rpi3 and Adafruit GPS Hat

[Ralph Smith]

Sent from my iPhone

>> On Jul 23, 2018, at 5:40 AM, Per olof Ljungmark <peo at nethead.se> wrote:
>> 
>> On 07/23/18 10:46, David Cornejo wrote:
>> this might be a little blasphemous, but for grins I tried an Oncore with
>> PPS to a GPIO and running the serial through a TTL-USB serial cable and
>> that seems to work ok.
>> 
>> there's probably some good reason that this is a bad idea.
> 
> Depends on what precision you are after, but for lowest possible jitter
> you need to use the uart, the difference is in magnitudes.

But he’s not running the PPS through the USB serial port, it is going directly to GPIO. Correct me if I’m wrong here, but once ntpd locks to the PPS there is no real difference between the two. I’m running that exact scenario here with two Pis, both Model 2. One has an Adafruit GPS HAT directly attached, and the other has the GPS connected to a USB-TTL serial adapter, with the PPS connected to the GPIO. Both sync to PPS with the offset typically around 4 μs and jitter around 2 μs. I may try it with an Oncore I have on hand but that will have to wait a month or so for other things to settle down. 

Ralph
>> On Sun, Jul 22, 2018 at 9:09 PM Per olof Ljungmark <peo at nethead.se
>> <mailto:peo at nethead.se>> wrote:
>> 
>>>>    On 07/18/18 19:01, Diane Bruce wrote:
>>>> On Wed, Jul 18, 2018 at 05:10:16PM +0200, Per olof Ljungmark wrote:
>>>> Being a complete newbie to arm I thought a nice project would be to
>>>> build a NTP server with the parts in the subject line.
>>>> 
>>>> Unfortunately I have almost no idea where to start, it seems
>>    FreeBSD for
>>>> arm have shifted around quite a bit, almost none of the googled
>>    pages I
>>>> find has relevance, and to add insult to injury, the Pi project
>>>> apparently shifted the serial ports around for the Pi3.
>>>> 
>>>> What I need to achieve,
>>>> 
>>>> - Stop the kernel to use the uart for console output (I have ethernet
>>>> and HDMI connected)
>>> 
>>> No need.
>>> 
>>> change your config.txt
>>> 
>>> #dtoverlay=pi3-disable-bt
>>> device_tree_address=0x4000
>>> kernel=u-boot.bin
>>> enable_uart=1
>>> 
>>> This moves the console port to the less capable micro uart port
>>> this will free up the good uart (the pl011 device) as /dev/ttyu0
>>> 
>>> Remove the pi3-disable-dt in config.txt
>>> enable_uart=1 is needed.
>>> 
>>>> VERY grateful if someone that knows better can give me a push in the
>>>> right direction for up to date information.
>>> ..
>>> 
>>> This is assuming you use FreeBSD-12 (Head of tree)
>> 
>>    Yes, 12.0-CURRENT #2 r336461.
>> 
>>    Unfortunately your advice did not solve the problem, when the hat is
>>    attached it sends NMEA sequences to the u-boot loader making it
>>    impossible to boot further, just like it is described in this thread:
>> 
>>    http://freebsd.1045724.x6.nabble.com/Adding-a-GPS-Module-hat-shield-on-a-Raspberry-Pi-td6236680.html
>> 
>> 
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>> 
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