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  #1 (permalink)  
Old 09-03-2004, 06:05 PM
Markus Meng
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Default [XC96xxXL] Maximum Value for the external Pull-Up resistor ...

Hi all,

I just wonder if someone else did make the same measurements.

We do have a system with a 9572XL having inputs with an external
47KOhm resistor. The design has done by another person. I prefer pull-up
resistors in the 3.3K...10K max value ranges.

We did have strange behavior until I did measure the voltage on
these inputs pins. After configuration through JTAG we do have
a clean 3.3VDC pulled up input value ...

Bringing one of these inputs to GND and releasing it again, shows
with the oscilloscope, that the voltage on these pins doese rise until
0.9Volt but not higher. Thought the external pull-up resistor should
bring these inputs - only - in the design back to 3.3VDC again. No
way, once these pins have been brought to GND, the release of it give
us a voltage rise up to 0.9VDC but not more.

This explains a lot, why even the simplest logic did give us a real blues ...

However reducing to the external pull-up resistors to below 10K, we
found everything is working as expected.
Checking the datasheet a see a max input current for the
inputs of +-10 uA. Multiplying this value with 47K gives us 0.47V max ...

It seems that this max value is somehow not the max value ....
(On these inputs, there is no additional cap ...)

Any idea?

Markus
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  #2 (permalink)  
Old 09-03-2004, 06:18 PM
Mike Lewis
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Default Re: [XC96xxXL] Maximum Value for the external Pull-Up resistor ...

Current flowing through the resistor results in the voltage
drop from 3.3v to 0.9v. The larger the resistance the greater
the voltage drop. I wouldn't think this to be an FPGA specific
issue but more a system issue. It would depend on what
devices are connected to the signal and what their current
draws are.

Mike


"Markus Meng" <[email protected]> wrote in message
news:[email protected] om...
> Hi all,
>
> I just wonder if someone else did make the same measurements.
>
> We do have a system with a 9572XL having inputs with an external
> 47KOhm resistor. The design has done by another person. I prefer pull-up
> resistors in the 3.3K...10K max value ranges.
>
> We did have strange behavior until I did measure the voltage on
> these inputs pins. After configuration through JTAG we do have
> a clean 3.3VDC pulled up input value ...
>
> Bringing one of these inputs to GND and releasing it again, shows
> with the oscilloscope, that the voltage on these pins doese rise until
> 0.9Volt but not higher. Thought the external pull-up resistor should
> bring these inputs - only - in the design back to 3.3VDC again. No
> way, once these pins have been brought to GND, the release of it give
> us a voltage rise up to 0.9VDC but not more.
>
> This explains a lot, why even the simplest logic did give us a real blues

....
>
> However reducing to the external pull-up resistors to below 10K, we
> found everything is working as expected.
> Checking the datasheet a see a max input current for the
> inputs of +-10 uA. Multiplying this value with 47K gives us 0.47V max ...
>
> It seems that this max value is somehow not the max value ....
> (On these inputs, there is no additional cap ...)
>
> Any idea?
>
> Markus



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  #3 (permalink)  
Old 09-03-2004, 06:20 PM
Peter Wallace
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Posts: n/a
Default Re: [XC96xxXL] Maximum Value for the external Pull-Up resistor ...

On Fri, 03 Sep 2004 10:05:33 -0700, Markus Meng wrote:

> Hi all,
>
> I just wonder if someone else did make the same measurements.
>
> We do have a system with a 9572XL having inputs with an external 47KOhm
> resistor. The design has done by another person. I prefer pull-up
> resistors in the 3.3K...10K max value ranges.
>
> We did have strange behavior until I did measure the voltage on these
> inputs pins. After configuration through JTAG we do have a clean 3.3VDC
> pulled up input value ...
>
> Bringing one of these inputs to GND and releasing it again, shows with
> the oscilloscope, that the voltage on these pins doese rise until
> 0.9Volt but not higher. Thought the external pull-up resistor should
> bring these inputs - only - in the design back to 3.3VDC again. No way,
> once these pins have been brought to GND, the release of it give us a
> voltage rise up to 0.9VDC but not more.
>
> This explains a lot, why even the simplest logic did give us a real
> blues ...
>
> However reducing to the external pull-up resistors to below 10K, we
> found everything is working as expected. Checking the datasheet a see a
> max input current for the inputs of +-10 uA. Multiplying this value with
> 47K gives us 0.47V max ...
>
> It seems that this max value is somehow not the max value .... (On these
> inputs, there is no additional cap ...)
>
> Any idea?
>
> Markus


XC9572XL have bus hold circuitry on inputs. Unless you overpower the bus
hold circuit with your pullup resistor, you will have funny I/O levels.

Not sure but maybe you can program input for pullups instead of buss
hold...


Peter Wallace
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  #4 (permalink)  
Old 09-03-2004, 11:20 PM
Mark Ng
Guest
 
Posts: n/a
Default Re: [XC96xxXL] Maximum Value for the external Pull-Up resistor ...

Hi Markus,

Peter is correct. Your external 47k ohm resistors are too weak to over
come the Bus Hold circuit in the 9500xl.

Your options are:

a) Use pullup resistors that are no greater than 10k

or
b) Float the IO's using the ISE GUI option. (Implement ->
Properties -> Fitting Tab -> Float)

I'd recommend option a), as floating IOs is not recommended unless you
have properly terminated all IO's...A floating input voltage will cause
the input buffer to potentially oscillate, and will cause the device to
draw unnecessary power....

Thanks,
Mark

Peter Wallace wrote:
> On Fri, 03 Sep 2004 10:05:33 -0700, Markus Meng wrote:
>
>
>>Hi all,
>>
>>I just wonder if someone else did make the same measurements.
>>
>>We do have a system with a 9572XL having inputs with an external 47KOhm
>>resistor. The design has done by another person. I prefer pull-up
>>resistors in the 3.3K...10K max value ranges.
>>
>>We did have strange behavior until I did measure the voltage on these
>>inputs pins. After configuration through JTAG we do have a clean 3.3VDC
>>pulled up input value ...
>>
>>Bringing one of these inputs to GND and releasing it again, shows with
>>the oscilloscope, that the voltage on these pins doese rise until
>>0.9Volt but not higher. Thought the external pull-up resistor should
>>bring these inputs - only - in the design back to 3.3VDC again. No way,
>>once these pins have been brought to GND, the release of it give us a
>>voltage rise up to 0.9VDC but not more.
>>
>>This explains a lot, why even the simplest logic did give us a real
>>blues ...
>>
>>However reducing to the external pull-up resistors to below 10K, we
>>found everything is working as expected. Checking the datasheet a see a
>>max input current for the inputs of +-10 uA. Multiplying this value with
>>47K gives us 0.47V max ...
>>
>>It seems that this max value is somehow not the max value .... (On these
>>inputs, there is no additional cap ...)
>>
>>Any idea?
>>
>>Markus

>
>
> XC9572XL have bus hold circuitry on inputs. Unless you overpower the bus
> hold circuit with your pullup resistor, you will have funny I/O levels.
>
> Not sure but maybe you can program input for pullups instead of buss
> hold...
>
>
> Peter Wallace

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