255)=result value=hex code（見編者按）。注意，在更高的十六進(jìn)制編碼轉(zhuǎn)換精度之前，必須保證轉(zhuǎn)換出的十六進(jìn)制值能夠覆蓋整個模擬信號范圍。下面的例子計算表明了使用已知電源電壓5V的微處理器時，測量輸入模擬信號電壓為1.6V的十六進(jìn)制編碼值：1.6V/(5V/255)=81.6=82，或者$52。

　　編者按：

　　作者還提供了兩個附件，一個查詢表（PDF格式）和一個Excel格式的表格，讀者可以對照這些附件與公式相互映證。最后，作者還在“反饋環(huán)”這篇文章為本篇文章提供了一些注釋。

英文原文：

　　Find hex-code values for microcontroller’s ADC voltages

　　Ease 8-bit microcontroller programming with a simple formula to calculate hex values of ADC output.

　　Harry Gibbens Jr, Deafworks, Provo, UT; Edited by Charles H Small and Brad Thompson -- EDN, 4/12/2007

　　This Design Idea is for low-end, eight-pin, flash-memory, 8-bit microcontrollers, such as the MC68HC908QT4A from Freescale, but it would apply to any 8-bit microcontrollers that use the ADC feature. In a nutshell, the ADC converts an input-analog-voltage level to a digital-signal format. The digital-signal format has an 8-bit hex-code value, such as $00. The microcontroller “sees” the input-analog-voltage level from its ADC ports ranging from $00 at VSS to $FF at VDD. Based on those hex-code values, there are a total of 256 ticks. The input voltages between VSS and VDD represent a straight-line linear conversion. In other words, the higher the input voltage, the higher the hex-code value.

　　The difficulty is that a programmer who needs to write assembly code for a programming algorithm must know what the hex-code value is for a different input-analog-voltage level—1.6V, for example. Referring to the microcontroller’s specs and even contacting its manufacturers do not yield satisfactory answers.

　　However, this Design Idea presents a solution to the problem. Given the microcontroller’s power operating-voltage source, VDD, use the following simple formula to obtain the hex-code value corresponding to an identified input-analog-voltage level: VIN/(VDD/255)=result value=hex code (see Editor's note). Note that you must round off the result value to a whole number before converting to a hex-code value for better accuracy. The following sample calculation finds the hex-code value for a measured input-analog-voltage level of 1.6V when using a known microcontroller’s VDD of 5V:1.6V/(5V/255)=81.6=82, or $52

　　Editor's Note:

　　Correction and addition (4/27/2007): Due to an editing error, we originally printed the formula as VIN×VIN/(VDD/255). In addition, the author has provided two attachments, a lookup table (PDF) and an Excel spreadsheet that allows you to interact with the formula. Finally, the author has posted some comments in the Feedback Loop section for this article.