Time transfer over USB can be improved by timestamping on both ends, then using a robust estimator for the clock offset. For example, imagine the USB is a small microprocessor peripheral. It has a local timer, freely incrementing, based on its local clock. When it gets a USB interrupt from the host, the timer state is read, and the USB message contains the host timestamp.
This is enough information for a single-shot clock comparison. It may be contaminated with operating-system latency or any number of other host latencies (bus, cache, etc.). But generally, with a lightly loaded host, the USB transaction goes as fast as it possibly can. A plot of the clock-pair points will show a heavy line with the best-case transfers and a smattering of latency events. A robust estimator will ignore the chaff. So I don't see a problem with submicrosecond time transfer over USB. (I tried this some time back as a quick hack, with a Teensy board.) Cheers, Peter _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
