Strictly these resistors should be present. However, under certain conditions, Its possible to get away with not putting them on.
On OGD1, we can only run the memories at DDR400 (200MHz) due to FPGA speed limitations. For SDRAM these days, that's pretty slow. Also, we only have to deal with point-to-point links on the data lines. Under these conditions, series termination at the source ends of the memory data lines is OK. There is an application note on the Micron site somewhere that details this approach and gives some test results. To cut down reflections, we use series resistors at the DRAM end to match the 50-ohm-or-so trace impedance and we control the driver impedance at the FPGA end by drive strength selection. Doing series termination is a big plus, because it cuts down the power. Parallel termination would have probably required a fan on the FPGA and heatsinks on the memories. On 10/16/07, Sebastien Bourdeauducq <[EMAIL PROTECTED]> wrote: > Hi, > > I've had a look at the board schematics from Transversal Technology, and I > wonder why termination resistors that connect the DDRAM signal lines to a > 1.25V (Vtt) supply are not present. From what can be read in various sources, > DDRAM uses SSTL-2 signaling which requires that resistor (see > http://download.micron.com/pdf/presentations/dram/plat7justin.pdf p.8 for > instance). > > Is that because the PCB traces are short enough that you can get rid of that > resistor and still get good signal integrity ? > > Regards, > > Sebastien > _______________________________________________ > Open-graphics mailing list > [email protected] > http://lists.duskglow.com/mailman/listinfo/open-graphics > List service provided by Duskglow Consulting, LLC (www.duskglow.com) > _______________________________________________ Open-graphics mailing list [email protected] http://lists.duskglow.com/mailman/listinfo/open-graphics List service provided by Duskglow Consulting, LLC (www.duskglow.com)
