[time-nuts] ``direct'' RS-232 vs. RS-232 via USB vs. PPSdecoding cards

Bob Camp kb8tq at n1k.org
Wed Feb 22 07:39:16 EST 2017


Hi


> On Feb 21, 2017, at 9:50 PM, Trevor N. <qb4 at comcast.net> wrote:
> 
> On Tue, 21 Feb 2017 07:37:34 -0500, BC wrote:
>> 
>> Some 1588 chip sets have (or had, I haven’t looked recently) external sync pins. 
>> This does get into the whole, what’s a motherboard / what’s a peripheral 
>> debate. Plugging in a 1588 card to get that pin probably no longer counts
>> as a simple solution. If plugging in a card *does* count then that opens up 
>> a lot of possible options. 
>> 
>> Bob
> 
> I found while looking at the datasheets for newer Intel server
> ethernet cards that they have the ability to timestamp GPIO pin
> transitions, but none of them have their internal timebase
> synchronized to a counter in the CPU. It looks like they are clocked
> from a separate XO on the card.

Everything on a motherboard traces back to this or that XO. None of the time 
sources are anything that would get you excited as a frequency standard. Linking
them together can be exciting. Replacing this or that one with a better frequency
source *is* possible in some cases. If you are going to replace one, why not
replace several :)

Bob

>  Maybe if it was synchronized to the
> PCIe clock / BCLK  they could take advantage of that new Always
> Running Timer in Skylake processors.  I'm surprised that Intel hasn't
> made a big deal about it. Support for it was added in the Linux e1000e
> driver early last year.
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