I am sending data over serial connection from a python script to my Arduino UNO. Now initially whenever I tried to send data to the arduino, it would reset. The fix was to put a 10 microFarad capacitor between the 5V and RESET pins. I did that and everything started working.

Now after while, mistakenly today I ran the same script without the capacitor between the 5V and RESET pins and for some reason it was working perfectly well. No more resetting when a value was received over serial communication.

Is something wrong with my Arduino?

  • 1
    The reset only occurs if you create a connection. So as long as the python script keeps the connection open, not further resets will occur.
    – Gerben
    Jul 1, 2016 at 12:30
  • Which OS are you using?
    – Majenko
    Jul 1, 2016 at 13:34
  • @Majenko Win 10 Home Premium
    – YaddyVirus
    Jul 1, 2016 at 16:41
  • This reset on connection behavior is ultimately under software control of the host operating system and is usually configurable. If you can still program the board through this connection (without your modification installed), the hardware is fine. Sep 1, 2016 at 15:03

1 Answer 1


Knowing the entire setup would help. For instance, knowing if the RS232 source will drive the Arduino input's dangerously past the Arduino's power rails.

If you have connected a real RS232 signal to the Arduino, the +12 to -12 volt swings could adversely affect the processor. Causing unexpected behavior. In such a case you would need a converter like this one.

Over driving inputs is discussed in this Stack Exchange question where the highest scoring answer says:

If you violate the spec sheet normal operating conditions requirements you can expect to experience abnormal operating conditions. These may range fro perfectly normal opration through complete maloperation...

That answer also links to this tutorial about logic level conversion.

Finally, there is this tutorial which specifically addresses converting RS232 to safe TTL level signals appropriate for a micro processor.

  • I am connected to a motor driver, which is connected in turn to a motor.
    – YaddyVirus
    Jul 1, 2016 at 17:07
  • We would need the specifications of the serrial interface. Maybe that could be foubd from the modle number of the motor driver? It is starting to sound like you need the proper interface.
    – st2000
    Jul 1, 2016 at 18:41
  • No its not like I'm having a problem here. In fact not having to connect a capacitor between the 5v and RESET pins saves me the hassle of connecting a mini breadboard. Thing is that how can all of a sudden the absolute need of a capacitor between 5V & RESET disappeared?
    – YaddyVirus
    Jul 1, 2016 at 20:30
  • When driving digital integrated circut inputs beyond the power supply rails, the state of the integrated circuit is indeterminate.
    – st2000
    Jul 1, 2016 at 23:54
  • 1
    There is no reason to believe the poster is using RS232 signalling at all, especially as if they were they would have had to explicitly hook up a connection between a modem control line and the reset to trigger this, rather than what they did do which was add components to defeat such an apparently forced connection. All evidence suggests they are using the Arduino's build in USB serial converter, which has such a modem line to reset connection as part of it. Sep 1, 2016 at 15:04

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