Re: [chrony-users] PPS Synchronization Verification

[Hal Murray <hmurray@xxxxxxxxxxxxxxx>]

mlichvar@xxxxxxxxxx said:
> Modern hardware seems to be worse. There are other delays than the CPU
> handling an interrupt, e.g. due to various bridges between the CPU and UART.
> On one machine I have measured an offset of about 27 microseconds between PPS
> on serial port and hardware-timestamped NTP (accurate to 1us). 

How did you measure that?

I have a hack that wiggles a modem output signal.  If that is jumpered over to 
DCD it can measure the time to get a signal out and back in to the PPS time 
stamping code.

I'm seeing a bimodal distribution.  One hump at 20-25 microseconds and another 
at 40-45 uSec.  I think the difference is whether the interrupt goes to an 
idle CPU that needs to warm up or goes to one that is already running.

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