What are Pull ups?

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Hi!

I am using SPARTAN 2 XC2S150 for my design. I have a question
regarding pull up resistors. I am using ISE5.1.

1)What is meant by optional pull ups? There are options in the
Constraints Editor to insert pull ups at the I/Os. Are these PERMANENT
pull ups or just during configuration? If I select these pull ups,
then i also need to set M2M1M010%0 ?

2)Alternatively if i DO NOT select those pull ups in the Constraints
editor, but i still set M2M1M010%0 then what will happen? Pull ups
will be used for configuration only?

3) Do i need to connect all my VCCO pins  to 3.3V and VCCINT to 2.5V?
All GND pins on the pacakge be grounded?

4)If I use one clock signal in my design, then it can be input on only
one IGCKO(input global clock buffer) or i need to connect all IGCK's
to this clk signal?

Regards
Rider

Re: What are Pull ups?



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Well, they're only permanent from the standpoint that they are there
after the device is configured.

You do not need M[2..0] set to 100 to get these pullups. They are in the
device configuration. If you set M[2..0] to be 100 you will also get
these pullups at power up and during configuration. If you set M[2..0]
to 000, then the pins will be three-state (hi-z) at power up and during
configuration.

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See previous answer.

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Yes, yes, and yes.

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You should input this on a single global clock. If you input this on
multiple clocks, even if you use a DLL, you may introduce additional
skew between the clocks.

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Re: What are Pull ups?
Rider,

See inline.

Regards, Wei
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The pullup that you set in constraints editor is a post-config setting only.
http://toolbox.xilinx.com/docsan/xilinx6/books/data/docs/lib/lib0338_306.html
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See above
Solution 17240
http://support.xilinx.com/xlnx/xil_ans_display.jsp?iLanguageID=1&iCountryID=1&getPagePath17%240
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As long as clock region rule is not violated. Check datasheet for more
info.
http://support.xilinx.com/xlnx/xil_ans_display.jsp?iLanguageID=1&iCountryID=1&getPagePath17%2401
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