FPGA Central

I am using a Spartan 3 Starter board from Digilent...

I am feeding a square wave to my D9 pin (I/O pin) and my board counts
the square wave, displaying the max count on the LED. However, when I
turn off the square wave and simply touch the output of the D9, the
FPGA counts be itself. Is there some touch sensitive stuff that I am
not aware about with the I/O pins ? How do I really know that the FPGA
is counting signals from the input or if the FPGA is just picking out
some external pulses...I feel that it really isn't counting the square
wave pulses...please help me with this occurance.

Ucehnna

Floating CMOS inputs do exactly what you just saw. It's not a function
of the FPGA, it's a function of CMOS architecture. I would suggest
either ensuring that the input is always driven or pulling the input
to a deterministic state. Xilinx FPGAs all have programmable pull-up
and pull-down resistors for input/output buffers. You can insert one
easily in the UFC file by adding a PULLUP or PULLDOWN constraint to
the pin.

- Nathan

On Oct 13, 4:42*pm, uraniumore...@gmail.com wrote:
> I am using a Spartan 3 Starter board from Digilent...
>
> I am feeding a square wave to my D9 pin (I/O pin) and my board counts
> the square wave, displaying the max count on the LED. However, when I
> turn off the square wave and simply touch the output of the D9, the
> FPGA counts be itself. Is there some touch sensitive stuff that I am
> not aware about with the I/O pins ? How do I really know that the FPGA
> is counting signals from the input or if the FPGA is just picking out
> some external pulses...I feel that it really isn't counting the square
> wave pulses...please help me with this occurance.
>
> Ucehnna

[email protected] wrote:
> I am using a Spartan 3 Starter board from Digilent...
>
> I am feeding a square wave to my D9 pin (I/O pin) and my board counts
> the square wave, displaying the max count on the LED. However, when I
> turn off the square wave and simply touch the output of the D9, the
> FPGA counts be itself. Is there some touch sensitive stuff that I am
> not aware about with the I/O pins ? How do I really know that the FPGA
> is counting signals from the input or if the FPGA is just picking out
> some external pulses...I feel that it really isn't counting the square
> wave pulses...please help me with this occurance.
>
> Ucehnna

There are concepts such as stored charge and static electricity that can
affect inputs - particularly floating inputs - and may damage your
device if you're not careful!

Inputs are intended to be driven by ground-referenced signals within a
defined voltage range. If you have a low impedance pull-up or pull-down
on your input, you will *typically* not see the signal change by
touching the signal pin PROVIDED THAT you have discharged your body to
ground first. If you don't discharge to ground - perhaps through a
grounding strap worn for static protection - you can damage sensitive
electronics!

On Oct 13, 5:53*pm, John_H <newsgr...@johnhandwork.com> wrote:
> uraniumore...@gmail.com wrote:
> > I am using a Spartan 3 Starter board from Digilent...
>
> > I am feeding a square wave to my D9 pin (I/O pin) and my board counts
> > the square wave, displaying the max count on the LED. However, when I
> > turn off the square wave and simply touch the output of the D9, the
> > FPGA counts be itself. Is there some touch sensitive stuff that I am
> > not aware about with the I/O pins ? How do I really know that the FPGA
> > is counting signals from the input or if the FPGA is just picking out
> > some external pulses...I feel that it really isn't counting the square
> > wave pulses...please help me with this occurance.
>
> > Ucehnna
>
> There are concepts such as stored charge and static electricity that can
> affect inputs - particularly floating inputs - and may damage your
> device if you're not careful!
>
> Inputs are intended to be driven by ground-referenced signals within a
> defined voltage range. *If you have a low impedance pull-up or pull-down
> on your input, you will *typically* not see the signal change by
> touching the signal pin PROVIDED THAT you have discharged your body to
> ground first. *If you don't discharge to ground - perhaps through a
> grounding strap worn for static protection - you can damage sensitive
> electronics!

Hi Guys!

Thanks for the help. So, I can insure myself that the FPGA is counting
as long as the FPGA is always driven by some input square wave and not
floting to some external squivering ? How would I verify that it is
actually counting (despite the fact that the count value got to the
max) ? Any ideas ?

On Oct 13, 10:36*pm, uraniumore...@gmail.com wrote:
> How would I verify that it is
> actually counting (despite the fact that the count value got to the
> max) ? Any ideas ?
>

Same way you would verify any function, give it a known stimulus and
check the result.

In your case...
1. Give it 10 (as an example) things to count and verify that you get
a count of 10.
2. Repeat step #1 a few times verifying that you get 10 counts every
time.
3. Repeat steps #1 and 2 but change '10' to be 'max - 1' (as another
example).

KJ

On Oct 13, 10:36 pm, uraniumore...@gmail.com wrote:

> Thanks for the help. So, I can insure myself that the FPGA is counting
> as long as the FPGA is always driven by some input square wave and not
> floting to some external squivering ? How would I verify that it is
> actually counting (despite the fact that the count value got to the
> max) ? Any ideas ?

One thing you could do is counter periods of a faster clock between
the puleses to measure the inter-arrival time. Any substantial
irregularities probably indicate glitches (or an unstable source) You
would then need a way to dump that data out. You can do that manually
through the serial port but it takes a fair amount of coding. Does
the chipscope logic analyzer work on the webpack tools? If so that
would be a good way to monitor the data from this type of experiment
(you could also just clock the analyzer with a faster clock and look
at the input and expect to see a square wave, or have it look at your
count register)

[email protected] wrote:
> Hi Guys!
>
> Thanks for the help. So, I can insure myself that the FPGA is counting
> as long as the FPGA is always driven by some input square wave and not
> floting to some external squivering ?

You can see the effect visually if you hold onto a scope probe tip

For reliable counting, the FPGA should have a fast-edge signal,
sufficently large.
If this is a generic IP, then an external schmitt would be a good
idea

> How would I verify that it is
> actually counting (despite the fact that the count value got to the
> max) ? Any ideas ?

Not clear what you are asking ?
If you are looking for a signal presence detector, that is often
a monostable and a led.
Or, if you know the appx expected frequency range, you can feed
an appropriate counter bit to an LED