Question about SSTL

I'm about to test a DDR2-SDRAM controller. I have a question regarding the use of SSTL IO standard in Virtex-4: Is it needed to connect the Vref? In the user guide for Virtex-4 (ug70.pdf the bottom of page 290) I don't understand the note n2 which is related to Vcco for inputs: "Differential inputs and inputs using VREF are powered from VCCAUX. However, pin voltage must not exceed VCCO , due to the presence of clamp diodes to VCCO."

Another question by the way: Which supply should Vccaux have? 2.5v?


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See below,


GaLaKtIkUs? wrote:

For a driver, no. For a receiver, yes.

This is telling you that the differential amplifier (comparator) and the Vref comparator are powered from Vccaux. That means that in order to meet the speed specifications for an input, the pin voltage should remain inside the limit of ground to Vccaux, better yet, 0.6 volts above ground, and no more than 0.6 volts below Vccaux. To go beyond this range means the full CMOS differential amplifier (comparator) may not be operating in its linear range. The comparator will function well beyond this range, but it is recommended to stay within the limits of the SSTL standard for pon voltages.

There are clamp diodes (intrinsic to the IO drivers) from ground to the pin, to Vcco.

gnd ----->|------pin------->|-----Vcco

Is that clear? These diodes are always there (that is what 'intrinsic' means: part of the device).

They have always been there: since Virtex II in the Virtex line. The top diode to Vcco was not there in Virtex, or Virtex E. That was a long time ago.

It has also been there in Spartan 3, and Spartan 3E, but not in Spartan

2, or 2E.

So, diodes = S3, S3E, V2, V2 Pro, V2 Pro-X, V4, V5.

Vccaux is always per the data sheet: 2.5 volts. In this case, 2.5 volts. Maybe someday it will change, but so far it has not changed since it was first introduced. In any event, table 2, section 3, of the data sheet is what is important. That is where you find the recommended operating voltage ranges.

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Austin Lesea

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