Re: Signal Integrity and series termination

On Aug 30, 2:34 am, Bhavani <bhavanire...@xxxxxxxxx> wrote:
I read somewhere on this forum that we need to have series termination
resistor as close as possible to the driver to avoid signal integrity
issues due to impedance mismatch. In my design I have a micro
controller interfaced with DDR2 SDRAM at 133Mhz. Since the data bus is
bi-directional, in one case micro is the driver and RAM is the load
(WRITE) and in the other case RAM is the driver and micro is the load
(READ) depending on the data flow direction. If I need to terminate
the data bus with 20ohm resistor pack How should I go about it? near
the micro or near the RAM and why?

Thanks
-Bhav

A couple of things...

1) Don't use 20 ohm resistor packs. Why? Because this is usually,
less a data integrety issue, than it is an EMC issue. You're going to
be in the EMC lab, trying to get this thing to pass compliance, and
you'll want the flexibility to tweak each resistor's value. You might
be able to put a 65 ohm in there. Not "theoretically correct", but if
you're scope tells you it's not rounding off your signal too much to
cause signal integrity issues, but is rounding enough so you pass
compliance, than that's what you'll end up with. You won't know
though, till build it and look at it on the scope.

2) Think about the use case. What's going to happen? Is it a data
logger, that writes to RAM constantly, but rarely reads from it? Or
is it program memory, where you're reading from RAM constantly, but
rarely writing to it. Position your resistors based on who drives the
bus most often. If it really is 50/50, then just pick one. It
doesn't really matter that much.
.

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