[time-nuts] A (slightly) different apu2 question

Jay Grizzard elfchief-timenuts at lupine.org
Wed Nov 16 18:24:06 EST 2016


So there's been a lot of discussion going around on how to do GPS foo on 
pcengines.ch's apu2 hardware, but there's one question I haven't seen 
discussed ... which I'm now going to discuss. Or at least ask about.

I can't find a public datasheet for the actual processor in these (a AMD 
GX-412TC SOC), but looking at datasheets for similar AMD chips, this SOC 
seems to use a single 48MHz external crystal from which all the other 
system clocks are derived (save for the 32.768kHz RTC).

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?

The CPU itself is four cores (no hyperthreading), so I'm figuring 
dedicate one core to PPS handling (should give really low-jitter 
interrupt handling), maybe one to ntpd, and combined with that precision 
reference, a pretty nice NTP/PTP server should pop out the other side. 
The ethernet on the apu2 even does hardware timestamping.

Can anyone think of a reason this wouldn't work, before I break out the 
rework gear?

-j



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