[time-nuts] NMEA time

[Christopher Hoover]

Hal Murray wrote:
> Yes, mostly.
>
> PCs have two nasty disadvantages in terms of time keeping.
>
> One is that the CPU clock is typically using spread spectrum to make the EMI 
> shielding (a lot) easier.
>
> The other is that the temperature at the crystal depends (a lot) on what the 
> CPU is doing.
>   
These are both problems, but the biggest issue in reducing jitter often 
is minimizing SMM[1].  Some SMI's can suck up 50 us at a go though most 
are less.  On some platforms you may see SMI's executing at a rate of 
more than 1 Hz.

-ch

[1] http://en.wikipedia.org/wiki/System_Management_Mode