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Re: [questions] GPS+PPS vs NTP server, why a huge offset ?

On 21/06/2022 01:31, chris wrote:
Sorry David, but that is incorrect. If you look at the app note you
posted, input capture section, quite clearly states that:

 > When the rising or falling edge is detected at the CCP1 pin,
 > the interrupt flag CCP1IF bit is set.

If you go to a more primary source <https://ww1.microchip.com/downloads/en/DeviceDoc/60001122G.pdf>, you will see, in the diagram on page 15-3, that the logic path for raising interrupts diverges from that for doing the capture, after the event detection, so the capture is not dependent on the interrupt occurring.

Also, on page 15-4, that the actual interrupt can be selectively disabled.

In my original response, I did meant to add that there would typically be an interrupt, but that the interrupt could be handled with low priority (you will get a good result any time until the timer actually wraps round. Even then, the use of an interrupt is not a fundamental requirement.

For a dedicated appliance, interrupts can actually be a liability, as they can introduce race conditions. Nonetheless, I think all current microcontrollers support them.

With precision time, over Ethernet, one would expect the normal Ethernet device driver interrupt handling, but, for example, the device driver might well service several incoming messages on one interrupt. However capturing the event time still means that nano-second accuracy is possible.
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