Data/Address line Matching

[eeh]

Hi,

I am going to design a DSP system which needs to interface with SDRAM.
The system clock is running at 200MHz. I have calculated the wavelength
to be 15mm.

The question is whether do I need to regard each data line and address
line as transmission line and do matching for each line?

Thanks!

 

[PeteS]

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

 

[eeh]

Thank you for your reply. Actually, I have basic electonic knowlege of
transmission line and the DSP chip I used have SDRAM interface.

I remembered that once time a friend told me the parallel port (PC)
cable needs termination matching resistors at the receiving side.
Otherwise the signal will be incorrect. So I raise this question for
200MHz data transmission.

Besides, I got an old SDRAM PC100 board and found there are 8 SDRAM
chips. For each SDRAM chip there is eight 10ohm resistor chips
connecting to it. I have not trace how they connect to the SDRAM chip.
Do anyone know what are their functions?

 

[PeteS]

You will find application data in the data sheets and related
application notes / tech articles from the manufacturers site. I
suggest looking around the major manufacturers (Samsung, Micron,
Hitachi [although that was spun off and I do not recall the new name])
and look at the data sheets.

The resistors will be series terminators and perhaps parallel ones - a
lot depends on the old unit.

I will repeat that this requires knowledge a little more advanced than
'basic', though

Cheers

PeteS