I have recently designed a shield to my Arduino Duemilanove, which has 8 LEDs, on the digital GPIO, from 0 to 7, but it won't program with the shield on because the voltage drop is too much on the RX and TX lines. My question is: can I have a LED on the TX/RX line with a minimum resistor value that allows me to program the Arduino?

  • how do you have the pins configured/wired? Pullups/pulldowns? Are you sinking current or providing current to the LEDs?
    – Madivad
    Commented Mar 29, 2014 at 1:42
  • I have from digital pin 0(RX)-->330R-->LED-->GND, and its pretty annoying on the developing fase to be constantly taking on and off the shield. Commented Mar 29, 2014 at 9:00
  • try something in the order of 5-15k and see how that goes. If brightness becomes an issue you might have to put a transistor in as well.
    – Madivad
    Commented Mar 29, 2014 at 9:27

1 Answer 1


I just found this reference (using ICSP for output as well as programming -- essentially the same thing), and it suggests a 4.7kOhm.

I didn't have a 4.7k handy, and used a 15k instead and it works.

In my scenario, I have run a sketch with a polarized buzzer connected between RX (digital 0) and GND. When connected directly (with no resistor) the sketch fails to load (but the buzzer works as expected).

If I put the resistor in series with it, the sketch uploads without fail.

You also probably realise, but mentioning for clarity/completeness, you will lose all serial port functionality for debugging purposes and won't be able to use Serial.[anything]. Make sure to not even initialise the port.

  • After trying several resistor sizes i got to the smallest possible size of 470R, now i can program whitout any problems at least for this kind of LED, it will probably change if you have an LED with a bigger voltage drop, thank you very much again for your help @Madivad :). Commented Mar 29, 2014 at 9:41
  • Oh well, that saves me from finishing this project. I was just putting together a breadboard to try different values myself. Glad I could help.
    – Madivad
    Commented Mar 29, 2014 at 9:52
  • Thats some dedicated fellow :), if your interested i can show you my shield it will be shared soon on OSH Park. Is there a way to contact people here with private messages ? Commented Mar 29, 2014 at 9:56
  • Well, I was using it as a learning experience for me as well. Which is why I answer a few XBee questions. As far as I know there is no PM system, but just below in the footer is a "chat", head over to "pin13"
    – Madivad
    Commented Mar 29, 2014 at 10:12

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