[Open-graphics] DDR Vtt termination ?

["Ing. Daniel Rozsnyó"]

Sebastien Bourdeauducq wrote:
>> P2P for me is a `direct chip to chip trace without branching`, in this
>> case also for 1 driver and 1 receiver per line (not counting address
>> lines now - they are a mystery even after reading the above pdfs, that
>> the termination is same for load of 2 (data) and 16 (addr) when using 2
>> modules with 8 chips..).
> 
> Maybe that's because the chip's input impedance on the address lines is high 
> compared to Rt ?

It should be no higher than the impendance on data inputs.. (from 
diagrams the sstl2 receiver uses an opamp/comparator).

Difference is that these pins are used as always IN, while data is 
bidirectional.

SSTL2 doc (from you previous link) shows termination as optional (pdf 
page7, section 1.2), unnecesary for short traces (page 16, section 4.1). 
Section 4.2 is given as an example of long lines - where they mention 
the use as for the address lines..

--
PS: That gives another point to try for your application: terminate 
first the address lines only, they are long (over 4 or 8 chips on the 
sodimm). The data lines are shorter, maybe it will work without Rt. The 
consumption should be in this partial termination reasonable.