From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756881AbXLDVie (ORCPT ); Tue, 4 Dec 2007 16:38:34 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754261AbXLDViM (ORCPT ); Tue, 4 Dec 2007 16:38:12 -0500 Received: from mail1.webmaster.com ([216.152.64.169]:3675 "EHLO mail1.webmaster.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753567AbXLDViJ (ORCPT ); Tue, 4 Dec 2007 16:38:09 -0500 From: "David Schwartz" To: Cc: Subject: RE: Is the PCI clock within the spec? Date: Tue, 4 Dec 2007 13:37:31 -0800 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook IMO, Build 9.0.6604 (9.0.2911.0) Importance: Normal In-Reply-To: <47556E48.40703@free.fr> X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3198 X-Authenticated-Sender: joelkatz@webmaster.com X-Spam-Processed: mail1.webmaster.com, Tue, 04 Dec 2007 13:38:48 -0800 (not processed: message from trusted or authenticated source) X-MDRemoteIP: 206.171.168.138 X-Return-Path: davids@webmaster.com X-MDaemon-Deliver-To: linux-kernel@vger.kernel.org Reply-To: davids@webmaster.com X-MDAV-Processed: mail1.webmaster.com, Tue, 04 Dec 2007 13:38:48 -0800 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > > A scope probe will allow you to see if there is > > a clock signal. That's all. You can't determine > > its quality. A 4-inch ground lead on the scope > > probe will result in 10-20% overshoot and undershoot > > being observed. > I don't understand this 10-20% figure. > (0V + 10-20% is still 0V.) If you're jumping from a 900 foot marker to a 910 foot marker, does a 10% overshoot mean you jumped 1 foot too far or 90 feet too far? The percentage is of the distance you were trying to go, not of where you started or where you ended up. > AFAIU, the nominal peak-to-peak voltage is 3.3V. The observed > peak-to-peak voltage is 6.08V (3.3V + 84%). So a 10% undershoot would mean that rather than going from 3.3V to 0V, you overshot 0V by 10% or the distance you travelled. The voltages could just as well be 100V and 103.3V, the transitions would still be the same. What you call zero is, at least in principle, arbitrary. DS