[ntp:questions] drift modeling question

David Woolley david at ex.djwhome.demon.co.uk.invalid
Sat Jul 19 00:02:33 UTC 2008

Unruh wrote:

> There is the rtc, and then there is the timer chip on the PC board. I
> thought that they were very different. The RTC is the on board real time
> clock powered by a battery on the cmos. The timer interrups are a divisor
> of the main bus clock that drives the computer bus I believe. I thought we
> were talking about the rtc, not the bus clock. ( int 8 not int 0)

That's what I was saying.  Martin was suggesting that the RTC was being 
used, and that alternative sources were more common than in reality. 
The RTC runs off the 32kHz crystal.

Note that on the PC/AT, for which I have detailed circuit diagrams, the 
main bus clock and the timer clock are distinct.  I think that is 
because the original clock was right for the PC but too slow for the AT. 
  They have merged again, because the crystal frequency is now 
multiplied to get the main bus clock.

HPET timers are recent, and generally use the non-32kHz crystal, i.e. 
they have the same temperature dependence as the counter timer interrupt 
and TSC, because they are derived from the same oscillator.

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