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I need to connect two boards using UART of AVR. I wanted to know how much can be the maximum length of the cable between two baords if VCC is 3.3V. I dont want to use rs232 here.

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    \$\begingroup\$ It would be largely dependent on the required baud rate. EDIT: Also the capacitive coupling between the signals is of real concern, since it can introduce unbelievable crosstalk. \$\endgroup\$ – Dzarda Dec 9 '13 at 7:31
  • \$\begingroup\$ baudrate will be 9600! can i safely use a 10 cm cable? \$\endgroup\$ – dmSherazi Dec 9 '13 at 8:12
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    \$\begingroup\$ @dmsherazi, you're better to edit that into the question rather than post as a comment. But I can't contemplate problems with a 10cm cable unless it's in a very electrically noisy environment. \$\endgroup\$ – PeterJ Dec 9 '13 at 8:16
  • \$\begingroup\$ Why don't you try it and, if it looks OK try sending data at a much higher rate to see if it still works. As an experiment it's barely longer to perform than reading answers and will give you some confidence to try with different cables to see if there are problems. Learning on the job is a good method. \$\endgroup\$ – Andy aka Dec 9 '13 at 9:16
  • \$\begingroup\$ No need for using RS232 (which is a mechanical and electrical spec too, not just timing) on a 10cm wire. Just use logic level UART and you'll be fine way above 9600Bd. \$\endgroup\$ – jippie Dec 9 '13 at 9:56
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I can't answer as to the maximum length, but at 9600 baud I wouldn't expect problems at a metre or two, let alone 10cm. Just ensure there is a good ground wire connecting the two boards.

And that assumes it's permanently wired, and working in a typical lab environment, or your home, rather than in the engine compartment, or in orbit or on a mountain top.

Even in the lab, if you plan to unplug and reconnect it regularly, add that info to the question and someone will advise on suitable ESD protection.

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