[ntp:questions] Linux 11-minute mode (RTC update)

Maciej W. Rozycki macro at linux-mips.org
Thu Apr 10 17:02:38 UTC 2008

Hello Serge,

> I guess this problem is due to the reset of the RTC oscillator, right?


> This holds the clock for up to a second, producing no more update nor
> alarm interrupts. But the oscillator still oscillates, so periodic
> interrupts should be undisturbed, during and after this procedure.
> Interrupts have to be enabled during step #4 of course.

 I am not sure what you mean -- according to the datasheet there is no way 
to prevent the update from being scheduled exactly 500ms after 010 (the 
pattern that enables normal operation of the RTC) has been written to the 
divider configuration bits of Register A and the periodic interrupt is 
synchronised to that as far as I know.  The only other patterns that do 
not reset the oscillator are 110 and 111, but they still do reset the 
countdown chain.  This is at least with the Dallas DS1287A chip used on 
the affected platforms.

 Besides it would be harmful if the timer tick was held off for a second 
as it is used for process scheduling (and some networking stuff too).

> I'm not aware of any tool using this method. What are those PIE-timer
> platforms? Do you have one?

 A number of DEC platforms have the RTC chip as their lone source of 
periodic interrupts available.  They include systems based on the VAX, 
MIPS and Alpha processors and I have a number of them (and try to support 
under Linux, time permitting).

 FYI, I had a look at NTP statistics a while ago when I worked on adding 
HPT support for some of these platforms that provide a high-resolution 
free-running counter in the chipset and it turned out these Dallas chips 
(as well as the piece of chipset in question), if left undisturbed by the 
11-minute update, have a pretty darn good precise oscillator.  I do not 
remember the exact numbers anymore, but I was quite astonished by the low 
value of dispersion reached.


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