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Re: [questions] GPS+PPS vs NTP server, why a huge offset ?




> On 19 Jun 2022, at 19:27, David Woolley <david@ex.djwhome.demon.invalid> wrote:
> 
> On 19/06/2022 07:38, Daniel O'Connor wrote:
>>> OK, then to which of the USB connector pins do you connect the PPS
>>> signal to get "PPS over USB"?
> 
>> You can connect them to CTS or RTS, on FreeBSD these can then hook into the kernel PPS API.
>> It works very well in practise, especially for the cost & effort required.
> 
> Taking the other side for this one, which of Ground, D+, D-, and VBus have alternative names of CTS and RTS? (I guess I should add StdA_SSRX-, StdA_SSRX+, StdA_SSTX+, and StdA_SSTX-, for USB 3.0, compatibility.)

None of course, but how do you think the CPU gets to know when a PPS edge appears on a 16550?

It is not magically wired into it, an IRQ is generated and has to be serviced before a timestamp can be taken. The bus it is on (LPC) is not designed for low latency and is not directly connected to the CPU..

--
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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