Re: Pull up resistors on Spartan 3 mode pins

Jim Granville wrote:
rickman wrote:

People here are driving me crazy insisting that the Xilinx factory has
told them that you *MUST* tie the mode pins to either Vaux or GND.
After finding all the info in the data *** and talking with support,
it looks pretty clear to me that the S3 parts have a very stiff
internal pull up and there is no need for an external pull up of any
kind, resistor or direct connection to Vaux.

Am I misunderstanding? Why did the factory tell us before that the
mode pins *MUST* be tied to Vaux? Did we misunderstand what they were
saying?

I promise this is the last time I will ask about this. I am totally
sick of going around this loop with everyone here.

Design the PCB with options for SMD resistors, that can also be
0-Ohm shunts, and say 'define in production for valid logic levels'.
That gets you past the design review, and closes the case :)

I started the design review process with pullups and pull downs of 10
kohms and was told I had to use direct connections. Of course they
could have been replaced with 0 ohm resistors at any time if needed.
Once I had researched the issue, I thought I had an FAE's blessing of
4.7 kohm resistors. I had found that the pull up resistors were rather
strong in the S3 parts, but did not find any indication that there were
internal pullups on those pins. Turns out I had missed the sentance
that told about them. Once I found that, it all started to make sense.
With internal pullups of 1-3 kohms I can see where a pull down
resistor would have to be low enough that there would not be much point
of using a value larger than 0 ohms. Being very cramped for space,
especially in the area of the mode pins, I removed the pullups all
together and am relying on the internal pullups. But some people here
("here" my work, not "here" the newsgroup) just won't let go of the
bone. They are insisting that the pins must be connected to Vaux
directly because of what they were told a year ago.

I don't want to put parts on the board that aren't required. It seems
pretty durn clear to me that pullup resistors are not needed on the
mode pins at any time, under any circumstances. The board is due to
come out of layout in a couple of days and I don't want to interrupt
progress. I don't understand why I can't get a clear answer to this
question. Peter replied, but didn't answer the question. I honestly
do not trust the support phone line (because I don't actually talk to
the person who gives the answer, just the gopher who relays the
message) and can't document what I am told verbally. I can't get an
account to work to get support by email. So I guess I'll have to
resort to working with the FAE who has already given me bad advice.

.

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