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11-04-2007

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.

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—1.6 V, 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 1.6 V when using a known microcontroller’s VDD of 5 V:1.6 V / (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.

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