Nano doesn't see I2C device

I have checked it with 3 boards. The DS1307 RTC I'm trying to connect seems to be OK. When connected to Uno, I2C scanner happily reports two devices found.

I’ve got Nanos running multiple I2C devices.
Check your wiring and pull ups.

pullups

oops... Never occurred I need those. How does one go about figuring out a value? According to Google 4K7 is popular, but I haven't found any explanations as to why.

Have tried 1K, 2K2, 4K7, and 10K. Nothing seems to work. I am supposed to put them between 5V and A4/A5, right? Is there anything else I can do except for order another batch of Nanos and pray?

cabarete_cub:

pullups

oops... Never occurred I need those. How does one go about figuring out a value? According to Google 4K7 is popular, but I haven't found any explanations as to why.

The "why" is related to bit rate. Faster bit rates require smaller pullup-resistors, which pull the line up faster.

:-)