Cyclone II SSTL-2 on-chip resistors

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 http://www.altera.com/products/devices/cyclone2/features/onchip/cy2-onchip.html)
while it should be 25 ohms (according to http://www.interfacebus.com/SSTL_Logic_Interface.html
- 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

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

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) ?