[ntp:questions] Reading the RTC

David Woolley david at ex.djwhome.demon.co.uk.invalid
Thu Sep 27 21:37:53 UTC 2007


In article <46f9361c$0$21478$426a74cc at news.free.fr>,
"Spoon <root at localhost>" wrote:

> systohw reads the current system time, then sleeps until the next whole 
> second, then writes that date to the RTC.

This is arguably off topic as manipulationg the RTC is incompatible with 
running ntpd on Linux, at least when using the kernel clock discipline code.
Under these conditions, the kernel updates the RTC every 11 minutes.

> However, it seems like the RTC raises the IRQ when the fraction of a 
> second is 0.5, not when it is 0.

You can see this 0.5 second offset in the kernel code that implements the 
11 minute update.  You can also look at the implementation of hwclock.

If you want the fine details, you should look at the data sheet for the
original MSI clock and memory chip.  Modern ASIC data sheets often assume
that you have access to the specifications of the components that are
being emulated.




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