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Signal integrity with multiple devices.

Discussion in 'Embedded' started by db, Sep 7, 2003.

  1. db

    db Guest

    I am attempting to learn more about signal integrity for a
    microcontroller design I am working on. I have done simple designs
    with just one device attached. Now I want to add more devices. I have
    done spice simulations and see the increase in noise and ringing as
    more devices are added. What I am not sure about is how to correct it.

    Simple series termination would not seem to be sufficient. However
    when I look at the few microcontroller designs that have schematics
    available, that is all that seems to be used. I have seen nothing that
    uses AC termination for example.

    I have tried to find information on designing for multiple items
    attached to a bus, but it seems to be hard to come by just searching
    on google. I find plenty of stuff for one or two devices, but what
    about three or more? Should I use tees or buffers or what. What about
    stability when component values are varied.

    Any recommendations on sites or books that can help me understand how
    to deal with the >3 case would be good.
     
    db, Sep 7, 2003
    #1
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  2. I presume we are talking about a databus system here. If this is for
    an RS485 style network then perhaps you should look at the application
    notes on Linear Technology's site. They are sometimes quite useful.

    If it is any other type of bus come back with more details and you'll
    get a number of others responding.


    --
    ********************************************************************
    Paul E. Bennett ....................<email://>
    Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
    Mob: +44 (0)7811-639972 .........NOW AVAILABLE:- HIDECS COURSE......
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    ********************************************************************
     
    Paul E. Bennett, Sep 8, 2003
    #2
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  3. db

    db Guest

    Yes this is for a databus. Basically there were between 4-6 devices on
    the bus. Specifically the microcontroller(Sharp 79520), sdram,sram ,
    flash, fpga,usb, compact flash and maybe a cpld.

    I have pulled values from the manufacturers sheet for pin
    characteristics and calculated for the impedence of the trace.

    So far I have tried series termination and schottky diode termination
    in the spice simulation. The results look pretty poor. The terminated
    end usually looks pretty good, but the middle would almost certainly
    generate double clocking.

    I did find a sheet from phillips (AN246) with a pretty good coverage
    of different termination issues in a multiport situation and what the
    simulations look like. It is good to see some confirmation of some of
    the waveforms I saw in my simulations with what is in the Philips app
    note.

    I guess one specific question I would have is what connection topology
    should I be trying to use. So far I have simulated a bus with the
    components stubbed in and also in parallel. Should I try some
    configuration of tees or a star topology.
     
    db, Sep 9, 2003
    #3
  4. [%X]
    So this is a serial device bus similar to IIC.

    Essentially keep the traces parallel and ensure they visit each device
    in turn (no spurs or T's). You should also attempt to keep each trace
    very similar in length. Terminate the end with a suitable termination
    network and you should minimise the reflections along the bus.

    There are probably app notes with various companies that do these type
    of devices. Does the Philips AppNote say anything useful about
    techniques?

    --
    ********************************************************************
    Paul E. Bennett ....................<email://>
    Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
    Mob: +44 (0)7811-639972 .........NOW AVAILABLE:- HIDECS COURSE......
    Tel: +44 (0)1235-811095 .... see http://www.feabhas.com for details.
    Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
    ********************************************************************
     
    Paul E. Bennett, Sep 9, 2003
    #4
  5. db

    db Guest

    I have been able to find several now that have been helpful and have
    been able to get better results from the simulations. One mistake I
    was makeing was not accounting for the capacitance of the attached
    devices and accounting for how that affects the impedence of the line.

    The best notes I found were
    Phillips
    AN246 Transmission lines and terminations with Philips advanced logic
    families

    Motorola
    AN1061 Reflecting on transmission line effects.
    AN1051 Transmission line effects in pcb applications.


    The boards will be quite small. I am shooting for the 3x4 inch size,
    so I may not need terminations at all. However I want to be real sure.
    I do not want to spend the money to build these boards and then
    discover they don't work because of sigint problems.

    I still have a more simulation to do, but the problems seem a little
    more manageable now.
    Based on what I have simulated so far AC termination seems to provide
    the best results. Parallel was good also, but I do not want the power
    drain. I saw a lot of ringing with diode termination. Series seemed to
    be the worst, but that does not seem to be recommended except in point
    to point situations.
     
    db, Sep 10, 2003
    #5
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