Virtex-4 pre-configuration pull-ups

[Database Administrator]

I can't seem to find any data on the value of the "weak" pull-ups
on the IO pins during configuration if the HSWAPEN pin is configured
to give them. Does anyone know approximately what value the pull
is? I need to counter it for some logic - needs pulling down during
configuration.

Also, I'm confused about the direction of the HSWAPEN pin - one
set of documentation says pull down to enable pullups, but another
implies pull down to disable pullups. Which is correct?

--
Thanks in advance.
Nobby Anderson

[Database Administrator]

HSWAPEN is the Input Dedicated pin. Active High input used to disable weak pre-configuration I/O pull-up resistors: 0 = weak pre-configuration I/O pull-up resistors enabled 1 = weak pre-configuration I/O pull-up resistors disabled HSWAPEN must be connected to either enable or disable the pull-up.

This is depends on your custo design, if you want to disable/enable the function.

Regards, Sachin resistors. HSWPEN has a weak pull-up prior to and during configuration.

[Database Administrator]

HSWAPEN is the Input Dedicated pin.

Active High input used to disable weak pre-configuration I/O pull-up resistors:

0 = weak pre-configuration I/O pull-up resistors enabled

1 = weak pre-configuration I/O pull-up resistors disabled HSWAPEN must be connected to either enable or disable the pull-up resistors.

HSWPEN has a weak pull-up prior to and during configuration.



This is depends on your custom design, if you want to disable/enable the function.

This means your all I/O pins would be pulled-up to VCCIO voltages during configuration. If you don't want to pulled-up ur all I/O's to Pulled-up to VCCIO then you can disable it.

Regards,

Sachin

[Database Administrator]

Sachin wrote:
> HSWAPEN is the Input Dedicated pin.
>
> Active High input used to disable weak pre-configuration I/O
> pull-up resistors:
>
> 0 = weak pre-configuration I/O pull-up resistors enabled
>
> 1 = weak pre-configuration I/O pull-up resistors disabled HSWAPEN
> must be connected to either enable or disable the pull-up resistors.
>
> HSWPEN has a weak pull-up prior to and during configuration.
>
>
>
> This is depends on your custom design, if you want to disable/enable
> the function.
>
> This means your all I/O pins would be pulled-up to VCCIO voltages
> during configuration. If you don't want to pulled-up ur all I/O's
> to Pulled-up to VCCIO then you can disable it.

Thanks for clarifying the direction of the pin. Do you have any idea
of the strength of the weak pullups? In most cases I want the pullup,
but for a couple IO pins I want to override it with a stronger pulldown
in the preconfigurationn time. If no one knows I guess I'll just have
to suck it and see, but I'm surprised there doesn't seem to be any
indication what "weak" means in the docs.

Cheers,
Nobby

--
Nobby Anderson

[Database Administrator]

Nobby,


The weak pullup is specified here:


http://direct.xilinx.com/bvdocs/publications/ds302.pdf

page 3 of 5.

From 40uA to 200 uA depending on Vcco.

200 uA @ 3.3 volts, 40 uA at 1.5 volts.

So if VccoConfig=2.5v, that is 120 uA. 2.5/120uA =~ 21 K ohms R min.

All of those are max values.

Min is 5 uA for any Vcco.

2.5/5 uA = 500 K ohms. Max R pullup.


Austin

[Database Administrator]

austin wrote:
> Nobby,
>
>
> The weak pullup is specified here:
>
>
> http://direct.xilinx.com/bvdocs/publications/ds302.pdf

Ah, I misunderstood that, I was looking for a different "weak" and
"normal" or "strong" value, or some such. I guess they really
mean that if there's a pullup, they're always week, and this
is the value. The documentation could be a little clearer, and
not use the term "weak" in some places and not in others.

Thanks for that, Austin, that's sorted me out.

Cheers,
Nobby

--
Nobby Anderson

[sreenivs]

Hi,

Thanks for this useful info. I have been trying to understand this concept for a long time too.

So to summarize:
By "Weak Pullup", do they mean weak current ?

I remember reading in one of Xilinx documents that, a weak pull-up doesn't always guarantee a logic '1'. Is that true?

[sreenivs]

My previous reply seems to have disappeared. I am sorry if you are seeing this response again.


I have been trying to understand the word "Weak" in "Weak pull up" for a long time. I am glad Nobby brought this up.

Based on your responses, this is what I understtod. Please correct me, if I am wrong.

1) Does the term "weak" actually mean weak current?

2) I remember reading the following quote in a Xilinx document: "a weak pull up doesn't always guarantee a logic '1' ". Is it true?

Thanks,
Murthy