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.

EDN

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