Handling of unconnected pins on circuitboard/ unused pins on microcontrollers (generally)

I design with lots of discrete parts. Somewhere around here I have a thermal image of a row of TinyLogic buffers glowing cheerfully in the thermal IR, because they were biased partially into their linear regions, specifically because they has 5-volt supplies but 3.3 volt logic inputs.

John

The problem of cross conduction is fairly overstated. A common HC logic gate @ 5V supply drains < 3mA at the worst case of cross conduction.

However, the random toggling of unconnected inputs (even if schmitt triggered) can considerably increase power consumption of micropower devices such as coin battery operated RKE, clocks, sensors, etc.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Agree, but the parts in my example are screaming-fast triple buffers in US8 packages. They were getting very hot. The fix was to add a diode drop of level shifting to split the difference, namely swing the inputs from roughly 0.7 to 4.0 We couldn't spare enough nanoseconds to do anything else.

John

If you need to do such dumb stunts, use 'HCT' or equivalents to ...Jim Thompson

minimize overlap currents.

(Completing my sentence. I don't know how I accidentally sent :) ...Jim Thompson

I wasn't worried about linearity - it's a knob that the user turns until it's "the right speed."

Thanks! Rich

It wasn't sloppy. Once in a while the ISR had a bunch of calibration stuff to do, so we let it. As I said, no harm was done.

John

Well, you've clarified what "let it go" means - you didn't really just "let it go", from what you've just said, you accommodated it.

Fair enough? :-)

Thanks, Rich

Yeah, now that you've mentioned it, you're both right. Mind you, I've never actually _designed_ a board with that cloooge, I've just read about it. Every board I've designed, I knew what each input and output was, and used pullups, etc. - no "expansion" pins. ;-)

Thanks, Rich

Expansion pins no, but I have been known to take pins out to headers. Only one row, not three, so it hardly takes up any space (relative to the DIP), and I can plug in at will, or poke jumpers into it, etc.

Tim