[time-nuts] A (slightly) different apu2 question
wojciech at owczarek.co.uk
Thu Nov 17 09:42:53 EST 2016
...do excuse the slightly off-topic 5(insert your preferred sub-currency
I think many people put a significant effort into getting a precise pulse
into a board like the APU2, losing a lot of precision and accuracy by
getting it into the O/S software clock, and then perfectly waste what's
left by then retransmitting it via NTP with software timestamping. I am not
saying that there is anything wrong with NTP, just that software packet
transmission and receipt timestamps ruin both precision and accuracy.
The APU2 is a real gem for many reasons, but one is of significance for
time sync. The board uses discrete i210 PHY/MAC chips which support
hardware timestamping (of all packets, not just PTP) - rather than using an
in-SOC Intel MAC paired with a different PHY like Marvell or, diety forbid,
Realtek. When designing the board, Pascal did the time nuts a huge favour
by presenting pins 60-63 of each i210 as test pads on the board, four per
chip. These pins are software-defined I/O pins which can be used for 1PPS
output, arbitrary square wave output, and input event timestamping - all
tied to the i210 hardware clocks.
I have exchanged a few e-mails with Pascal and he hinted that he will
consider breaking those i210 pads either as a pin header or possibly as
u.FL connectors - maybe not all, but say two per NIC. The only thing that
could make this board better (but then it would be a different board) is if
it used the latest generation of Intel Atom, where they implemented counter
correlation between the CPU and NICs, allowing for very tight O/S clock to
NIC clock sync. But that Atom was only recently announced (Skylake CPUs do
this already), and an AMD CPU was selected for the APU2 for specific
Another small board with similar potential is the upcoming Minnowboard
Turbot Dual-E (
Atom-based, small, with two i211 right on the board.
On 17 November 2016 at 13:04, Attila Kinali <attila at kinali.ch> wrote:
> On Wed, 16 Nov 2016 15:24:06 -0800
> Jay Grizzard <elfchief-timenuts at lupine.org> wrote:
> > On the apu2, this crystal is easily accessible (at least as easy as
> > anything SMD is). Can anyone think of a reason that it wouldn't be
> > feasible to replace this crystal with an external reference, à la the
> > widely known clockblock + Soekris net4501 hack (but with 64x the RAM)? I
> > figure the higher frequency might make it a bit trickier to get the
> > signal to the board intact, but is there any other good reason this
> > wouldn't work?
> You should check with Pascal Dornier (the guy behind pc-engines).
> He can exactly tell you what would work and what wouldn't.
> And he is generally very very helpful.
> Attila Kinali
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