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sharing the crystal — Parallax Forums

sharing the crystal

BebopALotBebopALot Posts: 79
edited 2006-03-22 08:10 in General Discussion
Hey folks,

Does anyone know if I can "tap" the same 75MHz TTL crystal to the SX for other ICs in my circuit? Are there any special restrictions, problems or results with this?

Comments

  • PJMontyPJMonty Posts: 983
    edited 2006-03-18 07:34
    Bebopalot,

    Given the (sometimes) fussiness of getting crystals to start up, especially at high frequencies like 75 MHz, I would think that buffering the oscillator circuit beofre passing it out to the chips in your circuit would be a good idea.
      Thanks, PeterM
  • Andrew BellingerAndrew Bellinger Posts: 7
    edited 2006-03-19 02:30
    Hi Bebopalot,

    I have a post - "oscillator considerations" - which raises the same question. Solutions I have been looking at include using a clock distribution IC which buffers a clock source and splits it into many channels. Pericom, Analog Devices, TI and other manufacturers make them. They are very specific however in what they will accept as a source and output specification (xtal, lvpecl, lvds etc...). I am looking for one that will work at TTL level and cover the SX operating range 4-100MHz. When I find something Ill let you know.

    Andrew

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  • Andrew BellingerAndrew Bellinger Posts: 7
    edited 2006-03-19 16:47
    Just an update to my last post-

    TI makes a line of clock distribution controllers that will output up to 8 TTL compatible clock signals. They all begin with the prefix CDC (clock distribution controller), and can be found on their product search page. Also, the outputs can be put in a high-Z state via a select line so the clock will not interfere with in circuit programmabillity of your SX.

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  • BebopALotBebopALot Posts: 79
    edited 2006-03-19 21:18
    I found them. Awesome, thank you Andrew. I'm looking at possibly six additional devices. Nice "timing"!

    Post Edited (BebopALot) : 3/19/2006 9:25:46 PM GMT
  • PJMontyPJMonty Posts: 983
    edited 2006-03-22 08:10
    Andrew and Bebopalot,

    Make sure you physically test the "Hi-Z won't interfere with SX programming" asumption before you make any PCBs. I made this assumption using TTL oscillators only to find out they have diodes in their output path which end up clamping the protgramming voltage generated by the SX-Key so that it can't program chips. I learned this after I made PCBs.
      Thanks, PeterM
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