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Newbie Question : Board level design

Discussion in 'Electronic Basics' started by Gman, Aug 19, 2005.

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  1. Gman

    Gman Guest

    Hi,

    I'm a newbie myself, so i'm probably not gonna help much ...but here
    goes.

    I am designing a fairly passive circuit right now (sensors, diodes,
    leds, resistors, etc) and I need to design the circuit to perform and
    all of the funtionality is built onto the board, so it only makes sense
    that I prototype the logic/schematic and thent he layout of the board.

    This changes if you are only going to be using certain pins on an IC
    dependent on their funtionality. So you will have to determine in
    advance if you want the option of using what pins based on programming
    that will be done.

    I can;t even begin to imagine the WiFi guys that are actually using the
    board as an antenea and how they do that and what the order of
    operations is, every component and even the placement of the trace
    maybe effecting the outcome on the performance of an antenea
    (guessing).

    Any way, my .0032 cents worth (after inflationary adjustments)
    Gman
     
  2. Rich Webb

    Rich Webb Guest

    The board *could* be designed first if all of the MCU pins had exactly
    one possible function. This is often the case with micro-PROCESSORS;
    this is rarely the case with micro-CONTROLLERS. The schematic (not
    necessarily the PCB layout) is often a compromise between what's easiest
    for the software and what's easiest for the hardware.

    The programmer may not be too involved with the translation from
    schematic to PCB layout. The design engineer (who may be the layout guy,
    as well) must be.
    There are design rules and thumb rules for power busses, ground planes,
    bypass caps, trace widths and shape, etc., etc., etc.
    Absolutely, if it is communicated to them what the purpose of each net
    and node may be.
    In-system programming is your friend.
    The other datasheet.
     
  3. zilinxchip

    zilinxchip Guest

    Hi,
    Upto now I've been building simple little projects on breadboards.
    Now I'd like to try my hand at designing a pcb board with an MCU onboard
    and all.

    I have a couple of questions which have puzzled me.

    1) Is the board designed first and then the software written for the MCU
    or is it the other way around. I had one old school electical engineer
    tell me the board is designed first whereas I thought the software and
    circuitry is prototyped in pieces first and then comes the completed
    schematic layout.

    2) How do board level designers (who may not know much about programming
    the MCU) know HOW to layout the board? Do they just look at application
    notes from the manufacturer and lay things out and get it right on the
    first shot!?

    3) Is it possible that a board level designer can layout a board without
    knowing anything about programming the chips onboard?

    4) For surface mount chips (not in a DIP format where you can plop it into
    a breadboard for trial purposes), how do you go about trying them out
    before actually committing them to be produced on a PCB?

    5) When you want to incorporate a chip into your design and hook it upto
    other chips onboard, what is the first thing you go searching for? The
    datasheet or...? How can you be sure it will work in harmony with what's
    already onboard (i.e. all the pin connections are correct)

    I got a few more questions but I'll stop here for now.

    Thanks
     
  4. PeteS

    PeteS Guest

    There have already been lots of replies, but I'll throw in my few
    comments...

    ::
    Hi,
    Upto now I've been building simple little projects on breadboards.
    Now I'd like to try my hand at designing a pcb board with an MCU
    onboard
    and all.
    ::

    Do you have any actual layout tools in mind? Although they all require
    the same basic skills, they all differ in implementation and difficulty

    ::
    I have a couple of questions which have puzzled me.

    1) Is the board designed first and then the software written for the
    MCU
    or is it the other way around. I had one old school electical engineer
    tell me the board is designed first whereas I thought the software and
    circuitry is prototyped in pieces first and then comes the completed
    schematic layout.
    ::
    In a perfect world (rarely realised) the board requirements are stated
    first (Someone mentioned a project manager - that's one option). If the
    requirements of the board are clear, the electrical design requirements
    are at least clearer. As noted, unless the board is for a single
    purpose or perhaps just a general purpose (with I/O connector
    positions, for example) it is *never* a good idea to try and design the
    board without knowing what the software must do.
    Often, all the design functions (electrical, firmware software, layout)
    reside in one person. I don't know many electrical engineers who have
    not written significant amounts of code - they *want* to know what the
    code must do.

    ::
    2) How do board level designers (who may not know much about
    programming
    the MCU) know HOW to layout the board? Do they just look at
    application
    notes from the manufacturer and lay things out and get it right on the
    first shot!?
    ::
    It is the task of the electrical engineer to guide the layout person
    (who may in fact be that same EE). We do schematic capture (with lots
    of notes), generate netlists and footprint requirements and pass them
    to the layout person with our notes. It is not unusual for the EE to
    sit with the layout person to deal with 'special' areas of the board.
    As to HOW to lay out the board - practise, practise, practise. A
    healthy does of aptitude helps, though.

    We don't just 'look at app notes', although that forms part of the
    design exercise. App notes live in a perfect, isolated world. We design
    their parts into a larger scheme, which requires us to know how to
    adapt the information in the app note to our current requirements.

