using pins 20 and 21 with 5V(on board) and 10k pullup. no external power. IT is used to drive AtTiny 4313 and ADS1115. It works. once in a while. it does not. suggest if this setup is right Thanks, yuva
I can't tell from here but it seems like maybe you've got a 5V pullup on a 3.3V pin? That's a bad idea.