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Altera_Forum
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14 years ago

Fitter and pin placements

I have a project where for once the PCB layout doesn't have to start right away so I have time to carefully think pin locations.

This leads me to my question: is it fundamentally always the best to let the fitter dictate where pins go? I have video data going in and out + ddr2 + spi and some other peripherals. Should I just try to finalize the design first without any port assignments, run the fitter and use those pins as an ideal starting point and them massage only if required by layout constraints.

3 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    14 years ago

    Usually the pcb layout contraints are more critical than those imposed by fitter and timing.

    Early FPGAs had a few routing resources, so it was recommended running the fitter before finalizing the pin placement. Modern FPGAs have a lot more routing resources and IMHO pin placement is a minor problem; at least I had no such problems in the last 10 years and I always privilege pcb routing. Clearly, it's recommended you follow the basic rules related different i/o banks, consecutive pins, etc. , especially if you use fast switching signals.
  • Altera_Forum's avatar
    Altera_Forum
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    14 years ago

    I agree... I think of the PCB layout first, printing the FPGA pin symbol from the pin planner and see how I can arrange my pinout to ease the PCB routing. Then I declare all my pins in the Quartus pin planner and run an I/O check, just to be sure I didn't violate any rule.

    I never had any problems with the fitter using this method.
  • Altera_Forum's avatar
    Altera_Forum
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    14 years ago

    --- Quote Start ---

    This leads me to my question: is it fundamentally always the best to let the fitter dictate where pins go?

    --- Quote End ---

    It depends, but usually the answer is no.

    If you have many tight timing constraints involving signals to or from pins, the Fitter is sometimes able to shuffle them around to improve timing. However, if you place the pins yourself and the Fitter does not issue any errors or warnings, and you successfully close timing, it means the placements you picked are probably just as good as what the Fitter would have done.

    Cheers, Adrian