    As to getting it right first shot, that takes a lot of practise (luck
    helps) and is usually a function of the complexity of the board,
    although classic neophyte problems abound for even the simplest of
    units.
    I had 5 in a row completely correct (i.e. the prototype is the shipping
    unit) for very complex boards (varied between really small and tight to
    big and hairy). That's the exception although we always try to get it
    right. It's the gotchas (which is why you should read the datasheets
    and app notes thoroughly) that catch you.
    ::

    3) Is it possible that a board level designer can layout a board
    without
    knowing anything about programming the chips onboard?
    ::
    If you are talking about the electrical designer, I would say *no*
    except for the simplest of devices. All newer processors and
    controllers have multi-use pins in these days, and a thorough knowledge
    of what the code/system requires is necessary to assign the correct
    pins to the correct board functions.
    If you are referring to the PCB layout person, then they don't
    necessarily have to know details, although we have to convey rules
    about the circuitry (which could be high currents, fast transients,
    high speed systems etc).

    ::
    4) For surface mount chips (not in a DIP format where you can plop it
    into
    a breadboard for trial purposes), how do you go about trying them out
    before actually committing them to be produced on a PCB?
    ::
    Others mentioned making a prototype board first - I have done that
    myself. On other cases, the SMD board *is* the prototype. For complex
    boards (or for analog sensitive boards) two different layouts will
    yield two different results, for an identical netlist.

    ::
    5) When you want to incorporate a chip into your design and hook it
    upto
    other chips onboard, what is the first thing you go searching for? The
    datasheet or...? How can you be sure it will work in harmony with
    what's
    already onboard (i.e. all the pin connections are correct)
    ::
    The datasheets, application notes, tecchnical articles from the device
    manufacturer. I also call the local FAEs to see if they have reference
    designs.

    ::
    I got a few more questions but I'll stop here for now.

    Thanks
    ::

    No problem

    Cheers

    PeteS
     
  5. John Larkin

    John Larkin Guest

    The first thing I do is write the manual. Then board layout, parts
    list, FPGA design, embedded software, test software.


    John
     
  6. PeteS

    PeteS Guest

    The esteemed John Larkin wrote:
    "The first thing I do is write the manual. Then board layout, parts
    list, FPGA design, embedded software, test software. "

    I completely agree - I wish more of my 'engineering managers' did.

    For the elucidation of the original askee:

    This is not merely a good idea, it's almost a requirement for any
    design if one wants to get it right. There have been many times I have
    heard (or have been known to comment / asked) "...but we thought the
    functionality was x". Alternatively "but we wanted to be able to use
    that pin functionality in a bidirectional way". More such comments
    abound.

    A written spec is more than just guidance, it makes sure everyone
    involved understands what the unit will do, and just as importantly,
    what it will *not* do.

    It takes a little time, but it takes more time to clear up
    misunderstandings later. The old saw that the further into the
    design/product process you are, the more expensive (in time and money)
    any change becomes is no more true than for this.

    A week (or maybe more - a lot more, depending on the product) up front
    saves a lot of time in the back end.

    Cheers

    PeteS
     
  7. Rich Webb

    Rich Webb Guest

    [snip...snip...]

    If I may make a suggestion that it would be a heck of a lot easier
    to follow your replies if you would employ the usual convention of
    prefixing each line of the embedded quote with a single character
    (usually a '>') in the first column.

    I understand that you're posting through Google and that the Google
    usenet interface is egregiously bad. "Do no evil." Feh!
     
  8. JeffM

    JeffM Guest

    PeteS wrote [with hand-generated context]:
    ..
    The esteemed John Larkin wrote:
    "The first thing I do is write the manual. Then board layout, parts
    list, FPGA design, embedded software, test software. "
    ------------
    There have already been lots of replies,
    but I'll throw in my few comments...
    ------------
    :
    :If I may make a suggestion that it would be a heck of a lot easier
    :to follow your replies if you would employ the usual convention of
    :prefixing each line of the embedded quote with a single character
    :(usually a '>') in the first column.
    :
    :I understand that you're posting through Google and that the Google
    :usenet interface is egregiously bad. "Do no evil." Feh!
    : Rich Webb

    Getting blockquotes on Google Groups (The Easy Way):
    http://groups-beta.google.com/group...Google+zzz+show-options+click-THAT-Reply-link
     
  9. Jasen Betts

    Jasen Betts Guest

    earlier this year I made my first MCU design, I used an off-the shelf
    stripboard designed for DIP ICs I first loaded the MCU with a simple program
    so I could drive the outputs and verify that the inputs functioned correctly
    and then after getting those parts working satisfactorilly I started on the
    software, because I kept adding features (and bugs) I exceeded the 1000
    write cycles the chips flash could handle and then I was glad I had used a
    socket for it,

    I think it would be hard to test the software without having a bouard to run
    it on.
    once the designer knows which pins will connect to which board features the
    layoy task would be similar to any other,
    as far as I can see, he need only know what the chip does, not how it has
    been implemented.
    my brother solderes them to a dip-shaped carrier using 10A fusewire,

    10A fuse wire is about the right gague for soldering to the terminals of
    surface mount ICs...

    look at the chips data sheet see which pins can do what.... the 90s2313 I
    used had one pin with a PWM output, I used this to drive a loudspeaker for
    simple polyphonic sound. another pin had an async serial output capability
    I reserved this function for debuggging output...

    Bye.
    Jasen
     
  10. Guest

    I find the board arrives entirely too fast. I haven't even ordered
    *all* the specialized parts yet (the assumption that everything's
    in stock fails with SMT parts), and the time was spent putting out
    the fires that began burning while the PCB-design push was underway.
     
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