Now that you have your whole analog system set up, what do you actually reference to when taking measurements?

Previously I didn't really care about how I was taking measurements because the ADC reading was always good enough - but now when it is actually used to convert into an actual voltage reading, precision and accuracy is incredibly important.

For the EFM8, by default it takes the VIO as the reference. This is great if you have a clean consistent voltage each time, but in this circuit, it is coming off a step-up voltage regulator, it can vary from 3.28 to 3.34 volts. Now that is problematic because the ADC uses this as reference voltage.

So for example if your reference is 10V, and you get 10 counts, then it is 1V/count. But if you set the reference to 9V, then it is 0.9V/count. But since you only get counts in your firmware code, so the same count would report a different voltage.

While trying to determine if I could control the output, I realized that this was a solved problem for EFM8 - just use the precision voltage reference!

It works superbly, but the only problem is that because I don't have it broken out, I need to solder a tiny bodge wire to a QFN pad....

A pretty masterful job if I do say so myself!