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Re: Timing via serial port

From: Kai Harrekilde-Petersen
Subject: Re: Timing via serial port
Date: Wed, 30 Sep 2020 14:19:34 +0200
User-agent: Android

I've measured the delay between the 1PPS signal and the TX uart output on the NEO-M9N board with a logic analyzer. I would need to find the or re-measure the data for you tonight.

Front edge of the UART frame is the most precise. Rear edge depends too much on the length of the message.

As I recall, the latency from the PPS to the front edge increases(!) with increasing update rate. I did not check for the variation between the 1PPS and the start of the UART message.



From: "O'Connor, Daniel"
Sent: Wed Sep 30 14:08:21 GMT+02:00 2020
To: Hal Murray
Subject: Re: Timing via serial port

On 30 Sep 2020, at 18:28, Hal Murray <> wrote:
Has anybody tested various GPS devices to see how good the timing via the
serial port is?

Some of the NMEA sentences have sub-second time stamps. Are any of them

Has anybody looked at whether the front of the sentence (or burst) has better
timing than the end?

I would say it depends how tight your requirements are.
I setup USB serial to a GPS engine and fed NTP with it and it seemed quite good - certainly NTP was happy to use it and all the numbers look good.

I imagine it would also depend a lot on your hardware and OS as to what sort of jitter you would get.

Daniel O'Connor
"The nice thing about standards is that there
are so many of them to choose from."
-- Andrew Tanenbaum

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