3.3V tolerant configuration interface Spartan 3

This is with reference to page 33 of Xilinx's Spartan 3 Family: Functional Description. Paraphrasing from the documentaion: "To power the Dual-purpose config. pins apply 3.3v to VCCO_4 and VCCO_5. For the Dedicated Configuration pins apply 2.5V to the VCCAUX lines. In order to acheive 3.3V tolerance, the dedicated inputs requires series resistors that limit the incoming current to 10mA or less".

OK.

So on my board, I have hard-wired all the VCCO banks to 3.3V, VCCAUX to

2.5V and VCCINT to 1.2V.

Am I am missing something here or why would use resistors on the Dedicated pins which are already operating at 2.5V while the dual-purpose pins are 3.3V.

I know it something really small print I have forgotten to catch.

I appriciate all your help.

Thanks Yaju N

------- BYU ECEN Dept. Provo Utah

Reply to
Yaju Nagaonkar
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Allrite. I guess I just needed to use Google rather than google groups to answer this. I found the answer on Xilinx Answers Database.

The resistor is used to drop voltages of the incoming signals to the dedicated pins (operating at 2.5V) which might come from a 3.3V source. The VCCO is kept at 3.3V so that the dual config pins can be operated at the corresponding voltage.

For reference of others the 3.3V tolerant configuration schematics are at :

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-Yaju

Would it hurt Xilinx to update its data sheets with all the relevant information? Maybe they should just release the Spartan -3 Architecture :). If it is protected by patents, why would it hurt to make it open source. Ok, don' answer that. That was my meek attempt at sarcasm. Sorry.

Reply to
Yaju Nagaonkar

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