Bizarre tlc5940 issues with arduino

Discussion in 'The Projects Forum' started by Bp_968, May 13, 2014.

  1. Bp_968

    Thread Starter New Member

    Dec 25, 2013
    I am trying to learn how to use the TLC5940 and having a very strange problem. Have a look at the image. It should be running the basic use script and lighting up the LEDs back and forth among other things. The problem is that unless I hold onto a wire (specifically the orange one leaving frame right)

    Any ideas?

  2. DickCappels


    Aug 21, 2008
    I think you meant to type that it only works when you hold the orange wire, with is Serial Out. That alone doesn't make sense, but you built this on a breadboard, and maybe that little electrolytic in the lower center is your decoupling capacitor.

    The timings associated with this chip is in the tens of nanoseconds. The minimum clock width is 10 ns. The electrolytic capacitor, even if it were mounted right on the chip has too much stray inductance for me to imagine your circuit would work correctly. You need a good quality RF bypass capacitor (1000 pf or so) right next to the chip, with short connections.

    The 10 ns clock pulse width suggest a large susceptibility to ringing on the clock line and its return. A ground plane would come in really hand here. Lacking that, you can try using a twisted pair of wires to carry the clock signal and its return to the chip. You might find that putting a small resistor (50 to 100 ohms) in series with the pin driving the twisted pair or in series with the clock input.

    Your data input line might also be having problems with ground, etc.

    Make sure you are observing the data setup and hold times noted on page three of the specification.

    Those plastic breadboards are generally not very good for use with modern high speed CMOS, and those interconnecting wires looping from here to there can't be helping at all.

    Good luck, if your problems persist, you might want to move to a circuit board with a solid ground system.
  3. Bp_968

    Thread Starter New Member

    Dec 25, 2013
    Actually it's plugged into the 4th pin from the bottom, GSCLK, but it does look like what you suggested (3rd pin from bottom). Regardless, I suspect your right. I'm a total newb to all this and have no EE formal education so ground planes and timing issues are all new to me! I was using much shorter wires cut to correct length but thought maybe it was a wiring issue so swapped it for wires I knew worked from other projects.

    I'll try your suggestions and post my results.

    Btw, I posted that message after taking ambian (for sleep). I fell asleep in the middle of typing that (lol) so don't mix forums and sleep medications! :D
  4. Bp_968

    Thread Starter New Member

    Dec 25, 2013
    The bypass capacitor seemed to fix the issue! Thanks again. Now I know a little more about what to look for and what problems to anticipate when using a breadboard.
  5. bertus


    Apr 5, 2008