Re: Configuration pins on Spartan-3

"rickman" <spamgoeshere4@xxxxxxxxx> wrote in message
news:1143833043.040499.197510@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I just had a design review on my board and I was zinged for using
resistors to pull the M[2:0] pins to power or ground. I have always
done it that way and do not see a reason to change. But the Xilinx
documents were shown to me, specifically XAPP453, where they clearly
show the pins being pulled hard to power or ground.

I can't find any info in the data *** on the threshold levels on
these pins (or JTAG), so I can't dispute the argument that I should
follow the app note.

The same person is saying that an XAPP (which I can't find) indicates
that the various JTAG signals need to be pulled low by resistors rather
than high. I have always used resistors to pull TCK and TMS high to
assure that the JTAG port was not put in an invalid state. The TDI and
TDO signals were not important. I am aware that these pins are 2.5
volts. Is that why they are shown pulled to ground, to avoid any
confusion about *which* high? Any official source of info on this?

I have looked on the Xilinx web site, but there are dozens of documents
that score a hit on JTAG and Spartan and I don't see any that answer
the questions.


Rick -

You're right about the M[2:0] pins. No reason to get rid of the resistors,
but they're not needed. The only caveat for pulling high is that these pins
are powered by VCCAUX, so pull (or tie) them to 2.5V. As for pull downs,
these pins are weakly pulled up internally, so if you pull them low just
don't use a huge value. Don't read too much into XAPP453. Those are just
logical drawings anyway. (By the way, in Spartan-3E the M[2:0] pins are
general-purpose I/O and are not powered by VCCAUX.)

For the JTAG pins, read record #11433 in the Xilinx answers database. It
says to pull TMS and TDI high through 4.7K and let TCK float. I usually see
TCK also pulled high, but with TMS high it shouldn't matter. Again, though,
the JTAG pins are powered by VCCAUX, so pull to +2.5V unless you follow the
guidelines to support 3.3V configuration.

Rob


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