From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailserv2.iuinc.com (qmailr@mailserv2.iuinc.com [206.245.164.55]) by puffin.external.hp.com (8.8.7/8.8.7) with SMTP id KAA27297 for ; Tue, 19 Oct 1999 10:53:21 -0600 Received: from chrome.rose.hp.com (kirkb@chrome.rose.hp.com [15.8.150.209]) by cosrel1.hp.com (8.8.6 (PHNE_17135)/8.8.5tis) with ESMTP id KAA22722 for ; Tue, 19 Oct 1999 10:54:10 -0600 (MDT) From: Kirk Bresniker Message-Id: <199910191655.JAA08169@chrome.rose.hp.com> Subject: Re: [parisc-linux] different PA2.0 CPUs in the same box? To: grundler@cup.hp.com Date: Tue, 19 Oct 1999 9:55:50 PDT Cc: parisc-linux@thepuffingroup.com In-Reply-To: <199910191604.JAA27382@milano.cup.hp.com>; from "Grant Grundler" at Oct 19, 99 9:04 am List-ID: | | The processors can run from a "system clock" and they all run synchronous. | No chance of "clock drift" since they run at exactly the same pace. | But on systems like 870, T-class and N-class, the processors are fed clock | signals from (similar but) different sources. Each frequency domain will | slowly drift from the "average" and thus the processor clocks need to be | resyncronized regularly. | Eh? I can't speak to the T-Class, but N-Class processors (as well as A,B,C,D,E,F,G,H,I,J,K, and L) all run from a single clock source. All of the fundamental problems of clock drift happen between systems, not interior to a system. The crystals used are usally 25~50ppm, which would amount to a second or two per day between systems, which is significant, but is easily sovled using NTP. There is no time clock in the processor itself, but rather a register which is incremented every clock tick. That is why you need to know the frequency of the processor to convert clock ticks to wall time. I believe that there are some wrinkles that can happen in sensing the rollover of the interval timer register under all interrupt conditions, and it may be that this is the 'synchronization' problem that you are thinking about. But, I don't think that it is due to drift on the hardware clock circuits. KMB -- +============================================================+ | Kirk Bresniker (916) 748-2393 | | 8000 Foothills Blvd | | Roseville, CA 95747-5649 | | kirkb@rose.hp.com |