Regarding your voltage swing, I suspect that what you are seeing is drive strength limitations of the driver. If you divide the voltage differences between the drive level and Vtt by the termination resistor, you will probably find the rated drive strength of the buffer. Once the low terminator is removed, the driver can drive to the rails. best regards, MG -----Original Message----- From: Aubrey_Sparkman@xxxxxxxx [mailto:Aubrey_Sparkman@xxxxxxxx] Sent: Tuesday, January 15, 2002 2:03 PM To: arsenault_brian@xxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Question about SSTL Termination Brian, Actually this does make sense. A 50 ohm resistor to the Vtt of 1.25 Volts is NOT a pull-up, but a "pull to center". So your swing to each rail is loaded equally and you are seeing that the pull-up and pull-down drive strengths aren't equal. And my GUESS is that you can use a series termination in some cases, but YOUR SIMULATIONS should guide your decision. Good Luck, Aubrey Sparkman Signal Integrity Aubrey_Sparkman@xxxxxxxx (512) 723-3592 -----Original Message----- From: arsenault, brian [mailto:arsenault_brian@xxxxxxx] Sent: Tuesday, January 15, 2002 12:50 PM To: 'si-list@xxxxxxxxxxxxx' Subject: [SI-LIST] Question about SSTL Termination Folks... I'm working on a design within our company that is utilizing SSTL buffers. We are working with a semiconductor vendor to add these buffers in as selectable vs. LVTTL. Since we are in the definition stage, we also requested that the termination needed for SSTL be built internal to the device, reducing the number of external discrete components. As a note, we are looking at SSTL2... One question that came up was, 'Do we really need the pullup resistor (50 ohms to a Vt of 1.25V) or can we live with just the series terminator?'. The simulations I have run with the pullup indicate a clean signal that swings between 600mV and 1.8V. When I remove the pullup, the signal now swings between 0V and 2.5V. My question is, does this make sense? I have tried models from two different vendors (Xilinx and LSI Logic). After reading the JEDEC spec on SSTL, and also the vendor datasheets, I can't come up with an explaination. Appreciate the help. Brian Brian Arsenault Sr. Signal Integrity Engineer EMC Corporation Hopkinton, MA 01748 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu