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AM571x PTP - CPTS and programmable oscillator output

David Cemin
Intellectual 400 points

The AM571x TRM explains how the CPTS works with regards to how to fetch the timestamps from the TX / RX packets. It also explains how it can select the clock input. It does not explain very much though what is the underlying programmable oscillator that it uses.

Would a PTP application running on the top of this hardware control the Programmable Real-Time Unit (explained on chapter 30) ? On this case, the PRU outputs a clock that is the clock used on the CPTS? 

Aside of that, I need a clock reference that is synchronized to PTP ? What should I use? If it is using the PRU, it looks like the eCAP module could be used for this purpose. Please, i need some help understanding how the underlying system works with regards to PTP and how can I output a clock from it that is synchronous to PTP.

Thank you

  • 0
    TI__Guru**** 393215 points
    The Ethernet experts have been notified. They will respond here.
  • 0
    Intellectual 400 points

    Since nobody replied to it, I found some information on the web.

    The following ppt has some explanation of how PTP works regarding to the clocks:

    www.google.ca/url

    1) I still fail to understand how the clock synthonization works. It looks like CPTS has 2 32-bit timers that are maintained as a 64-bit clock. Also, the epoch is calculated based on Linux time (slide #17 above). Is this assumption correct? 

    2) http://processors.wiki.ti.com/index.php/ICSS_PTP_1588_Developer_Guide#Design 

    1 PPS Output

    IEP has an additional hardware to generate a programmable sync output which is tied to the IEP counter. This is called the Sync0 unit. For this signal generation CMP1 is programmed to a value ranging from 1ms to 1 second. PRU0 checks this event in firmware and re-programs it after every hit to ensure that accurate sync pulses are generated. This sync is equivalent to the 1PPS output and should not be confused with PTP Sync frame." 

    So, it looks like we have a up to 1 kHz output clock synced to PTP. If this is true:

    a) What is the physical pin I should get this output from

    b) Is it possible to have other frequencies higher than 1kHz ? (obviously not with the sync0 signal, but with another signal instead (that I dont know if exists). 

    Thank you. 

  • 0 in reply to David Cemin
    Intellectual 400 points
    It looks like the AM571X doesnt have GPIOs routed out to the pins, whereas the AM572X does:
    www.ti.com/.../spruhz6g.pdf - Page 7290
    www.ti.com/.../spruhz7c.pdf - Page 7168

    Can one of this GPIO pins be used to output a clock of lets say 30MHz synchronous to the PTP clock that the PRU is controlling ?