Electronic – When is a short not really a short


I'm debugging a PCB which has a AT Mega 128L and two CPLDs mounted on it. The microcontroller actually works OK but I'm unable to program it via JTAG (and I don't have ISP or any other headers on the board). I was obviously able to program it earlier.

When the problem began, I checked for continutity between all JTAG pins. The Reset pin was problematic and I soldered a wire onto the Reset pin and connected the other end onto the JTAG header. I assumed this would have fixed the problem, but nope. I still get the same error: Failed To Identify Target.

But while doing the continuity checks, I came across something more alarming. The meter told me I had continuity between my 3.3V rail and Ground! Not only that, it also beeped when I checked for continuity between 3.3V and 1.8V. I am quite sure that a large current is not flowing because the power supply isn't hot – I know I should have measured it but I had to leave work for other purposes. But none of the chips on the board are hot either.

My colleague whose working with me on this board told me this is actually normal and its actually because the some pins are internally shorted in some of the chips. He doesn't seem to think this could be the cause for our JTAG problems. I'm quite skeptical of this claim because when is it good to have continuity between Vcc and Ground?! But I am quite inexperienced with such things and hence I'm asking this site. Is it normal to have such continuity?

Note: I know that capacitors will intermittently give a beep but once charged, its essentially an open circuit. In this case, I've held the DMM leads onto the pads for for quite some time and it gives a continuos beep.

Best Answer

Swap your positive and negative meter leads. Do you still read a "short"?

Check the power consumption of your board when powered, or better yet, put it on an adjustable current supply with the current limit all the way down and see how far you have to turn it up to get out of current regulation mode and into voltage regulation mode.

Most likely your jtag problem is unrelated to the meter reading. If you can't pull some of the parts out of sockets, try slowing the jtag clock.