Brief digital high on start up

Using Decimilenove and noticed that on every start up or Serial Monitor select, the digitals briefly flash high - even does it in Blink, regardless of whether the digital is selected or not (i.e. plug an LED into pin 7 instead of 13), and that flashes as well. Tried forcing outputs low, but still get it. I haven't tried measuring duration or magnitude.

I want run some mains equipment via relay eventually, so I don't want any chattering every time I look on Serial Monitor.

It might only be a ghost transient and may disappear when pin is connected to real equipment - as all real life operations are slow (seconds, not milliseconds), could the output be "slugged" with a small capacitor?

Any ideas appreciated.

Starting the Serial Monitor resets the Arduino. This can be disabled, but loading a new firmware is less automatic (see The high pulse on reset is probably the result of your initialization routine. On reset all ports are in 'input mode'.

You might want to install pulldown resistors on your relay control pins to keep control signals low if that's what you need on startup.

do you see the outputs flash high when you just press the onboard reset button?

Tried forcing outputs low, but still get it.

With hardware or software?

During RESET all I/O pins default to INPUT which is high impedance. It is not uncommon for a floating pin to briefly appear high. Does the same behavior happen when you use an external pull-down (or pull-up) resistor?

I was seeing brief outputs on pins when loading code onto an uno.
I was told the problem is a board design issue and was pointed towards this thread which you might want to look at;,64256.0.html

aha, i was one of the contributors to that thread :slight_smile: if you'd replied that a simple reset didn't produce the same results then i'd have pointed you to it...

Hello all,
Thanks for all the replies and help.
It does seem to be OK on board reset (10 out of 10 resets fine).
It is behaving impeccably at the moment so I will need to see what I have done in the meantime (guilty of throwing solutions at it). I will definitely look at the other threads and will implement external pull-ups - I suspect floating pins might have something to do with it.
"Forcing low" was simply digitalWrite (LOW) to start with.
Sorry Rozee - only spotted Radman's post after I pushed the post button.
Looking for perfection in an imperfect world.

"will implement external pull-ups"
That will definitely make them High - did you mean pulldowns to keep them low?