Cyclone II SSTL-2 on-chip resistors

Oct 10, 2007 2 Replies
S

Hi,



I'm trying to build a board that will use a DDRAM PC2700 memory module connected to a Cyclone II FPGA.



It uses SSTL-2 signaling, and one thing I really don't understand is why the on-chip series resistor for SSTL-2 seems to be 50 ohms (according to

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while it should be 25 ohms (according to
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- I've also measured the series resistors of my memory module and they are about 25 ohms). I think I'm going to use SSTL-2 Class I for simplicity (btw I wonder why Class II exists ? it just uses more components and sucks more power).



Any suggestion ?



Thanks,



Sebastien



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A

Sebastien,

The class II is bidirectional.

Class I is unidirectional.

I have no comment on the Altera part, the series component is not supposed to be 50, but as you note, 25 ohms.

I suspect it is a typographic error, you should report it to them.

Austin

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S

Ok, thanks for the info.

So I think I should use Class II for data lines (two 50 ohm terminations connected to Vtt, at both ends), and Class I (one 50 ohm termination connected to Vtt, at the receiver) for all the others ; and use the on-chip 25 ohm series resistors at the FPGA, and the 25- ohm series resistors included on my DDRAM module. No additional termination is required for clock lines (except the FPGA's on chip series resistor) as these differential signals already have a 120 ohm resistor across them on the DDRAM module. Is that correct (given the price of 4-layer prototype boards and the difficulty in reworking BGA packages, I'd like to be absolutely certain) ?

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