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Comment by michaelt | Hacker News Reader

One thing I always wonder about with these small scale NTP projects is: they often reject getting 1PPS through a USB-to-serial converter, because USB is polled and just has too much jitter - then they'll choose a Raspberry Pi 1 and serve NTP over... a USB to ethernet converter. Or sometimes even USB-to-wifi!

One would think both interfaces would suffer the same problems, after all.

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I built one using the Adafruit GPS hat to get the PPS through a GPIO pin, no latency at all.

The Adafruit HAT was out of stock earlier this year so I ended up with a USB-to-serial for everything but PPS (using GPIO) setup.

It's not the prettiest but works brilliantly.

Now just waiting for Raspbian to respin to be Bullseye based so that I can update to a version of chrony with NTS support.

You can just build chrony yourself.

(And I think you can upgrade RpiOS to bullseye right now. It's not "officially" out but it seems to work anyway.)

You’re not wrong, anything older than a Pi 4 is a bit flawed as an accurate time server. Even the Pi 4 needs a stable temperature or its clock speed will be unstable, as there’s no temperature adjusted clocksource, it just uses cpu.

On older Pi versions USB is run in kernel instead of on chip, causing thousands of cpu interrupts per second. Also its on board ethernet is run through USB.

This (very old) guide covers how to build ntpd with ATOM kernel-mode polling of the serial pins. Does this not sidestep the issue?

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