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Find hex-code values for microcontroller’s ADC voltages

This Design Idea is for low-end, eight-pin, flash-memory, 8-bit microcontrollers, such as the MC68HC908QT4A from Freescale (www.freescale.com), but it would also apply to any 8-bit microcontroller that uses the ADC feature. In a nutshell, the ADC converts an inputanalog- 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 inputanalog- voltage level from its ADC ports in a range from $00 at V_SS to $FF at V_DD. Based on those hex-code values, there is a total of 256 ticks. The input voltages between V_SS and V_DD represent a straight-line linear conversion. In other words, the higher the input voltage, the higher thehex-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 manufacturer do not yield satisfactoryanswers.

However, this Design Idea presents a solution to the problem. Given the microcontroller’s power operatingvoltage source, VDD, use the following simple formula to obtain the hexcode value that corresponds to an identified input-analog-voltage level: V_INV_IN/(V_DD/255)=result value= hex code. 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 hexcode value for a measured input-analog- voltage level of 1.6V when using a known microcontroller’s V_DD of 5V:1.6V/(5V/255)=81.6=82, or $52.