WS2811 with Custom Board

Hi,

I have an issue driving RGB LED's using a custom board I designed. I'll explain the setup and then the issue.

Topology:
There are three PCB's in the setup. The primary board has a linear regulated power supply and an arduino which controls the LED's. There are two daughter boards which each have 24 WS2811 drivers which connect to the primary PCB. The data output of the arduino connects to daughter board #1 then through each WS2811 which at the last driver the signal then returns to the primary board. A trace on the primary board carries the signal to the second daughter board which is identical to the first. The last driver data line in the second daughter board is left floating

Issue:
When operating the 'string' of LED's the first daughter board operates as expected. The second daughter board misses changes in color and flickers. The symptom gets worse the further towards the end of the second string.

I removed the connection which jumpers the two daughter board data lines through the primary PCB and directly connected the signal out from daughter #1 to input of daughter #2. When I do this all LED's function as expected.

Scoping the waveform does not reveal anything, the input of the first LED of daughter #2 looks the same with each setup. This is making it really difficult to understand what the problem is. Any suggestions what else to look for?

Thanks!

Can you post a schematic please, and some pictures of each board. We need to see what components you are using, for example caps across the power lines to the led boards, series resistors on the data line etc.

Have you got a 0.1uF ceramic capacitor across the power pins of each WS2811?

I’ve attached screen snips of the first and last WS2811 drivers on the LED driver board and a snip of the control board which has the interconnect between the two boards.

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Why the resistor on RGB_DOUT? That is not going to help. The same goes for the resistor in the supply line to each LED.

These should not be included, replace them with wire links or zero ohm resistors.

I replaced the VDD resistor, RGB_DIN and RGB_DOUT resistors with zero ohm resistors but it made no difference.

The reason for the resistors is application circuit suggestions from the WS2811 spec sheet.

Any other ideas why using the bypass wire works but the PCB trace has signal integrity issues?

Thanks.

The reason for the resistors is application circuit suggestions from the WS2811 spec sheet.

I think you might have misunderstood something.

You still have not answered the question if reply #2

Yes, there is a 100nF capacitor across each WS2811.

Is the first LED of the second board working OK or do you get problems there?

If the first LED is fine then that regenerates the data signal for the rest of the LEDs. So if you are having problems with that then it can only come from the power supply / decoupling. I would increase the bulk decoupling capacitors and maybe add one or two extra along the length of the second board.
I would also try star wiring the power rails. That is connect the two strips to one common 5V and ground instead of chaining them.