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LMX2594: Which signals does need length match?

Part Number: LMX2594
Other Parts Discussed in Thread: LMK04828, TIDA-01024,


I checked some documents for JESD204B clocking and they recommend all devclk and sysref should be length match.

My application is similar to TIDA-01024. Two LMK04828 are connected as daisy cain and supply devclk and sysref to FPGAs and multiple PLLs.

PLL produces devclk_dac and sysref_dac for a DAC.

From LMK04828 to FPGA, there are devlck_fpga and sysref_fpga.

From LMK04828 to PLL, there are devlck, sysrefreq and sync.

From PLL to DAc, there are devclk_dac, sysref_dac

Length match on is mentioned only for DCLKout12 and SDCLKout13 of first LMK for daisy chain on TIDA-01024 user guide.

How about other signals devclk_fpga, sysref_fpga, devclk, sysrefreq, sync, devclk_dac and sysref_dac?

All devclk_dac and sysref_dac signals from PLLs to DACs will be routed as same length.

1> Are devclk_fpga and sysref_fpga should be length matched with devclk, sysrefreq and sync? 

Even though devclk_fpga and sysref_fpga are matched length with devclk, sysrefreq and sync, I am not sure if phase of sysref_dac and sysref_fpga will be aligned.

2> Which case is better for sysref_dac.

One is that LMK produces sysref_dac or the other s that PLL produces sysref_dac by sysrefreq from LMK?


  • Hi,

    TIDA-01024 reference design uses the same HW as TIDA-01021 design, which explains all the required length matched signals.

    For JESD204B clocks, devclk_dac has way higher frequency than devclk_fpga and it doesn't necessary to have length matched but the multiple data converters devclk_dac and sysref_dac pairs should be length matched to have deterministic / synced output. similarly, if there are multiple FPGAs, their devclk_fpga and sysref_fpga pairs signal lengths should be matched.

    sysref_dac should be possible to have from LMK or LMX, based on delay adjustment needed. LMK04828 have SYSREF analog delay in the step to 150ps, whereas LMX2594 has SYSREF delay step of 9ps. Based on devclk_dac frequency can choose SYSREF output from any of the device.


    Ajeet Pal

  • Hi Ajeet,

    I have read SYSRefReq should meet the setup and hold time of the OSCin on this E2E webpage.

    The OSCin clock of LMX2594 is 300MHz. The setup and hold time of SysRefReq pin of LMX datasheet is 2.5 ns + 2ns, and this 4.5ns is longer than period of 300MHz.

    If there can be timing issue, how about SYNC signal? The setup and hold time of SYNC pin is also 2.5ns + 2ns.

    Is there no problem to aling phase of RFoutA when SYNC pin is asserted?



  • Hi Yoonsun,

    There are 2 different requirements for SYNC and SYSREFREQ input signals.

    Regarding the SYNC input setup and hold time, LMX2594 support sync feature and it can have different categories for SYNC, where SYNC pin may or may not required SYNC input based on categories mentioned Figure 27 flow chart in LMX2594 datasheet.

    If device operates in SYNC category 1 and 2, SYNC input timing is not critical (setup and hold time requirement not needed). but for category 3, SYCN timing is critical and should meet setup and hold time requirement and in this condition OSCin input frequency is limited to 100MHz and it should have sufficient valid window (10ns - (2.5ns + 2ns) = 4.5ns) to meet the requirements.

    Similarly for SYSREFREQ input setup and hold time, for SYSREF pulse in generation mode and SYSREF in repeater mode, SYSREFREQ input timing is critical. SYSREF continuous in generation mode doesn't necessary raising edge and would need only SysRefReq pin high, whereas SYSREF outputs in pulse mode and repeater mode would re-clock with interpolator clock (generated inside) and should meet the setup and hold time to realign with OSCin input frequency. 



    Ajeet Pal

  • Hi Ajeet

    I checked LMX2594 datasheet. 

    OSCin is 300MHz and RFoutA is 12GHz, thus I understand that LMX runs in category 1 and SYNC mode not required.

    By the way, SYSREF is output in repeater mode and setup and hold time(4.5ns) is not sufficient for 300MHz OSCin.

    This mean that SYSREF by LMK04828 is better than SYSREF by LMX2594 in repeater mode Am I right?



  • Hi Yoonsun,

    That's correct. If LMX2594 output frequency is VCO, it doesn't necessary to have SYNC mode.

    It's better to keep SYSREF out from LMK04828, to avoid any re-timing issue of SYSREF from LMX2594. But these outputs should be from same source and have deterministic delay.



    Ajeet Pal