Ken I have never seen a 5V tolerant 3.3V device that had a clamp device tied to 3.3V. I have seen a few 5V tolerant parts that had clamps that turned on past 5.5V. 99.999% of the devices we have measured have ground clamps. If you have overshoot (defined as a short duration spike above the normal high or low level of the driver) of 2 Volts in the rising direction you will likely also have it on the falling edge. You will need to control it in both directions. Tom Dagostino Teraspeed(R) Labs 13610 SW Harness Lane Beaverton, OR 97008 503-430-1065 tom@xxxxxxxxxxxxx www.teraspeed.com Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI 02882 401-284-1827 -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Ken Willis Sent: Friday, January 13, 2006 6:25 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Overshoot / Undershoot Hi, I had a slightly different question related to this. Some 3.3V devices are 5V-tolerant, and per their specs can tolerate quite a bit of overshoot. Let's assume for a minute that the overshoot you will see is within the device's spec, and is such that the timing will still be OK. So everything should be functional and within spec for SI/timing purposes. Some of the EMI folks I have worked with in hardware development have expressed concern about leaving a bus like this with excessive (ex. 2v) overshoot, even if it is within the device specs. The thinking here is that every time the bus switches, it can have 64 (or however many) bits with overshoot at their receivers, turning on their associated clamp diodes, and shooting nice spikes of current into the power/ground system. Some view this as a bunch of little noise sources, exciting the planes, and potentially lighting heatsinks, cables, other critical components, etc. nearby. So for EM compliance reasons and general robustness of the system, it seems like it may be desireable to terminate buses of this nature, even if it is OK by traditional SI/timing points of view. I was curious to see if folks are generally content to let buses like this thump away with the overshoot, or if they usually strive to terminate them in their typical methodologies. Ken -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Bill.Cohen@xxxxxxxxxxxxxxxx Sent: Thursday, January 12, 2006 7:45 PM To: andrew.seddon@xxxxxxxxxxxx Cc: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Overshoot / Undershoot Andrew, JEDEC has an overshoot/undershoot specification for DDR2 a 1.8V technology. Older technologies relied on an absolute voltage but the newer technologies work on an absolute maximum and an area (above/below power) of stress. This area limits the overshoot stress into a maximum stress for a period of time. The long time reliability of the gate oxide is at issue here and the duty cycle of the signal also comes into play. Look at the DDR2 datasheet (JEDEC.org) and look at the stress model they have for undershoot/overshoot of input signals. We have incorporated this model into our latest specifications. Best Regards, -------------------------------------------------------------- | Bill Cohen | Toshiba America Electronic Components | Mixed Signal Design Group -------------------------------------------------------------- =20 "Andrew Seddon" <andrew.seddon@ca msig.co.uk> To=20 Sent by: <si-list@xxxxxxxxxxxxx> si-list-bounce@fr cc=20 eelists.org =20 Subject=20 [SI-LIST] Overshoot / Undershoot 01/12/2006 07:30 PM =20 =20 Please respond to andrew.seddon@cam sig.co.uk =20 =20 Hello, I was wondering if anybody had an idea of what is a typically acceptable bad but workable overshoot/undershoot on a 3.3V system? For example I see some memory IC's can take transients upto 5.5v where as the datasheet max is say +0.3. Obviously this figure is based on DC. I presume the major effect's of over/under shoot are to reduce working life of the IC and make the circuit potentially fail at temperature extremes? So when you guys analyse overshoot/undershoot how do you decide if it's acceptable? Kind regards, Andrew ------------------------------------------------------------------ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: =20 //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 =20 ------------------------------------------------------------------ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org 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