[time-nuts] Re: Can Anyone Help Me Get PPS Into This Mini PC?

[Bob Camp]

Hi

Like it or not, pretty much all these small (and many not so small)  MCU’s are a bit challenged in terms of single digit nanosecond time capture. It simply isn’t a design priority for these folks. If the hardware is challenged, then whatever goes on top of that will simply add more issues. 

If you really need single digit sort of accuracy, some sort of outboard “gizmo” is likely to be part of the design. 

At this point it is a bit unclear if the goal is +/- 1 ns accuracy. +/- 10 ns resolution, or something else entirely different. There’s a really wide range between those two limits. Both *might* be called “I need nanoseconds”. 

If the goal is something like the PPS pulse out of a typical modern GPS module, a goal below 200ps accuracy just might be what somebody would look for. That (at least to me) would be well outside the “I need nanoseconds” range. Once you add a TDC, it’s not that much harder than 2 ns accuracy.

Bob

> On Feb 12, 2024, at 7:17 PM, Ed Marciniak via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Depending on the exact architecture of an embedded PC, a serial port might have hundreds of nanoseconds jitter added by a PCIe switch or things like an LPC interface that maps to a 16 bit 8.33 MHz bus in between.
> 
> In an ideal world, a network interface with IEEE-1588 PTP support would directly get a PPS to synchronize a counter
> ________________________________
> From: Denis Dowling via time-nuts <time-nuts at lists.febo.com>
> Sent: Monday, February 12, 2024 5:14:06 PM
> To: time-nuts at lists.febo.com <time-nuts at lists.febo.com>
> Cc: Denis Dowling <dpd at opsol.com.au>
> Subject: [time-nuts] Re: Can Anyone Help Me Get PPS Into This Mini PC?
> 
> Hi Ed,
> 
> I can see how you would want something like this to replace the RPi as
> it looks a lot more capable. There does not seem to be a lot of
> information about the GPIO port on this board. From previous experience
> with Advantech equipment the GPIO ports can be difficult to find
> information on. A better option would be to use the COM port also on the
> board. This is likely mapped to /dev/ttyS0 on boot. You might have to
> enable it in the BIOS first. It should then be possible to use the RS232
> handshake lines for the PPS input to the Linux pps-tools. You can also
> feed in GPS NMEA to this same communication port that will mean no
> external NTP connections are needed.
> 
> Regards,
> Denis
> 
> 
> On 10/02/2024 9:10 am, Ed Armstrong via time-nuts wrote:
>> I recently purchased one of these mini PCs to be used as my router.
>> https://urldefense.proofpoint.com/v2/url?u=https-3A__cwwk.net_products_cwwk-2Dx86-2Dp5-2Dsuper-2Dmini-2Drouter-2D12th-2Dgen-2Dintel-2Dn100-2Dddr5-2D4800mhz-2Dfirewall-2Dpc-2D2x-2Di226-2Dv-2D2-2D5g-2Dlan-2Dfanless-2Dmini-2Dpc-3Fvariant-3D44732374352104&d=DwIGaQ&c=euGZstcaTDllvimEN8b7jXrwqOf-v5A_CdpgnVfiiMM&r=JsDsKeR7cZC8wbZhIlxxBQ&m=JBs9b5cAEBCI0RcPG_SBLalBkFpA8vxUbfgk7fZGN1gP2uLM8TTE12Ls33A0gRnn&s=9JUsRvAgIvKeYLFylZeCPkwW7ImbHurHBMg3mbTe4Qo&e=
>> 
>> 
>> 
>> I am going to be running the new 64-bit Intel release of the excellent
>> "Tomato" router firmwareTomato64 <https://urldefense.proofpoint.com/v2/url?u=https-3A__tomato64.org_&d=DwIGaQ&c=euGZstcaTDllvimEN8b7jXrwqOf-v5A_CdpgnVfiiMM&r=JsDsKeR7cZC8wbZhIlxxBQ&m=JBs9b5cAEBCI0RcPG_SBLalBkFpA8vxUbfgk7fZGN1gP2uLM8TTE12Ls33A0gRnn&s=HkPJNn78J7mrok__SJQFksbEuxiR87zBhndqPpe8VGo&e=>, which can be
>> installed either "bare-metal" or as a virtual machine. I am going the
>> virtual machine route using proxmox v8.1.3. The tiny PC idles at about
>> 9 W and maxes out around 18 W, and has considerably more power than I
>> need for my routing purposes. So, I also put my FreePBX on a separate
>> virtual machine, also located on this mini PC. But that is probably
>> not very interesting to anyone here, nor is it really part of my
>> question.
>> 
>> Proxmox is a Linux based OS which is designed for the express purpose
>> of running virtual machines. I suspect many of you on this mailing
>> list may already be using it yourself. I am currently running my own
>> stratum one NTP server on a Raspberry Pi 3 which is getting its PPS
>> from a GPSDO. well, proxmox comes with chrony already installed, and
>> I'm sure I could change it over to ntpd if I wished to do so. If any
>> of you follow my first link, you will see the little mini PC has a
>> header for GPIO. It is a 2x5 header with I believe a 2 mm pitch.
>> According to the listing, there are four input and four output pins. I
>> assume the other two pins are either both grounds or both positive,
>> not really sure.
>> 
>> Now comes my question. Can any of you tell me how to use these GPIO
>> pins? I can find tons of information very easily on setting this thing
>> up as a router, NAS, or as a desktop computer. I have not been able to
>> find a single post related to those GPIO pins. I can't find the pin
>> out, can't find out what voltage they are supposed to work at, nor any
>> information about communicating with them in Linux. I queried the
>> system, hoping I could look up the motherboard online, and this is
>> what I got:
>> 
>> root at Proxmox:~# dmidecode -t 2
>> # dmidecode 3.4
>> Getting SMBIOS data from sysfs.
>> SMBIOS 3.6.0 present.
>> # SMBIOS implementations newer than version 3.5.0 are not
>> # fully supported by this version of dmidecode.
>> 
>> Handle 0x0002, DMI type 2, 15 bytes
>> Base Board Information
>>        Manufacturer: Default string
>>        Product Name: Default string
>>        Version: Default string
>>        Serial Number: Default string
>>        Asset Tag: Default string
>>        Features:
>>                Board is a hosting board
>>                Board is replaceable
>>        Location In Chassis: Default string
>>        Chassis Handle: 0x0003
>>        Type: Motherboard
>>        Contained Object Handles: 0
>> 
>> root at Proxmox:~#
>> 
>> I also tried dmidecode -t baseboard, this obviously gave more details,
>> but most devices were just listed as "other". I haven't found this to
>> be terribly useful.
>> 
>> My desire is to replace the Raspberry Pi by syncing chrony or ntpd to
>> the PPS in proxmox and using that instead. It would save a tiny amount
>> of energy, and would remove the slight jitter caused by the ethernet
>> port being on USP, I believe. Can anybody on here give me some idea on
>> how to do this, or perhaps point me in the right direction to find the
>> information I need.
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