Uno resetting

Hi All,

I'm a complete novice with the Arduino, RPi etc. I've just bought an Arduino Uno and a Sparkfun Red Board, and I've been working through the first projects in the Spark Fun SIK and John Boxalls 'Arduino Workshop' book I.e 'Blink', sequence 5 LED's, etc, easy, simple sketches.

This problem applies to the Uno AND the Red Board. 1. I upload a simple sketch and it starts to run. After about 5 or so seconds the board appears to reset - the LED of pin 13 flashes, the blinking or sequence stops (depending on which sketch I've uploaded) briefly, and then the sketch starts to run again. 5 or so seconds later the pin 13 LED flashes, blinking stops or the sequence stops and so on......this just repeats.

One way I discovered that I can stop this apparent resetting is to disconnect the PC - a 2009 Mac Book Pro running MAC OS 10.6.8 Snow Leopard. This is mildly inconvenient, but once I advanced to the sketches that require you to open serial.begin() then obviously I can't do that if I've disconnected the MAC and if I leave the MAC connected I get a few serial port readings but on the first 'reset' they stop and don' start again when the sketch does, so effectively I can't use the serial port readings.

I've asked about this on the Spark Fun forum and the consensus was a too much current/power issue. I've tried running the boards straight off the MAC USB port with the MAC on battery, on mains, and with the MAC and boards connected up to a powered hub. None of that makes any difference.

My plan next is: uninstall from the MAC, re-install. Try it on a Windows PC running Win7, I've got access to both 32 and 64 bit versions.

Any help would be greatly appreciated as this, especially the serial port issue is spoiling what could be a very interesting experience. Regards,


Had a similar problem using a PC. Computer talk via serial can reset the board. (to make upload easier) A PC will check the USB-connection every 30 sek or so. I guess your MAC does similar. Does this also happen if there is no 'Serial.begin' ?


Yes it does. If it didn’t I wouldn’t be too bothered. When/If I get onto more advanced projects I’m going to need the feedback from the serial port.

Bit of an update, last night I installed the IDE on a Win 7 PC - the Red Board would not work at all, Win said there was no driver required or found! So tried the kosher Uno - works perfectly, no resetting, no unplugging the USB. Last test will be uploading a sketch that requies the serial.begin().
If that’s passed then I’m using Win PC.

Might try it on a flavour of Linux.


I acknowledge I'm replying to an old post but it came up in my searches so might come up in others. I had a very similar problem with two different Arduino's resetting every 5 seconds or so on two macs but not a third. This may seem really obvious to some - but took me a while to realise that some background software for my GPS watch (Suunto Ambit) which I use regularly to track cycling exploits runs automatically every time I boot was checking for a watch at the USB ports every few seconds. Quit the software and the regular reset has stopped and I can get on with developing....

I came across this post by Google search and can confirm simonlittler's observation. The Suunto Ambit app (Movescount) on macOS triggers are reboot of the Arduino board every five seconds.

Thanks simonlittler, nice find!