leaving pin floating

Yes, I guess we are talking about the same thing just using different terms. When you say output you mean a programmable pin that is set to an output. Naturally you will need to handle that time the pin is an input and in fact I am discussing that with Luhan in another thread.

Jim

Holly Smoke, this subject is hot! :))

But I should've been more accurate in my request. Sorry about that.

What if the PIC (16F628) is kept in reset state for a long time, say or hours or days and the floating pins are PORTA pins. These pins have Schmitt trigger inputs. A middle point threshold should not promote oscillation or current increase, is'nt it... or is it?

Al

I would still have a problem with keeping any high impedance input just hanging there, just from an ESD suseptability stand point, it is a bad idea. I have this thing about the electrons going where I want them to when I want them to. I guess I'm just old school and those habits stuck with me and have served me well.

Jim

Mmmmh! Floating pins seems to make most people unconfortable. For sure, adding resistors is cheap. I now consider adding few 4x100K SIP resistors soldered directly to the PIC under the PCB, the common leg just falling nicely on a Vss pin (or Vcc pin) and the other legs to I/O pins.

For ESD, these "floatting" pins are not used and no PCB trace go to them. Al

That's exactly what I do for a proto. The little 10 pin pull up packs can be stuck under the IC. Trust me about ESD. It doesn't matter that the pin has no copper touching it. There is enouch there just counting the pin and the bonding wire inside. Some of our stuff has to meet some pretty big discharge tests and every little thing hanging out there can be a problem. Some of those high V discharges don't care about insulation and "isolation". I'll bet you never had much of a problem shocking all kinds of "insulated" things using the 'ol rub your feet across the carpet in the winter.

Jim

But what does ESD have to do with a pin having a pullup, or leaving floating?

Gives the juice somwhere to go other than the guts of the IC. If eveything is at 10KV it is the same as having nothing at 10KV. Sure there are other things to consider when making industrial strength ESD protection, but not having any loose pins is a great first step.

Jim

But I don't think this is true. It will go through the IC *before* it goes through a 10k (or 1k) pullup. Through the input protection diodes, for a start.

I don't see what benefit it has at all for ESD, in fact I would expect it to make things worse. Before, you just had a tiny exposed pin. Now you have the pin, and the tracking, and the resistor body to worry about. On the other hand, connecting the unused pin direct to the groundplane would work.

This is what I usually do, for production boards.

In my experience you need a combination of parallel transient suppression *and* series current limiting, for external signals.

________ I/P o-------[ R ]--------|CPU | | - TVS ^ | ======================== Groundplane

So you think having a highZ input just floating there is better than pulling it somewhere? Fine, do it your way. I agree grounding the line is best, (depending on how "good" the ground is) but I don't like programmable pins connected directly to power or ground. Like I said, there is more to good ESD supression than what I went over, I just didn't want to take the thread any further off topic.

Jim

No, not at all, just that I could not see the difference *from the point of view of ESD*. The main problem I have *actually* found, when leaving floating, is that the pin may turn out to have some configuration function that is read at power-on! Other hazards would be an interrupt left enabled, connected to a floating input.

The point about increased power when floating is interesting, but I think that switching the pin to output mode should eliminate this. The brief window after reset would not cause a problem in my applications, but perhaps in very low power work it might become important.

Yes, good point, I usually only do this for inputs (e.g. unused ADC inputs).