Forum Rule: Always post complete source code & details to reproduce any issue!
Results 1 to 13 of 13

Thread: need help with large screen

  1. #1
    Junior Member
    Join Date
    Apr 2018
    Location
    liverpool
    Posts
    4

    need help with large screen

    hi
    I am new to this site and to teensy
    I am building a 2 meter x 2 meter led screen using ws2812b led strips 120 led's per 2 meter
    I was wondering how many strips the teensy 3.2 with the octows2811 adaptor would work
    or if there anything more powerful I could use
    any help would be greatly appreciated
    regards
    ste

  2. #2
    Senior Member+ MichaelMeissner's Avatar
    Join Date
    Nov 2012
    Location
    Ayer Massachussetts
    Posts
    4,100
    Well, the product page for the octows2811 (https://www.pjrc.com/store/octo28_adaptor.html) gives these limits:

    • 4,416: 60 HZ refresh
    • 6,000: 120V USA power (approximate, based on typical power supply efficiency and standard 15 amp circuit breakers)
    • 8,800: 30 Hz refresh (uses 86% of Teensy 3.2's available RAM)


    Even though the octows2811 does things in 8 parallel streams, sooner or later you run into the issue that the timing is fixed for each pixel in the stream. I imagine when you start to go much past the 4,000 LED size, you probably want to think about breaking it up so that you have one teensy per a smaller size, and possibly a teensy to coordinate all of the teensys. By the time you get up to that level, the cost of the extra teensys are dwarfed by the sheer cost of the lights.

    That being said, I personally have only done ws2812b's at 100 LEDs or less, so I don't have any experience building the large rigs.

    I also tend to wonder whether going with an HDMI based display and using Raspberry Pi or similar might be cheaper as you try to scale things up (perhaps using a projector instead of a large display).

  3. #3
    Junior Member
    Join Date
    Apr 2018
    Location
    liverpool
    Posts
    4
    thanks you have gave me something else to consider

  4. #4
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    Posts
    25,234
    I've updated the page with the 4th absolute maximum limit of 10920 LEDs which is imposed by how the DMA hardware works. It can't do a transfer of more than 32K bits, and since each LED needs 24 bits of RGB data, that means you can't have more than 1365 LEDs per pin. With 8 pins, the absolute maximum is 10920 LEDs.

  5. #5
    Junior Member
    Join Date
    Apr 2018
    Location
    liverpool
    Posts
    4
    thanks paul do you have any recommendations for what would be best to use

  6. #6
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    Posts
    25,234
    The recommendation depends on the number of LEDs. For a "small" number, use WS2812Serial. For "large" projects, use OctoWS2811.

    Usually under 400 would be "small" and over 800 would be "large", and between those two is kind of a gray region where either would be worthy of recommending. But there's certainly more overlap, so don't consider these numbers to be hard limits, as WS2812Serial can be pushed to larger numbers if you're not looking for rapid update, and OctoWS2811 can be used for even very small projects if you're ok with the notion of "overkill".

  7. #7
    Junior Member
    Join Date
    Apr 2018
    Location
    liverpool
    Posts
    4
    Thanks Paul I am using 14400 led' s so I would need 2 octows2811 and sinc them is that correct

  8. #8
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    Posts
    25,234
    Yes, at least 2 would be needed. 3 might be easier, depending on how much processing you're doing for each pixel.

  9. #9
    Junior Member
    Join Date
    Sep 2015
    Posts
    19
    Quote Originally Posted by PaulStoffregen View Post
    I've updated the page with the 4th absolute maximum limit of 10920 LEDs which is imposed by how the DMA hardware works. It can't do a transfer of more than 32K bits, and since each LED needs 24 bits of RGB data, that means you can't have more than 1365 LEDs per pin. With 8 pins, the absolute maximum is 10920 LEDs.
    I'm working on a library based on OctoWS2811 on the Teensy 3.6, and I'm trying to figure out what exactly is causing this 32K bit limit and if there would be a way to circumvent it.
    Would it for example be possible to chain two transfers after another? or start two DMA transfers simultaneously and use 16 ports instead of 8?

  10. #10
    Junior Member
    Join Date
    Sep 2015
    Posts
    19
    Quote Originally Posted by emiel.h View Post
    I'm working on a library based on OctoWS2811 on the Teensy 3.6, and I'm trying to figure out what exactly is causing this 32K bit limit and if there would be a way to circumvent it.
    Would it for example be possible to chain two transfers after another? or start two DMA transfers simultaneously and use 16 ports instead of 8?
    After a bit of digging around I found that the reason for the limit is probably explained by the 15 bit limit of the Current Major Iteration Count (CITER) (24.3.28 p.561 of the k66 manual).
    I guess this means that the minor loop technique used by https://github.com/raplin/HexaWS2811 actually allows for twice the amount of total LEDs to be addressed, as twice the amount of data is transferred per major iteration.

  11. #11
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    Posts
    25,234
    Yes, the minor loop could be used to write to another port, for 16 outputs.

    Another idea might involve the interrupt quickly starting another DMA transfer. Some WS2812B look for a 50 us gap, others a 300 us gap. So if you could start another transfer in much less than 50 us, the LEDs would probably see it as one continuous transfer.

  12. #12
    Junior Member
    Join Date
    Mar 2016
    Posts
    5
    Now it appears sk6812 led,it has the same protocol with ws2812b.
    SK6812 has RGB,RGBW,WWA,SK6812 rgb is similar with ws2812b led,the main difference between the two drivers is that the SK6812 has voltage-independent color and brightness over a wide voltage range, which means that the colors of the LEDs should not be affected by a drop in the supply voltage as much as they are on the WS2812B.In addition , in our test , SK6812 LED quality is much better , when the temperature is high,ws2812b is much easy to be dead than SK6812.
    So i advice using sk6812 led strip to make screen.

  13. #13
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    Posts
    25,234
    Quote Originally Posted by daisy1988 View Post
    Now it appears sk6812 ledthe main difference between the two drivers is that the SK6812 has voltage-independent color and brightness over a wide voltage range
    Really? Where are you finding this info?

    I'm not saying that's necessarily wrong, but it is rather surprising....

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •