[SI-LIST] Re: Data Bus termination

Dear Ravinder,

Thank you for your comments.

A rule of thumb I know related to series termination of a bidirectional
point-to-point
topology can be summarized as:

For a short bus with slow edges (weak drivers) a single resistor at the
middle of transmission line can
prove sufficient; otherwise, two terminators may be required (one near =
each
agent).

However, as indicated by your email the more accurate way for =
determining
the requirements for
one or two resistors is by means of simulation.

Best Regards,

Abe=20

=20

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
On
Behalf Of Ravinder.Ajmani@xxxxxxxxxxxxxx
Sent: Wednesday, June 29, 2005 1:10 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Data Bus termination


Abe,
You are correct that for a bidirectional bus, series terminations may be =

required at the both ends of the bus.  Depending on the driver =
impedance,=20
one may be able to manage with only one termination.  I was simulating =
the=20
interface between our memory controller ASIC and a single DDR2 DRAM.  =
The=20
memory interface impedance can be programmed to 20 ohms or 40 ohms.  I=20
found out that with 40 ohm impedance driver, I could manage with only =
one=20
termination at the DRAM end.  However, with 20 ohm driver, I needed=20
terminations at both ends.  I tried moving the single termination around =

(at either end as well as in the middle), but the overshoot was always=20
excessive.=20

Regards, Ravinder
Server PCB Development
Hitachi Global Storage Technologies

Email: Ravinder.Ajmani@xxxxxxxxxxxxxx


"ariazi" <ariazi@xxxxxxxxxxxxxxx>=20
Sent by: si-list-bounce@xxxxxxxxxxxxx
06/29/2005 11:27 AM
Please respond to
ariazi@xxxxxxxxxxxxxxx


To
si-list@xxxxxxxxxxxxx
cc

Subject
[SI-LIST] Re: Data Bus termination

Dear Mahesh,

Series termination is the most common selection for a point-to-point
topology.

When the bus is unidirectional, the ideal placement for the source
terminator is close to the driver. For series termination of a =
bidirectional
bus two resistors=20
may
be required
(one near each agent) or just a single resistor some where in the middle =

of
the line.

There exist rules of thumb and mathematical formulas for determining the
effective=20
locations and values for series terminators. However, the need for and =
the
optimum=20
termination solution is usually ascertained with aid of simulation.=20
=20
Best Regards,

Abe Riazi
ServerWorks
=20

=20
=20


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