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Topic: Brief digital high on start up (Read 661 times) previous topic - next topic


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 http://www.arduino.cc/en/Main/ArduinoBoardDuemilanove). 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.
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robert rozee

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

James C4S

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?
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