"druck"  wrote in message
news:qhrg3s$tef$1@dont-email.me...
> On 29/07/2019 18:10, A. Dumas wrote:
>>  wrote:
>>> can anyone tell me if the new RaspberryPi4 is capable of hardware
>>> timestamping according to IEEE1588/PTP?
>>
>> I'm sure it can be a slave, it's just a Linux computer, but the Pi (any
>> version) doesn't even have a hardware clock, so probably useless as a ptp
>> master.
>
> There is a battery backed RTC module for the Pi, and a number of people
> have interfaced to GPS based time signals. So there is no reason why a Pi
> can't be used for this.

I wonder how much extra it would add to the purchase price of a Pi to
include an RTC powered by a capacitor to keep things going until the Pi has
rebooted. If you have a permanent internet connection, it's no problem
because as soon as the Pi boots it resyncs with a time source on the
internet, but it can be a problem if a Pi is used with an intermittent
internet connection.

OK, so we had a very unusual case: no ADSL/VDSL internet connection in our
new house for a few weeks, so I was relying on tethering my PC and Pi to the
mobile phone internet. That was fine until my Pi rebooted when there was no
mobile in range, and its time reset itself to an hour or so earlier. That
screwed up the data for my weather station logging that my Pi does: I had to
go in and hand-modify the time stamps on some readings until I realised the
problem and synced the Pi to NTP.

Given the very portable nature of a Pi, and therefore the chance that it may
be used on projects that don't have an internet connection, it needs to be
resilient to reboots when it's not able to see an NTP source or when the
date/time can't be modified manually.

But maybe an external RTC for those special cases is more cost-effective
than adding it to the Pi where it is not needed for situations where there
is access to NTP.

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