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TMS320C6678: TMS320C6678: PCIe interface impedance

Danny Ayers
Prodigy 130 points
Part Number: TMS320C6678

Tool/software:

The SPRABC1 SERDES Implementation Guide specifies 100 ohm differential traces for the PCIe TX and RX pairs.  

The device I am interfacing with specifies 85 ohms differential impedance for these signals.

Can the TMS320C6678 operate with 85 ohms, or how should this situation be handled?

Thank You,

Danny

  • 0
    Prodigy 130 points

    Also, one more detail I omitted:  the two devices connect directly via pcb traces and AC coupling capacitors, no connectors.  The connection path is < 2 inches.

  • +1 in reply to Danny Ayers
    TI__Guru 61950 points

    Danny,

    The differential trace impedance guidelines are given to minimize the signal reflections that happen due to the impedance discontinuities.
    There are tolerances for the impedance since an exact 100ohm differential impedance may not be met. 85ohm differential impedance can be considered within the allowed tolerance level. We would also suggest to do any timing/ high speed signal simulations on your schematic/layout to improve the chances of first pass success. Given that your PCB trace is <2inches, I would think you should be good.