[time-nuts] First few measurements of my Arduino Due GPSDO

Andrew Rodland andrew at cleverdomain.org
Thu Sep 25 15:00:03 EDT 2014

On Thu, Sep 25, 2014 at 3:34 AM, Neil Schroeder <gigneil at gmail.com> wrote:
> Are all the devices you're using or considering capable of hardware
> timestamps?  Or are you doing it in software today?

The PPS from the GPS is timestamped fully in hardware using the "input
capture" function of the hardware timer to copy its value to another
register on a signal edge. The outgoing PPS is generated by the same

The NTP stuff relies on software; the receive timestamp is captured
from the timer as early as possible in the Ethernet ISR, and the
transmit timestamp is captured as the last thing before sending the
packet to the MAC to be transmitted. It's not perfect, but it's as
good as I'm likely to get right now. Some day I might look into
whether I can integrate a PTP-capable Ethernet PHY :)

Right now I have the UDOO board (which is no longer serving as the
mainboard of the clock) running ntp on Linux with both the PPS and the
NTP as sources; numbers from that forthcoming when I have them.


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