LED Cube: Finished 3x3x3 prototype circuit, first hardware issue

In the last few days I've spent some time in the evenings/nights to set up a complete prototype circuit of the 3x3x3 cube on breadboard and create the first parts of the driver software.

There have been some minor issues (e.g. realizing that the larger breadboard has a different layout and connection scheme than the small one or running out of wire jumpers and cables) but in the end I managed to build the circuit on the available larger breadboard space in its full beauty.

From LED Cube Images

Starting the tiny test application which I wrote for the first IC test circuit even brought up the desired result on the first attempt so I've been pretty happy with the initial outcome.

From LED Cube Images

The next logical step was of course to enhance the driver software to allow for more sophisticated patterns and also do measurements on how big the impact on the multiplexing performance would be. Well, before I could finally finish that measurements the first non-expected issue manifested and left me puzzling.

I set up the cube pattern to light up the voxel (1,1) on the first layer, (2,2) on the second and (3,3) on the third layer and removed all delays to perform the multiplexing at maximum speed. Let's say the result did not exactly represent what I had in mind.

From LED Cube Images

When I limited the multiplexing to ~10Hz everything looked ok and only one LED per layer was lit but already at 100Hz additional LEDs became dimly lit and with 1kHz it became pretty clear that there was a speed-dependend issue, as on every currently active layer the corresponding LED which was active on the previous layer still received some current which increased with multiplexing speed.

From LED Cube Images

I re-checked the circuit for obvious and not-so-obvious problems (using my limited knowledge) but there were no aparent issues. Since the LED driver ICs are rated up to 30MHz my only possible explaination of this effect is that the transistors for some reason do not turn off fast enough after a layer switch. I'm somewhat sure that it's not based on the transistor type itself as I can't imagine that a standard transistor can't even manage to switch fast enough to produce a clean 1kHz signal. These are PNP transistors, so it allows current to flow E->C as long as B is connected to GND (or at least much lower than E). The LED driver IC is, according to its data sheet, fast enough when disconnecting the OUT pins from GND so that there should be no recognizable delay caused by this component. So my current understanding brings me to the conclusion that there is a small timespan where there is still current flowing out of the B of the transistor even if the OUT pin on the IC is not active anymore. Has simply the wire itself a capacitance which is just large enough to cause this?

In the meantime I've found a more-or-less acceptable workaround for this (thanks to my work colleagues for throwing some ideas back and forth with me), but more on that in a later posting. Yet, the underlying cause for this strange behaviour is still not clear and I'm determined to understand and resolve it eventually.


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