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jitter-sensitive multi-output clk distribution for multi-gigabit-transceivers

Discussion in 'Embedded' started by Toni Merwec, Nov 16, 2007.

  1. Toni Merwec

    Toni Merwec Guest

    Hi there,

    I am currently designing an FPGA board, featuring two Xilinx Virtex-4 FPGAs.
    I've already posted another question concerning a correct JTAG chain
    implementation a few days ago and gained pretty good response. But some
    problems remain... although on another topic:

    I'll be using the Xilinx Virtex-4 FX series FPGAs featuring the high-speed
    MGTs. The MGTs are located in two rows on each FPGA, each requiring their
    own MGT reference clock, i.e. four reference clock inputs have to be fed
    altogether plus at least one additional clock input for the core logic per
    FPGA. Because the FPGAs have to exchange data synchronously (via the
    standard GPIOs) I thought about using the same reference clocks for both
    FPGA and making them the same as the MGT reference. Unfortunately that leads
    to a clock signal that has to be distributed to at least 6 FPGA clock

    I don't think that a regular low-jitter clock device (and it HAS to be
    low-jitter as for the reference for the MGTs) can drive 6 inputs over
    several centimeters. I already used the ICS843020 clock synthesizer in
    several other projects and wanted to use it again. Reason for the ICS is
    that it features a programmable output frequency in the range of 35 - 700
    MHz. Problem is, the ICS843020 has only two outputs. The Epson EG2121CA
    device that is proposed in the Virtex-4 MGT user guide is not suitable
    because these devices are restricted to one fixed frequency.

    Maybe a clock buffer or multi-output clock distribution device is the
    solution here, but I am afraid every additional device in the clock network
    would introduce additional jitter which is the most critical aspect in this
    application. Therefore I woul prefer a solution without those kind of
    devices... if possible.

    Has anyone ever had a similar problem and knows about an adequate solution?
    Any help (if possible) is much appreciated. Thanks!

    Regards Toni
    Toni Merwec, Nov 16, 2007
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  2. Toni Merwec

    bill.sloman Guest


    ON semiconductor list some 272 clock buffers - this is one of them.
    bill.sloman, Nov 16, 2007
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  3. Check out ICS843001-21:

    It's a frequency synthesizer with very low jitter. It's used on Xilinx
    ML505 board for RocketIOs. Then I'm pretty sure that you can find a
    low skew clock buffer to drive your 6 clocks.

    Patrick Dubois, Nov 16, 2007
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