I’m using an Arduino Micro connected to an Arduino TFT display to build a car computer that shows clock, ambient temperature and various other things. Attached you will find the schematics. Most important thing for now is that ACC (coming from the car) is connected to A0 via a voltage divider. Additionally, the signal is smoothed using a 6.8uF capacitor. For testing purposes, ACC and VCC are connected to a lab power supply such that the Arduino can measure its own Vin voltage at A0.
For some reason, the measured value fluctuates badly by about .7V, respectively, .2V directly at PIN A0. I do not have an ocilloscope available, however, my multimeter shows a very stable voltage at A0. Now the strange things: If I decrease the lab power supply voltage to about 3 to 4V, such that there is just enough power to drive the logics and the TFT starts dimming, the values become stable. If I then increase the supply voltage again, everthing stays stable and there is no fluctuation at all.
I then took another older Arduino Micro from another project, flashed it with exactly the same code and replaced the current one. Now, there are no fluctuations from the beginning. But what I wondered about: From powering on, the older Micro takes about 11s until the TFT startes showing values. The new Micro takes only about 3s.
Both boards look very similar, both being of R3. However, one of the boards is more blueish (the older) and the new one is more greenish. Is is possible that the new board is broken? Or is one of the boards possibly not original? Or is the newer board just equipped with a newer bootloader that is quicker and leads to some errors reading from ADC?
schematics.pdf (659 KB)
code.ino (1.95 KB)