Reason for digital Input short circuit to Ground ?

My since month super-proper-phantastic working Arduino Mega 128 all of a sudden is making a problem: dig. Input 28 now is permanently pulled low on ground. I took off all the wireing and measured zero resistance between pin 28 and Grd. - strange... The pin uses the inside pullup resistor and is triggered by a simple pushbutton to GRd. Anybody any idea or similar experience?

thinking out loud

  • broken
  • dirt, mousture on the board ==> magnifying glass
  • was the switch connected when measuring? ==> switch broken?

measured zero resistance between pin 28 and Grd

This reminds me of a problem we ran into when students tried to check a very low current fuse with an ohmmeter. Every single one tested bad.

Don

This reminds me of a problem we ran into when students tried to check a very low current fuse with an ohmmeter. Every single one tested bad.

Wow, that must have been quite the robust ohm meter? How low were the fuses rated for?

Lefty

I don't remember the exact fuse value, but they were designed to protect laboratory micro-ammeters. I could probably find one in my garage if I looked hard enough. The ohmmeters were an RCA version of a Simpson 260.

Don

If you haven't guessed already don't do this:-

I took off all the wireing and measured zero resistance between pin 28 and Grd.

It tells you nothing if the chip is un powered or potentially can damage it if the board is powered. My Avo Minor meter gives a positive voltage out of the negative terminal when measuring resistance.