To make a long story short, my program gathers a bunch of ADC values and then puts them into a large string and outputs them with Particle.publish. One day, I changed the program to use Serial.println and now my ADC values fluctuate +/- 10% per minute whereas before they only fluctuated 1%. Is there something wrong with the UART over USB, or should I look really hard at my program again?
Strange, have you measured the values with another tool?
I would expect the data output via serial to be quicker and thus have fresher data and the update frequency could be higher also.
In my experience with UART over USB if there is an issue you get garbage data, if the data is arriving intact than its probably not a hardware issue but a code issue.
If you are happy to share the code it might help others help you find a solution.
Question: If you revert back to the previous code does the behavior go back to fluctuating by only 1%? In other words, is this behavior reproducible?
I believe it was my fault–a circuit problem. I received the same results when using Particle.publish instead of Serial.println