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Original question:

LMK00334: LMK00334 || PCIe module not detecting

LMK00334: Blocking caps at the input

keith leisses
Prodigy 170 points

Part Number: LMK00334

Tool/software:

Wondering if I can remove the blocking caps I have in place on the CLK0 inputs from the PCIe interface [gold finders in this case].   

Also the downstream connections will have blocking caps in place - on those modules [XMCs].   I have 49.9 pull-downs in the circuit located at the LMK00334.   Pretty standard and would expect to work but figured I would ask.

Last, I put in a filter ->  3.3V 10uF-600 ohm /.5 ohm - 1 uF to feed all of the pins with individual .1 uF at each pin.   Figured a mini-plane under the device would be a good way to make this happen.  Caps on the back, via at each of the 8 power pins.  I saw in the diagrams on this one that there are separate circuits for each power type.   Is that really necessary?

  • 0
    TI__Mastermind 32152 points

    Hi Keith, 
    Yes, the VDD pins require the bypass capacitors be placed close to each pin. 

    Regarding the AC coupling caps for CLKIN0 (I believe you meant CLKIN0) - the answer is yes if the reference clk is HCSL and you have the 50Ohm PD on each intrapair you can DC couple the LMK0334. 

    Best regards ,

    Vicente 

  • +1 in reply to vicente flores prado
    Prodigy 170 points

    Perhaps I was not clear.  I was asking about the input side - do we need blocking caps at the clock inputs when used in a standard PCIe system.  Gold fingers directly connected to the buffer or via blocking caps?

    Also my question about the decoupling was not about the .1 caps at the pins.   The question was about separation - do the supplies need separate filters or can I use one filter to feed the power pins with the .1s at each pin?

  • 0 in reply to keith leisses
    TI__Mastermind 32152 points

    Hi Keith, 
    PCIe Ref Clk is usally HCSL or LP-HCSL. 
    If this is the signal you're trying to buffer out is an HCSL/LP-HCSL than the answer is no - you do not need to AC couple the input. 
    You would follow the termination scheme I attached above (figure 9-6) if the RefClk is HCSL or you would simply DC couple from the gold fingers directly to LMK00334 if the signal is LP-HCSL which means the driver has the 50Ohm to GND terminations internally. 


    One filter should be enough if your supply is clean - just ensure each VCC/VCCO pin has the corresponding decoupling cap near pin. 

    Best regards, 

    Vicente