LM75, I2C bad temperature read problem
well the problem was fake/defective chips. despite I knew they couldn't be originals, but I thought the whole difference would be a few degrees in temperature that wasn't very bad for my purpose. but I didn't expect a total I2C failure in 5 new chips in a row! if I didn't change one of them with another batch by chance, I wouldn't know and I would have wasted 3 weeks diggin the whole hardware and software from the ground up. lesson learned.