[time-nuts] NIST time services

Mike George mgeorge at tuffmail.us
Sat Mar 22 16:23:31 EDT 2014

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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