I have two boards with atmega8 on each. Both have programmed to support UART at baudrate 115K. I have cross connected RX TX lines of the boards. And I have also connected GND line. Only the first board has DC supply. The second has no DC supply.
When I switch the power on the LED on the second board blinks!
This LED indicates that something is transmitting over the UART.
How come the LED blinks?
Is it possible that atgema8 migth be powered from RX or TX lines?
it’s possible you are back-feeding power from your TXD line of the first board into the power rails of the unpowerd board through its RXD line. There are effectively zener diodes on each input of many microcontrollers that protect them from small ESD events. if you don’t have the device powered though they can allow current to flow to the power rails because they become forward biased. if you can’t ensure both boards get powered together the easiest way to limit this is to put resistors in series with the TXD and RXD lines, something around 10K is usually fine.
Thank you for your reply. I have got the problem. I will add the resistors.
Actually it is only theoretical configuration, in real life both boards have power.
Hmmm… 
Hi,
In this you have problem into the receiver side because
in the receiver side it can have zener diode it has emmited voltage.
It can produce the voltage so that for that configuration your LED is blinking.
Thanks.
