CPU to RAM routing

Discussion in 'General Electronics Chat' started by rybo, Mar 13, 2007.

  1. rybo

    Thread Starter New Member

    Mar 13, 2007
    9
    0
    Hi folks,

    This is my first experience trying to route CPU to external memory IC's with more than 1 memory chip. Even worse, there is 2 SDRAM + 2 Flash IC's.

    [​IMG]
    6ML


    Well, the question is fundamental.. Will this routing actually work? :confused: I'm feeling not sure because some traces are about 4-5" long with not really much space between them, which result in impendance, resistance, capacitance, bla bla...

    Any suggestions or hints?

    Thanks!
     
  2. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    Frequency and part numbers please...
     
  3. rybo

    Thread Starter New Member

    Mar 13, 2007
    9
    0
    CPU is ARM9 @ 180 MHz
    SDRAM : 2 x Micron MT48LC16M16A2 (256Mbit, 7.5ns/CL=2 -> PC133)
    FLASH : 2 x Atmel AT49BV642D (64Mbit, 70ns)

    In fact, the access times may be configured at CPU.
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    I'm taking wild guesses at trace width, separation, and thickness. I'm therefore guessing half a microhenry per meter, or 76 ohms between worst case traces.

    For a more accurate answer, go here.
     
  5. rybo

    Thread Starter New Member

    Mar 13, 2007
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    0
    Btw, layer stack-up for PCB:

    1 - TOP
    2 - SIGNAL
    3 - PWR
    4 - GND
    5 - SIGNAL
    6 - BOTTOM

    After resolving few questions, I think this board will have in result 8 layers for 2 additional ground planes because of layer to layer impendance on top and bottom.

    1 - TOP
    2 - GND
    3 - SIGNAL
    4 - GND
    5 - PWR
    6 - SIGNAL
    7 - GND
    8 - BOTTOM

    I was said that if traces are less for 6" (delay of 1ns) this will work.

    What do you think, people?
     
  6. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    If you are going to end up with 8 layers anyway, I think you should revise the layout. Move the chips in-line and closer together, avoid changing layers on the bus and go with vertical and horizontal layers. Try to make the tracks somewhat similar in length, it's a good idea to do this even with PC133.

    What layout package are you using?

    You could always loosen the timing if they didn't work :D
     
  7. rybo

    Thread Starter New Member

    Mar 13, 2007
    9
    0
    Well, the situation is that there is not much time left for this project, that's why I don't really want to revise layout and reroute this again :(

    Btw, the difference between shortest(1044mil) and longest(5198mil) track is about 4154mil (4.154"). Impendance variation is 79.93-55.03=24.09Ohm.

    Rising edge flight time is 1.48ns and falling is 4.4ns. Signal integrity says this will this work without loosing access time?

    Altium Designer 6. Not seen better at this moment.
     
  8. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    If you have not much time left, then you could go ahead with the current layout. As I said previously, if things do indeed go wrong, you have an option to loosen the timing.

    The signal integrity on Altium is pretty good, you could rely on it most of the time.

    How do you cope with the un-intuitive interface? It totally put me off that context menus, or double clicks resulted in totally different things than what's normally expected. And the pain to change components properties or values. But that's just me :D
     
  9. rybo

    Thread Starter New Member

    Mar 13, 2007
    9
    0
    Thanks very much! This is what I wanted to hear. After I'll get assemblied pcb, I'll let know about the results.

    Well, I had to :D In time You'll get quite familiar and fast enough, believe me. For example, changing properties/values for a lot of components at the same time is possible within Tools>Parameter Manager>Parts. Just highlight few rows, right click, edit, ..., accept changes. PCB filters is also awesome for changing, for example, all the ground vias hole diameter.
    Indeed, the interface is really complicated and it is easy to run deep into forest. But You know, there is also a lot of things hidden there :)
     
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