[SI-LIST] Re: Signal at the driver

  • From: "Carrier, Patrick" <Patrick_Carrier@xxxxxxxxxx>
  • To: "shilpa.p@xxxxxxxxxxxxxx" <shilpa.p@xxxxxxxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Mon, 17 Sep 2012 18:23:07 +0000

Hello Shilpa--
This is a fundamental property of any bus that does not have the driver, 
T-line, and receiver perfectly matched (which is pretty much any bus) - there 
are reflections.
In fact, DDR3 has fast enough edges that you will see a difference in the 
receiver waveform at the pin versus the waveform at the die.  The shape of the 
waveform is determined by how those reflections line up, and will look the best 
at the endpoint of the bus (the die).

We walk through an example of propagating reflections and the resulting 
waveform shapes in our Transmission Lines, Reflections, and Terminations 
Webinar available at:
http://www.mentor.com/products/pcb-system-design/series/si-basics-series

Because of reflections, it is important to note that the waveform viewed at the 
driver (in measurement or simulation) isn't the same waveform that is "sent 
out" onto the bus.  That is why timing is measured from the driver into a 
standard test load, and should not be measured from driver waveform to receiver 
waveform.  This topic is discussed in the Timing Webinar at the link above.

--Pat



BTW, you may also be interested in our webinar on designing DDRx busses:
http://www.mentor.com/products/pcb-system-design/multimedia/ddrx-design-webinar/


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Shilpa Patil
Sent: Sunday, September 16, 2012 11:46 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Signal at the driver

Hi SI Experts,
I have been carrying out the analysis for DDR3 memory. My query is that at  the 
driver end  I can see a signal with a dip and shorter peak-peak the signal is 
not a pulse  even though I have  given a pulse, but at the receiver the signal 
is good. So why is the signal at the driver so not a proper pulse? 

Regards,
Shilpa P
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