[ntp:questions] how do I lock in average frequency correction

unruh unruh at invalid.ca
Tue Feb 14 17:46:58 UTC 2012

On 2012-02-14, Ron Frazier (NTP) <timekeepingntplist at c3energy.com> wrote:
> On 2/14/2012 9:36 AM, unruh wrote:
>> On 2012-02-14, Ron Frazier (NTP)<timekeepingntplist at c3energy.com>  wrote:
>>> On 2/13/2012 3:25 PM, Chris Albertson wrote:
>>>> On Mon, Feb 13, 2012 at 10:59 AM, Chuck Swiger<cswiger at mac.com>   wrote:
>>> Perhaps a silly question, but, does the "tick" that drives the OS
>>> software clock originate from the RTC or from the CPU master clock at 2
>>> GHz or whatever?  Just trying to understand how this stuff works.
>> Not clear what thei question has to do with frequency stablizing a xtal,
>> but the system clock is linked to the cpu clock.
> I was simply wondering which crystal is the perpetrator.  I always 
> thought the CPU clock was highly accurate.

Accurate it is not. It does not need to be. All it has to do is tick,
and the cpu really does not care ( within bounds) what rate it is
ticking at. It also does not care if that rate changes in time. Ie, it
does not really need to be a good clock. In fact on some modern
machines, its rate purposely fluctuates. 
It is certainly sensitive to temperature fluctuations. The cpu clock
drives the interrupts on the timer interrupt chip. The cpu clock also
drives the counter in the cpu which is used to interpolate the fine time
between the major clock ticks from the timer interrupt. 
In many ways it is astonishing that the PC architecure allows timing
down to the usec level with any accuracy at all. 

> Ron

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