[time-nuts] GPS for ntp

Simon Marsh subscriptions at burble.com
Fri Oct 24 17:44:47 EDT 2014

On 24/10/2014 21:33, Joseph Gray wrote:
> Timestamping a message to the nearest 100 ns is what I am after. If I can't
> do it directly with the ublox, I'll have to look at the PRU on the
> Beaglebone. It has a 200 Mhz clock and single cycle instructions.

The BBB has no less than 8 peripherals that will do event capture at 
under 100ns resolution, no need to mess around with the PRU. To be fair, 
pretty much any microprocessor would be capable of doing this, and an 
arduino or pic could be a lot cheaper way to go depending on your 

If you need to stick to a BBB:

DMTIMER's 4 to 7 will do hardware capture, but they operate at 'only' 

The SoC also contains 3 eCAP units which operate at the OCP bus speed 
(100mhz / 10ns resolution) and can store up to 4 events.

The last unit is also an eCAP unit, but it's a PRUSS peripheral and 
operates at the PRUSS clock speed (200mhz / 5ns resolution). The 
surprise is you _don't_ actually need to write PRU code to access it, 
all the PRUSS peripherals can also be controlled from the main ARM core 
(and similarly the PRU can access all the SoC peripherals).

Using two hardware capture units, you can implement a simple (or as 
complex as you want to make it) interval timer and follow Tom's posting 
from a few days ago:




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