Jeff Brower wrote:
> Steve-
>
> > Until the POR is released, all I/Os not actively involved in
> > configuration are high-impedance. The HSWAP_EN pin controls whether or
> > not internal pull-ups are applied to these I/Os. When HSWAP_EN = High,
> > the I/Os are turned off. Also, the pull-ups connect to their
> > associated power rail so you won't see the effect until VCCO ramps up.
>
> Except for the S3 errata about "if HSWAP_EN input is high, pull-up
> resistors are momentarily enabled on User-I/O at end of Configuration"
> which I believe only hints at the magnitude of the problem we saw on
> Spartan 3s in mid-2005. On one board with ACQ revision XC3S1500-676,
> the I/O pins are forced high enough to produce a 1.5V output on lines
> with 1k pull-down resistors, and this condition lasted more than 100
> msec prior to DONE assertion.

Spartan-3 pull-up and pull-down resistors are much stronger than in
previous FPGA families. In previous families, they were on the order
of 20-50k ohms.

>
> On boards with revision ECQ and later parts, we don't have the problem
> so it looks like it has definitely been fixed.

>From the XC3S1500 errata notice ...
http://www.xilinx.com/xlnx/xweb/xil_...egory=-1210888

.... it appears that the "pull-ups active during end of configuration"
issue was fixed on parts marked with either "AGQ" or "EGQ". In other
words, essentially anything built with the "GQ" process/fab code. The
errata also has diagrams indicating how to determine which device you
have. The "EGQ" is the current silicon. Table 3 in the errata notice
describes which issues were in the early and later silicon revisions.

> But this brings up a question we've had about S3s for a long time:
> what is the actual pull-up R value? I had heard from our local FAE
> that the pull-ups are not true Rs, but a "pseudo-transistor" method is
> used.

The equivalent resistance is actually specified in the data sheet. See
Table 32 on page 56 in specific.
http://www.xilinx.com/bvdocs/publications/ds099.pdf

The actual measurement is a current, which equates to a resistance.
The "resistor" is as Peter described in ...
http://groups.google.com/group/comp....2b86b865e6c456

For example, the equivalent pull-up "resistor" when powering a bank for
3.3V, is between 1.27k and 4.11k ohms. The equivalent pull-down
"resistor" is between 1.75k and 9.35k ohms.

---------------------------------
Steven K. Knapp
Applications Manager, Xilinx Inc.
General Products Division
Spartan-3/-3E FPGAs
http://www.xilinx.com/spartan3e
---------------------------------
The Spartan(tm)-3 Generation: The World's Lowest-Cost FPGAs.