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digital RGB LED question

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glenenglish:
HI
I'm troubleshooting some RF problems with digital LEDs (single wire DIN DOUT cascade) . The design of the product is lacking much engineering rigour, but its what I have to work with from the client.
The designer has used an opamp to drive several meters of cable with a string of cascaded digital LEDs on, them a total RFI and EMC nightmare
ie a) low speed opamp cant make square waves, so it starts off bad, and b) no shields anywhere, RF gets into it very easily and muds up the edges.

QUESTION : The date bit rate in the datasheet is given as 1.25uS typical. (800kHz)
This is what it is running at.  If it ran at 1/10th of that, or 1/100th of that, will the chips still work?- IE are they edge triggered, or is there some asynchronous timing activity internally ???
SLowing the clock would permit the opamp to make decent looking waveforms. and permit some scope for EMI filtering from 10-200 MHz RF.

I cant just try it, have to go back to the client to get a change. There isnt time budget for me to dick around with the led's myself. Hmm maybe I can program up my AWG to do this...... That might work
-glen

ataradov:

--- Quote from: glenenglish on April 20, 2024, 11:40:18 pm ---are they edge triggered, or is there some asynchronous timing activity internally ???

--- End quote ---
They are asynchronous. You can't run them slower at all. In fact, long low level is the reset signal.

SiliconWizard:
So, no, you can't. There is some tolerance, but not enough to make a difference in your case.
(This tolerance can be leveraged though to use some "spread spectrum" approach to lower EMI.)

The datasheet for the WS2812B specifies 1.25µs +/- 600ns, which is about +/-50%.

glenenglish:
thanks both for the info.  :)

tooki:
Given that, as I understand it, each LED is retransmitting the data (after stripping off the packet it will display), I don’t think the issue is the driver, because only the first LED in the string sees it. Could the problems actually be due to current spikes on the power supply rail? You could try putting caps near the LEDs and seeing if that helps.

Or is the issue several meters of cable between the driver and the first LED?

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