[time-nuts] Re: PRS-10 sawtooth correction with microcontroller

[Bob Camp]

Hi

My thought was that there must be a command to set/modify the “position” of the output 
PPS without doing anything else. Based on a quick re-read of the commands, it was the
only candidate I saw. You kinda sorta need to have a command like that to get the output 
lined up to the “right” on time tick time ( if that’s the sort of application you have ….). 

Bob

> On Dec 22, 2022, at 10:06 AM, Azelio Boriani via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> The PRS10 manual offers no clue about when the offset (TO command) is
> added to the measured time-tag (TT? command) so the only way to know
> is to try (with the input filter off, LM0 command) and see if feeding
> the sawtooth correction once a second has any effect on the PPS output
> and frequency output. In my opinion the PRS10 input filter makes the
> sawtooth correction unnecessary. other than taking care of occasional
> hanging bridges.
> 
> On Thu, Dec 22, 2022 at 1:15 PM James Wilson via time-nuts
> <time-nuts at lists.febo.com> wrote:
>> 
>> Thanks Jim - I wouldn’t mind having a look at the code you wrote to see if I understand the ublox commands correctly.
>> 
>> I’ve done the survey-in step with the LEA-M8T module I have - I’ll probably have this program do it as part of startup if the GNSS module is on a cold boot and doesn’t have saved survey data.
>> 
>>> On Dec 21, 2022, at 16:42, Jim Harman via time-nuts <time-nuts at lists.febo.com> wrote:
>>> 
>>> I have not interfaced to the PRS-10, but I have Arduino code to extract the
>>> qErr data from the UBX-TIM-TP messages. It also extracts the clock time
>>> from that message in case that is of interest . It is implemented as a C++
>>> Class and is easily extensible if you need to read more uBlox messages.
>>> 
>>> One thing you also may want to consider in working with the uBlox T series
>>> modules is the need to do a Survey-in to determine the exact location and
>>> put the module in Fixed mode, to reduce the errors in the timepulse.
>>> 
>>> I would be happy to share my code and setup instructions for the uBlox
>>> modules if that would be helpful.
>>> 
>>> On Wed, Dec 21, 2022 at 6:49 PM James Wilson via time-nuts <
>>> time-nuts at lists.febo.com> wrote:
>>> 
>>>> After running in many of the familiar problems of connecting a GPS timing
>>>> module 1PPS signal to a PRS-10, I want try building a software sawtooth
>>>> correction board. I’ve seen previous discussion of some of the commands to
>>>> use for adjusting the PLL and PPS control on the PRS-10 and getting the
>>>> sawtooth error from ublox GNSS modules, but not a start-to-finish
>>>> description of how to put everything together.
>>>> 
>>>> My idea to use a microcontroller with multiple UART ports; one connected
>>>> to the ublox module and the other connected to the PRS-10, with level
>>>> shifters as needed. The microcontroller sits between the two devices, and
>>>> gets the PPS sawtooth correction info from the ublox (specifically via the
>>>> UBX-TIM-TP message and its qErr field), and then adjusts the PRS-10’s PPS
>>>> time offset by this value to null out the sawtooth for the next pulse.
>>>> 
>>>> 1. At startup, configure the timing module to periodically send UBX-TIM-TP
>>>> messages every second. From the PRS-10, get the current time offset value
>>>> with the TO? command.
>>>> 
>>>> 2. Listen for UBX-TIM-TP messages, and extract the qErr field. If the
>>>> qErrInvalid bit is clear, compute qErr/1000 (convert from ps to ns) and add
>>>> this to the stored time offset value. Push the adjusted time offset to the
>>>> PRS-10 via the TO command.
>>>> 
>>>> Has anyone built sawtooth correction for the PRS-10 and can comment on
>>>> this?
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>>> 
>>> 
>>> 
>>> --
>>> 
>>> --Jim Harman
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