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Thread: WS2811 functionality

  1. #1
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    Default WS2811 functionality

    I have a little side project I am noodling through and have some questions on the functionality of the WS2811 (the chip, not the LEDs themselves).

    I am not concerned with the data lines, just how the outputs work.

    If I am reading the datasheet right, the outputs are constant current sinks of about 16.5mA right?

    And that the way that they control the LED brightness is by using a PWM on the current sink to get its 256 levels of brightness.

    So my question for those who have delved deeper into the chip's functionality is how accurate is the PWM (assuming I am understanding the chip right)? It is an 400kHz PWM, is that clock fairly accurate?

    I am trying to simulate it (while I wait on some chips to come in), and I am just trying to understand it better. Ultimately, I want to be able to be able to trigger on whether the current is above a certain threshold. I am considering low pass filtering the current sink, but I am still not 100% confident that I am grasping its functionality yet.

    Thanks!

    Sent from my SM-G955U using Tapatalk
    ~Jason
    [URL]http://www.tooz.us[/URL]

  2. #2
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    Default Re: WS2811 functionality

    The 2811 is a constant current sink, with three outputs. The outputs R G B get shorted to ground at a max of ~18ma. The shorting turns on and off very quickly (I forget the frequency). As far as clock accuracy goes, it's not consistent like a crystal clock would be. It can vary +- 10% easily as it's just an RC circuit running it (btw: it varies with temperature too.) But there's no reason to care that the frequency can vary. What's important is the Red, Green, and Blue are mixed in the same proportions. If I have 10% red and 90% green, the pulse widths will be 10% and 90% whether the frequency is 400Khz, 600khz, 60khz, etc. The PWM accuracy is what you care about once you are at a frequency high enough you can't see blinking (100hz, or clock of 25khz -- eyes only it can go as low as 20hz).

    The chip shorts to ground at a varying pulsewidth you give it over the data pin. Since all three outputs have the same clock, the color mixing is the same regardless of the clock frequency.

  3. #3
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    Default Re: WS2811 functionality

    Hi, it sounds like you have another interesting project on the go. As normal in this hobby things are not quite as straight forward as you hope but may be I can save you a bit of pain.

    There are two types of WS2811 kicking about at the moment; the old ones that have the 50uS update time and a new one that has the 280uS update time. Let’s call them WS2811(50) and WS2811(280).

    The WS2811(50) that you quoted from the data sheet in your first post has a Pixel data speed of 400khz or 800Khz user settable and a PWM frequency of 400hz not 400Khz as you posted.

    The WS2811(280) has the same Pixel data speed of 400khz or 800Khz user settable and a PWM frequency of 2Khz.

    The next part as to why this changed is mostly speculation on my part. World Semi and the WS2811(50) where starting to be old hat as their competitors now offered faster PWM which wasn’t visible in videos and dual data lines to prevent the loss of large sections of strings if a single pixel failed. To resolve this they produced the WS2813 which has 2Khz PWM and dual data, this left them needing to manufacture two ICs the old WS2811(50) and the WS2813. I suspect they stopped production of the WS2811(50) and now use the WS2813 and only bring out one of the data pins which is why we all had to adjust the refresh to 280uS for our WS2811(280)s.

    Hope this helps.
    Last edited by Barnabybear; 08-01-2018 at 04:35 AM.

  4. #4
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    Default Re: WS2811 functionality

    Thanks for jumping in guys. I have some ICs in hand now, so I need to throw them onto a breakout board, and I can finally do some scoping! I've never played with constant current before, so I've been noodling over the best way to handle it for what I want to do.

    I have 2 projects that need to be completed for my own yard, but I have a long commute, so I was dreaming up a little board that I thought might be useful to others as well.

    I wanted to be able to tie into a show, so I figured the WS2811 was the most common variant out there. I was unaware that there was 2 different types of 2811s, so that stinks. Is there any visual way to know which you have?

    Anyway, all I really want is an on/off trigger from the show. So I figured that I throw the IC on there, have the user set red to be 255 or 0, and just let the IC pass the data down the line as intended. I was going to low pass filter the PWM signal with a resistor and cap, and use that as my on off trigger.

    Now, all that said, I don't run a "show" like I think this would be useful for (the animated light shows), so if someone thinks there is a different direction I should go that ties into what most of the people are doing out there, I am all ears. I just figured 2811 covered a large swath of people and I would just need the 4 connections to get tied into the larger show.

    Sent from my SM-G955U using Tapatalk
    ~Jason
    [URL]http://www.tooz.us[/URL]

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