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PCB Layout Care for designing a custom NOR cape with Beagle Bone Black

ARM arm
Expert 2630 points
Hi,

I am designing PCB layout for a custom NOR cape [2 NOR Flashes] with Beagle Bone Black. Schematic attached herewith..
My address data bus length on the GPMC lines of NOR Flashes is coming around 4 inches for one and 5 inches for the other [including the BBB GPMC length lines], do I need impedance control on such length? Please help or guide in this whatever brief possible.

I know that for FR4, transmission time or propagation delay = 137ps/inch and for 5 inch it is coming around 800 ps. Now I do not know the slew rate of AM335X [How much does fast mean] but i know that this 800ps should be at least half of slew rate or even lesser. please help with some thumb rule.

Regards.

  • 0
    TI__Guru**** 393215 points

    Hi,

    Important thing here would be to equalize trace lengths, because this is a parallel interface. I would say 5-10mils skew would be enough. Signal traces on Sitara boards are usually done as 50Ohm single-ended impedance lines. Impedance control is not necessary for this interface.

    Slew rate is not defined. You can see section 7.7.1 of the AM335X Datasheet Rev. H for GPMC signal rise and fall times.

  • 0 in reply to Biser Gatchev-XID
    Expert 2630 points

    Hi Biser,

    We are using asynchronous NOR Flash where the read and write access time is >=35ns. Still is that much length matching required? We have made the length match within +/-150mil till now..

    Further will 5 inches be a long distance between AD bus between NOR Flash and ARM Controller on BBB w/o any buffer in between or shall it be ok? Actually these points have cropped after the design and layout is done.

    We know that you may not see the board files but I am attaching it in case your cursory look might reveal a thing or two.

    abcd4.brd

    regards

  • 0 in reply to ARM arm
    TI__Guru**** 393215 points

    MOHIT HADA said:
    We have made the length match within +/-150mil till now..

    Yeah, I actually overdid this. 150mils should be OK. This will give 20-25ps delay between signals on longest and shortest trace. Should be no issue.

    MOHIT HADA said:
    Further will 5 inches be a long distance between AD bus between NOR Flash and ARM Controller on BBB w/o any buffer in between or shall it be ok? Actually these points have cropped after the design and layout is done.

    This should be OK without buffer.