[time-nuts] NIST time services

Mike George mgeorge at tuffmail.us
Sat Mar 22 19:25:43 EDT 2014

PRU appears to be unique to TI.

I have only used Raspberry Pi, Beaglebone, and Cubieboard.
The Cubieboard (Allwinner CPU) has a lot of IO pins like the
Beaglebone but nothing like a PRU.

Mike George

On 3/22/2014 16:53, Chris Albertson wrote:
> Thanks,  Yes of course "ARM" refers only to "ARM"
> Would you know which other systems include the PRUs?  Is it only in
> the TI products?   It seems like an ideal solution to the problem of
> non-deterministic latency.
> This may not even be required.  There is no point to extreme levels of
> accuracy because the weak link with any NTP server is the Internet.
> NTP's purpose is to transfer time over unreliable data links and these
> links will always be the limiting factor.
> On Sat, Mar 22, 2014 at 1:23 PM, Mike George <mgeorge at tuffmail.us> wrote:
>> The PRUs (Programmable Realtime Unit) aren't a feature of ARM in general
>> (they are not present
>> on the Raspberry Pi for instance).  The BeagleBone has 2 PRUs as you
>> describe.  It uses the TI Siatra
>> ARM variant.
>> ARM just describes the core architecture.  Manufacturers tack on all sorts
>> of proprietary peripherals
>> depending on what they envision as it's primary target market.
>> Mike George
>> On 3/22/2014 15:54, Chris Albertson wrote:
>>> On Sat, Mar 22, 2014 at 12:24 PM, Chuck Forsberg WA7KGX <caf at omen.com>
>>> wrote:
>>>> I can see a use for an inexpensive GPSDO with a built-in
>>>> gigabit ethernet or USB3  port powering an NTP server.
>>> Neither of those is a good way to transfer time to an NTP server.
>>> Both Ethernet and USB are packetized.   The best way is with a simple
>>> wire with a square wave pulse on it that pulses ones per second.
>>> Nothing can be more simple or accurate.
>>> The trick is to build an NTP server that can react deterministically
>>> to the pulse.   I think an ARM based system could far outperform an
>>> Intel based one.   ARM has two independent PRUs.  These are little
>>> 32-bit processes each with 4K of memory that are build right on the
>>> same chip as the main ARM CPU.   The PRUs purpose built for real time
>>> task and can handle nanosecond level timing.   In most existing system
>>> the PRUs are ignored and everything is done using the ARM.
>>> The other way to improve things even better is to not even bother to
>>> have a link from the GPSDO to the NTP server.  Why not simply run the
>>> NTP server software on the same processor as the GPSDO?   Just one of
>>> the little PRUs is more than powerful enough to run a GPSDO.  They are
>>> a 32-bit uP that runs at 200MHz, one instruction per clock.  The PRUs
>>> don't run any operating system code but have access to all of the
>>> ARM's memory and interrupts.  A PRU is way-overkill for a GPSDO.
>>> Doing this eliminates the link cable from the GPSDO to the NTP server.
>>>    If the ARM CPU can't handle 6 billion requests per day then buy many
>>> copies the ARM based systems.  They are cheap.
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