I am using a potentiometer with 5% tolerance as a voltage divider in order to send voltage signals to a microcontroller. The mcu reads the voltage and converts it to digital numbers which is then converted to time. The problem that I am having is that when the system resets, the clock changes its reading, I don't really know why this is happening but it happens often.
Using a POT for ADC
potentiometer
Related Solutions
Here is snip showing a "G" curve from this Panasonic document (warning - opens pdf)
A potentiometer works in the same way as a simple two resistor potential divider, so here on the left is the equivalent of the schematic on the right showing a potentiometer. The middle output being the wiper of the potentiometer. On the physical potentiometer itself the middle pin is usually the wiper but I would recommend that you check that with a data sheet for the potentiometer.
I hope that this clears up the connections and just for extra knowledge here is a formula which will work to calculate the output voltage from the resistance of the two resistors either side of the output:
$$\mathrm{Voltage\space Out} = \mathrm{Voltage\space In} \times \left(\frac{R_2}{R_1+R_2}\right)$$
simulate this circuit – Schematic created using CircuitLab
Best Answer
I'd say you have a precision error. The microcontroller reads the voltage from the potentiometer with a reference to something, usually Vcc but it can also be referencing ground, or some other internal voltage.
If your voltage changes, the value converted by the ADC will change as well. Voltage changes can come about from many things, including but not limited to ambient temperature changes, power supply load, component temperatures, etc.
If everything is fairly consistent, you may just have a rounding error. For example prior to reset you might get an arbitrary value of 10.4 (for example), but after reset or a second ADC read you get 10.5. The latter would be rounded up to 11 if using integer math.
You might want to modify your code to have fewer or broader "steps" to convert to time, so that input values do not necessarily have to be as precise.