[ntp:questions] drift modeling question

David Woolley david at ex.djwhome.demon.co.uk.invalid
Sun Jul 13 11:34:17 UTC 2008

Hal Murray wrote:
> Most PCs have 2 xtals.  One at 14.xxx MHz (cheap, 4X color burst)
> that drives the CPU and most motherboard logic through a magic clock
> generator (PLL) chip, and another that is a 32 KHz watch crystal for
> keeping time when the CPU is off.  The latter also makes interrupts
> for the scheduler.

Historically interrupts from the 32kHz clock have not been used, except, 
possibly, in powered down states to initiate a restart from suspend or 
hibernate.  It is possible that has changed very recently, but they 
certainly weren't used historically.

> I had the temperature probe on the 14 MHz xtal.  It didn't work very well.
> I was assuming that the system used something like a cycle counter
> (TSC) for timekeeping.  That has troubles in multi-CPU systems.

I think the most common way of doing timekeeping at the 1ms and higher 
level is to use the counter timer, which is driven from a signal divided 
down to approximately 1MHz.  TSC is mainly used to interpolate between 
ticks or to detect missed ticks.

> The code I was actually running used the timer interrupts for
> timekeeping and the TSC to interpolate between ticks.

but note that the timer interrupts are not based on the 32kHz 
oscillator, in the typical system.

More information about the questions mailing list