Timekeeping oddities on MacMini G4s

Achim Gratz Stromeko at nexgo.de
Sun Feb 5 10:48:57 UTC 2017

Hal Murray writes:
> I think the system bus runs at 166 MHz and the time keeping register
> runs at 1/4 of that.  That would be 41.5.

If they really run it on the system bus frequency, you can generally
expect that to be spread-spectrum modulated for EMI reduction.  Most
common approach is a 0.5% downward spread with a modulation frequency in
the mid-tens of kHz.  Most clock generators can switch between different
spread-spectrum modulations (including none) these days.

> There is a potential problem in this area.  166.666 is really 6
> nanoseconds.  But you order crystals by frequency rather than cycle
> time.  So the crystal is probably 166 or 166.6 depending on how many
> digits the order form had.  Maybe 167 or 166.7 if somebody rounded up.
> But maybe they wouldn't round up because that might push something
> over a timing spec.  Mumble.  If it were easier to take apart, I'd
> look inside to see if I could find the crystal and see what was
> printed on it.

The crystal probably isn't 166MHz or anywhere near that.  The clocks are
generally generated by a system clock generator IC via PLL from some
reference frequency.  I haven't found pictures of the mainboard with
high enough resolution to read the markings, but on the lowres ones you
can make out two oscillators on the board (one metal can and one SMD).
I'd guess that the SMD would be 100MHz and is used as a reference for
all things hanging off the system bus (25MHz Ethernet, 48MHz USB, 166MHz
bus, etc.).  It could also be a dedicated clock for PCI-to-USB bridge
that's next to it (should be either 24MHz or 48MHz in this case), but
then there'd most likely be a third one hidden someplace else (under the
CPU module for instance). The metal can one presumably drives the RTC,
so most likely 32678kHz.  But then it could be related to the modem
nearby (unlikely, I'd expect that clock to sit on the modem card).

There's an instructable on iFixit on how to get these open and the board
out, but I'm not sure you'd want to do that while it still works.

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