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Noise problem I think

Discussion in 'Electronic Basics' started by Chris W, Mar 29, 2005.

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  1. Chris W

    Chris W Guest

    I have 4 of these 16 bit serial LED drivers hooked together on
    solderless proto boards.
    http://www.allegromicro.com/datafile/6276.pdf

    I have a RCM3110 module sending data to them and everything works great
    till I have just over half of the LEDs on at a time. If I just turn
    them all on and leave them on it's fine but if over half are on and some
    are flashing, I get a few others near the ones that are supposed to be
    flashing also flashing in what appears to be a random manor. I have
    tried slowing down the clock pulses and increasing the time between data
    changes and the clock going high, but that has had no effect. I don't
    think it is the clock speed though since with less than half of them on
    at a time I can have flashing sequences going VERY fast with no
    problems. When I say fast I mean the data is clocked into the chips as
    fast as the RCM3110 can send them. The scope meter tells me that with
    no code to slow anything down, the clock is high for 2uS and then low
    for about 15uS, giving a total of about 17uS per cycle or 58 kHz. I
    don't know for sure how slow I have taken it, but I would guess that it
    was far less than 1 kHz, increasing the high and low time as well as the
    time the data was set before the clock goes high. I have done extensive
    testing and debugging to confirm that the code is not the cause of the
    problem. .

    Is this something that will just go away when I put it on a PCB and the
    lines between the pins are a lot shorter than the wires I am using now
    to hook 4 of the together on 2 different proto boards? Or do I need to
    add something to help prevent noise? I think this is only happening
    when there are several (more than 32) 1s in a row but I'm not sure. Any
    ideas?

    --
    Chris W

    Gift Giving Made Easy
    Get the gifts you want &
    give the gifts they want
    http://thewishzone.com
     
  2. It could go away, but that would depend on your
    PCB layout. The same effects can be brought on
    by poor ground (and power) distribution.
    You could add redundant ground wires and a gob
    of extra bypass capacitors to your proto board.
    Given your description, I would expect to see the
    threshold number of driven LEDs go up. If that
    happens, you will know "ground bounce" is hard
    at work bedeviling your circuit.

    The solderless proto boards are often a waste of
    time. They induce or force poor layout, introduce
    a few pF between adjacent pins, and inevitably
    have excessive coupling due to all the air wires
    in proximity to each other. When I was starting
    my electronics education, it went faster once I
    realized that my proto boards should be shelved
    for use only in DC or near DC circuits.

    As ICs have become faster, those proto boards
    have become all the more useless for the kinds
    of circuits that are interesting by being busy.
     
  3. Chris W

    Chris W Guest

    I don't have much in the way of capacitors around so I added 2, 2.2uFs
    that I had around, as close to the Vcc and GND pins on 2 of the 4 LED
    drivers as I could. That solved the problem, I could have almost all
    of the LEDs on and just a few flashing (so I could detect a errors) and
    didn't see any.

    After that, it occurred to me to check the voltage drop from the PS.
    The prototype board for the RCM3100 has a 5V source I was using to power
    the LED drivers. After some testing I found that if I had more than
    about 6 LED's on at once, the voltage from the PS started dropping. By
    the time I had all the LEDs on the voltage was down to 4.8V and the
    regulator chip was starting to get fairly warm. So I disconnected the
    LED drivers from the prototype board of the RCM3100 and used a 4 cell
    NiCd pack to power the LED drivers and LEDs. With that set up even with
    out the capacitors it worked fine.

    Makes for a good reminder why I never skimp when buying a power supply
    for my computer. A bad power supply, or one that just isn't regulating
    as well as it should, can cause all kind of random errors and it's one
    of the last things most people think to check.

    --
    Chris W

    Gift Giving Made Easy
    Get the gifts you want &
    give the gifts they want
    http://thewishzone.com
     
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