[SI-LIST] Re: series resistor on SSTL-2

Jeremy-

The in-line (series) resistor is for source termination of the bus. It 
is placed at the driving end of the line close to the driver.
The sum of the output resistance of the driver and the series resistor 
is approx. 50 ohms and makes the driver end load look matched with 
respect to the transmission line. So as you surmised, it is indeed there 
for SI reasons. You'll also find that the series resistor and the shunt 
resistor tied to Vtt at the receiver form a voltage divider that serves 
to ensure that the proper sstl signaling levels are presented to the 
receiver.

-Ray

Leong wrote:

>Hi all
>I am not sure why there is a in line resistor (Rs) on SSTL-2 design and this 
>in-line resistor was placed close to the DDR SDRAM DIMM.
>
>I understand DDR protocol and still I cannot figure out the purpose of this 
>in-line resistor (Rs). Is that something have to do with signal integrity ? If 
>it is, why don't we place this resistor close to the driver side (North 
>bridge) ?
>
>I am lost, can someone please do a brief explanation or send me a link so I 
>can read more on it.
>
>Thanks in advance
>
>Jeremy
>
>  
>
.

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