Arduino Mega I2C without USB - problem

I've connected RTC PCF8563 and LCD display with Arduino Mega. I'm reading date&time from RTC every second and displaying it on the LCD display.
Reading between Mega and RTC is correct when Mega is connected to the PC via USB cable, but when I connect it to external power supply (5,3V/1A) communication with RTC is slowed down - display is refreshed on every 2 second (instead od every 1 second). I thought it could be related to the external power supply, so i used my tablet power supply (5V/2A) to power up Mega with USB connector, but problem persist - data from RTC are read every 2 seconds.

Could someone explain me if I2C communication is somehow related to the USB connection please?
Thanks in advance

That seems to be a power problem.Try to power it with batteries to see if the problem disappear.Those power sources could have a lot of ripple or noisy outputs maybe.

If you are connecting 5.3vdc via the external power or Vin pin then you are using too low a voltage. From the product specs:

The board can operate on an external supply of 6 to 20 volts. If supplied with less than 7V, however, the 5V pin may supply less than five volts and the board may be unstable. If using more than 12V, the voltage regulator may overheat and damage the board. The recommended range is 7 to 12 volts.

Thanks for hints. I've changed 5V ext. power supply to 12V, but the problem is still there. Meanwhile I've found, that if I touch USB connector on Arduino with hand, communication is correct. I have to check Arduino PCB probably :slight_smile: