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mixing LVDS data

Hello,

I have a newbie problem that is probably easy to solve.

I have 4 data streams entering Virtex II FPGA through LVDS standard (each stream is 4 wires, 2 clock and 2 data). This is fine as I am able to receive the signals each individually i.e. data from each stream is clock by only their respective clock and the data does not interact with each other.

What I want to do is to treat each stream as a bit of a word i.e. have them all clocked by a single clock. Because these streams are independent clock phases and probably clock jitter is not equal, though frequency should nominally be equal.

Is there a way to generate a single clock and/or correct for phase differences on the data streams so that can can treat the data as a single "word"? How should I look at this problem?

Herwin

Reply to
Herwin
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Does your system meet the following requirements?

  1. The average frequency is exactly the same on all four channels. (You said "nominally equal" but that could mean anything.)

  1. You can put an upper limit on the instantaneous phase difference between channels.

  2. Each channel contains a unique data pattern (frame marker, etc.) that would allow you to work out the phase difference between the channels.

If so, then I recommend the following:

Have four independent FIFOs. Each FIFO takes its input from one of the channels. The output of all FIFOs are clocked from the same source, which would be the input clock on *one* of the FIFOs, or any other clock with the same frequency (e.g. one of the input clocks that has been filtered by a PLL).

The FIFO size is determined by the maximum phase difference from #2 (above).

Have a control input on the FIFO that allows you to adjust the depth (by fiddling the pointers - it doesn't matter if this trashes the data briefly during the adjustment), and control this such that the frame markers (from #3, above) line up on the outputs of all four FIFOs.

BTW, this is very common technique. It is done in some variants of

10G Ethernet (that recombine four 3.125Gbps 8B10B streams into one 10Gbps stream).

Regards, Allan.

Reply to
Allan Herriman

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