XC9500XL keeper ?

I just migrated a project from the XCV9500 to the XC9500XL, and am having some problems with keepers on the used inputs. Looking at Xilinx' docs on the 9500XL series, it looks like the keeper may be left on ALL the inputs, not just UNUSED pins. You seem to have a choice between a keeper and a pulldown (GND) option. Is there a way to turn off the keeper on selected inputs? (Seems like the answer is no, but just checking.) I have a reset pushbutton with a debounce RC and a crystal oscillator that need to be tweaked to work OK with the keeper in the circuit. I'm using web pack 10.1 in Linux, if that makes a difference.

Thanks for any suggestions!

Jon

Reply to
Jon Elson
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It was my understanding that the Keeper vs Pulldown is a global option, but between that and tristate can be selected pin by pin. Used input pins normally have to have a keeper or pulldown constraint in the .ucf file. With neither constraint the input should be high impedance. You will get an error if you try to have keeper and pulldown in the same project, but you should be able to have keepers on just selected inputs. If you need to make individual settings to unused pins, you need to add the pins to the design.

-- Gabor

Reply to
Gabor

Well, that's the quirk! As far as I can tell, it has a keeper on all USED inputs, and I am not specifying that - I think. Digital inputs are not bothered by the keeper, but it is interfering with the input pin of the clock oscillator and the reset RC circuit. I seem to have the impedances of these circuits pulled down low enough now that it is working, but I worry I may have reduced the margin on those circuits. What I'd like would be a "no-keeper" option in the UCF file, but I don't see any such thing.

Thanks,

Jon

Reply to
Jon Elson

Maybe the message here is reset supervisor chip and oscillator.

--
Michael Karas
Carousel Design Solutions
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Reply to
Michael Karas

I Agree.

You really have to be careful when your digital IC vendor tells you that the pads have "pullups and pulldowns". They don't, they have current sources and current sinks.

The PC board guys love to have them because they think that they don't have to put real ones on the PCA but if you look at the current spec they are usually speced to +100 -50 %. If you try to put any sort of analog filtering on the pad then this range will make it hard to add any hi-impedance circuitry.

If you can't disable them then you must us an external buffer

John Eaton

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Reply to
jt_eaton

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RC circuits on reset lines are very bad ideas anyway. They may work well if the circuit if off for some amount of time, but if you get a momentary glitch they can fail to reset at all leaving the circuit in an undefined state because the power glitch disrupted the circuitry.

Rick

Reply to
rickman

With todays chips there is also the problem with separate pad and core voltages. The RC is on the pad supply and may create a reset that is long gone before the core voltage can ramp up.

You have to have a voltage supervisor on ALL of the supplies.

John Eaton

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jt_eaton

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