On 20/06/2022 15:54, chris wrote:
Still doesn't explain how the pps state change gets into the system at hardware level. Devil in the detail, as usual. It sounds like a polled system, not an interrupt driven one,
Interrupt system are I think pretty much always polled at the hardware level.
which can never be as accurate as the latter. Assuming a poll rate of say, 1mS, not unreasonable for a modern cpu, Then if the pps state change is not seen at one poll, but is seen at the next, then the uncertainty can never be better than that
The uncertainty is the clock period in the capture hardware, because the hardware counts and returns the the number of its clock cycles, so you know very precisely how long it was from the the PPS to when you polled. Actually I think something like that is pretty standard for most microcontrollers.
I believe there are Ethernet cards that do this to support, I think, Precision Time Protocol, which I suspect is what high frequency traders use.
1mS, since it cannot be known when during that 1 mS the state transition occurred. Best results for ntp really need a true real time os, but it's far more common to use general purpose os's because of the out of the box functionality., but not ideal.
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