From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <3A9F225D.7040807@bigpond.com> Date: Fri, 02 Mar 2001 15:32:29 +1100 From: Brendan J Simon Reply-To: brendan.simon@bigpond.com MIME-Version: 1.0 Cc: linuxppc-embedded Subject: Re: linuxppc-embedded: mpc8260 bus frequency calculations References: <3A9EFA26.8030805@bigpond.com> <20010301202024.A7876@lx.c-side.com> Content-Type: text/plain; charset=us-ascii; format=flowed Sender: owner-linuxppc-embedded@lists.linuxppc.org List-Id: I agree that most systems have some error, but about 1 second every minute is not good :( This error can be substantially reduced by more accurately specifying the clock frequency. We have a custom board with a 66.6666MHz oscillator module on it. There is no way to specify this accurately with the current interpretation of the board_info structure. The best one can do is specify 66,000,000 or 67,000,000 Hz :( I would like to see the board_info structure members be interpreted as Hz rather than MHz. I think this must have been the case earlier because I saw some code that was commented out which looked something like "/*** / 1000000 ***/". ie. the Hz value was being divided by 1000000 to display MHz. Now someone (Dan ?) changed the interpretation of the board_info structure for a reason, and I'm wondering if I will break anything if I change the meaning to Hz. I'm sure there would be some easy work arounds by just dividing by 1000000 where necessary. Brendan. Brendan. Neil Russell wrote: > If bi_busfreq is set to 66.0 and the crystal is 66.6666, then the system > clock should appear to go too fast. Sounds like what you have is the other > way around. I've seen plenty of crystals that are running at 66.0 MHz. > Do you have one of these? > > The system time is set once at boot up from the RTC. If the system > gains/looses time by some fraction of a percent, then this may well > be just error in the crystal. Most systems I've seen have some error. > The program "adjtimex" exists to correct this. > > > Neil. > > > On Fri, Mar 02, 2001 at 12:40:54PM +1100, Brendan J Simon wrote: > >> I've come accross a problem where our custom embedded 8260 board is >> losing time. I've tracked it down to the bi_busfreq value being set to >> 66 instead of the true value of >> 66.6666666666666666666666666666666666666. One solution is to have the >> board_info fields in units of Hz instead of MHz. I could then set the >> bi_busfreq value to 66666667 for much better accuracy. >> >> Another solution might be to regularly read the real time clock. Does >> linux do this normally or does it just read the RTC once at boot up and >> then just rely on it's internal timers for the system time. I can't see >> how this can be very accurate. >> >> Thanks, >> Brendan Simon. >> >> > ** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/