uP/uC Floating Pins?

All bidirectional pins *typically* configured as inputs to avoid conflicts. However this is NOT a rule. Always check with datasheets.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

That depends on the part.

I have yet to run across a chip with a pin designated as 'output' that isn't low impedance all the time, but I would expect that whether it is low, high, or oscillating randomly would depend on the part.

You do mean _output_ pins, not general purpose I/O that might be configured for either input or output -- right?

Tip: Open drain

VLV

AFTER your boat is out of the water...

Well, yes, but that'll be mentioned. Like you said: read the data sheet.

Preferably in a state as specified in the datasheet.

-- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico@nctdevpuntnl (punt=.)

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I guess I could be driven by an I/O pin. Is it possible to program those as a "LOW" during POR? ...Jim Thompson

What are you _really_ trying to do?

I gather that you want to drive some bit of circuitry during POR, but for some reason you can't just monitor the reset line. Do you want to _drive_ it or do you just want to signal that a reset is occurring?

_Some_ microcontrollers will drive their outputs to some defined state, or will put weak pull-up or pull-down devices on their GPIO pins during POR. If you don't mind the low speed and low current capability, you could use one of these 'weak pull-up' pins to indicate a reset, then have software clear the pin when it wakes up.

If you're trying to make some generic peripheral that'll work on every microprocessor in existence, and have the correct POR behavior, you're screwed -- none of them act the same, and (as I think you've pointed out yourself) not all of them even behave _appropriately_ in response to a brownout event.

Read the data sheet.

In general, the junk I/O pins float. You can't "program" them from software during reset because the software isn't running yet.

Some chips look at a few input pins to select a mode. Some modes might drive some pins.

The normal trick is to add a weak pulldown if you need a particular floating signal to be low during reset.

That's what I'm concluding. I've found a way to cope with a floating input that I'm adding into the design ...Jim Thompson

Most microcontrollers float their I/O pins (various possibilities as to what is connected inside, could be ST digital input could be an inverter digital input, could be analog circuitry) while the reset is active.

Since most of them can be configured as either an input or an output, it's safest to have them power up as inputs so they don't do silly things like turning motors on or relays if reset is held active. Sometimes they have internal programmable pull-ups or pull-downs, but they often don't get configured as such until the user program runs for a bit.

A few older ones might do interesting things like become pseudo-birectional outputs with high frequency signals on them.

What if it hasn't been written yet?

Why are you designing something for a chip that doesn't exist?

Ask the guy who is trying to sell you the chip? Ask the guy who told you about the chip? Use another chip.

Jim designs chips. There are no data sheets until the designs are complete.

I don't design uP's or uC's, at least not complete ones... small local controls, occasionally.

Here I have a analog situation where a floating input can cause high currents (like 250mA) to flow, so I just wanted to make sure it was a real problem before I added-in a fix. It's done, took me 4 MOS transistors :-) ...Jim Thompson

I thought you had done some analog parts of ASIC that had a stock CPU design?

In message , Michael A. Terrell writes

Yeah, but in this case, he's designing what sounds like a peripheral for an existing chip. If it was a design of Jim's then I'd not have expected him to need to ask the question.

Ever hear of flaky (*) customers? Many weird, non-physical requests and lots of misinformation. So I ask here to get the true scoop :-)

• Like a customer who labels the R and C for a triangle wave oscillator on his "specification" as "TANK1" and "TANK2"... I kid you not :-( ...Jim Thompson