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J722SXH01EVM: Question for extenal clock source of PCIe

Ryuuichi machida
Guru 12715 points
Part Number: J722SXH01EVM
Other Parts Discussed in Thread: AM67A, LMK3H0102,

Tool/software:

Hi,

Customer will try to use external reference clock source ("SERDESx_REFCLKx_P/N" pin of AM67A) for PCIe.
They are going to refer EVM schematic to understand recommended circuit. You do not implement external clock (LMK3H102) on lastest EVM(PROC170E3B), but we know you have implemented this clock device on previous EVM (now we refer "PROC170E1".)

When I checked this previous EVM schematic, you have implemented 33 ohm series resistor(R40, R39) on clock line.


They will plan to use same device(LMK3H102) as EVM to their custom board.
(Output setting of clock soruce will be "LP-HCSL". Maybe same as EVM.)
Then I have following question.

* Should we need to implement "33ohm" series resistor on reference clock line ?
(I think these resistors are for damping resistor, but, if possible, please tell what purpose these are implmented and how you decide value.)

Best Regards, 

  • 0
    TI__Guru 52525 points

    The REFCLK inputs can accept HCSL drivers, and thus the recommended config with LMK3H0102 is shown below (from LMK datasheet).

    I don't think the J722SXH01EVM clock circuit shown has been tested, as that SERDES is using internal clock generation for REFCLK.  The EVM provides multiple different configuration/population options, and I think the selected population values are not correct.  The 33-ohms resistors should be 0-ohms for HCSL source.