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LMX2594: Category 3 Synchronization

Wouter van Verre
Prodigy 30 points
Part Number: LMX2594
Other Parts Discussed in Thread: LMX2694-EP, LMK04828, LMK04826

Hello,

I am looking to synchronise multiple LMX2594 ICs, and following the flowchart in the datasheet, I think I will require category 3 synchronisation. 

Elsewhere on the forum it was mentioned that for such cases it is common to use a SYSREF output from another LMX/LMK product. 

E.g. here: https://e2e.ti.com/support/clock-and-timing/f/48/t/769595?tisearch=e2e-quicksearch&keymatch=category%203%20sync

Unfortunately there is not much detail on how the SYSREF signal should be generated. Is a single pulse sufficient, or would it require multiple pulses / continuous SYSREF?

What is a typical LMX/LMK that can be used for this purpose?

An additional question I have is whether this all changes if the LMX2594 ICs are on separate PCBs in separate modules? 

I am assuming that in that case the SYSREF generators on each PCB would have to be synchronized somehow?

Kind regards,

Wouter

  • 0
    TI__Guru** 110530 points

    Hi Wouter,

    Since there is a timing requirement to the SYNC signal, we suggest use LMK device to generate a single pulse signal (this is possible with the SYSREF output) which is synchronous to its main clock output. The timing requirement is specified in the LMX2594 datasheet section 6.6, a timing diagram is available in LMX2694-EP datasheet Figure 1. 

    There is an example in LMK04826 and LMK04828 EVM user's guide () how to generate SYSREF pulse.

    Right, all the SYNC pulses should be synchronous for all LMX devices.

  • 0 in reply to Noel Fung
    Prodigy 30 points

    Hi Noel,

    That's great, many thanks.

    I'd like to ask a follow-up question, but I don't know if this is more appropriate for a separate topic, so please let me know if you'd like me to move it.

    It looks like the LMK04826 or LMK04828 are good options for me, but I would just like to confirm a few points:

    1. Is it possible to synchronize the output clocks from these devices to the input clock, so that I can synchronise all my modules, which would each have one LMK0482x device? I think this is possible by choosing one of the zero-delay modes?

    2. If I understood the LMK0428x datasheet and EVM user's guide properly, I would need to use the SYNC/SYSREF pin for two functions? 

    a. I would first use it to synchronize the output clocks, 

    b. and I would then change some register settings so that I can generate one SYSREF pulse using the SYNC pin?

    I think it is necessary to use the SYNC/SYSREF pin rather than SPI programming to make sure everything stays synchronous across multiple modules with multiple LMK0482x devices?

    Many thanks for you help,

    Wouter

  • 0 in reply to Wouter van Verre
    TI__Guru** 110530 points

    Hi Wouter,

    1. yes, this is possible in LMK0482x in zero-delay mode.

    2. correct. 

    Use SYNC pin to phase align multiple LMX devices is recommended unless all LMX devices are in Cat 1 or 2 sync. 

    To phase align all LMK devices, software sync is enough.