What are Pull ups?

Sep 12, 2003 2 Replies
R

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 M2M1M0=100 ?


2)Alternatively if i DO NOT select those pull ups in the Constraints editor, but i still set M2M1M0=100 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


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M

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.

See previous answer.

Yes, yes, and yes.

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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C

Rider,

See inline.

Regards, Wei

The pullup that you set in constraints editor is a post-config setting only.

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See above

Solution 17240

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As long as clock region rule is not violated. Check datasheet for more info.

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