The short answer is you need to design the system as a transmission
line system.
I don't know of any standard SDRAM devices rated at 200MHz - for those
speeds you need either DDR / DDRII / some other later spec. The notes
below, however, still fully apply as much at 50MHz as at 200 (except
the timing budget is easier).
If you have never designed length matched / impedance controlled
systems, then I suggest allowing yourself at least a month (maybe 2) to
understand the implications. There are specific relationships involved
between clocks, address, data and controls, and you will also have a
system timing budget. DDR in particular is quite challenging in this
regard, as the data strobes are mastered by both the controller and the
DDR device itself (Controller owns them during write, memory device
owns them during read).
If you do not understand all the things I just mentioned, I would
strongly advise you to consider carefully whether you need to interface
as you mentioned. If the answer is Yes, then I suggest looking at the
copious resources for such things on the web and particularly at Micron
(
www.micron.com).
You will also need a capture/layout tool that can length match to
within 0.030 inch (if you are particularly fortunate in your choice of
device, or perhaps tighter if you are using a module).
Interfacing onto SDRAM/DDR-SDRAM is a time consuming and non-trivial
task, so be prepared for a large number of calculations, which although
simple, can seem rather tedious.
I have put SDRAM and DDR systems down on a circuit board both in
modules and as ICs, and I can assure you it is not 'simple'. Much
depends on the timing budget of the SDRAM controller (in the host), and
by the time you take the varables from that and the devices themselves,
it can be quite a challenge to get SDRAM running at the speed you want
Cheers
PeteS
