Ping Pong Screen - 24x24 synchronous controller

I document and reverse-engineer a LED screen controller I designed for a custom art piece, that uses some sick hacks and tricks

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Back in 2015/2016 I designed and built 2 boards that control a LED screen made of 24×24 synchronous RGB LED.
Cost, reliability and serviceability are huge concerns so the solution was utter simplicity. A PIC16F818, a 16MB serial Flash chip, some TTL and I could avoid any concern with OS, updates, failures, ...
But the custom design means it is frozen for ever and the extremely tight schedule worked against the proper documentation. Many files were lost with a hard disk and I have to reverse-engineer the system from what I have gathered...
Maybe this could inspire people who don't want to use an Arduino or a Raspberry Pi for such a simple project ?

This project is featured at the beginning of this video :

Two ping pong screens (24×24 RGB LED) display alternating videos in an endless loop. The circuits must be reliable, simple, cheap, and start immediately when powered up. The other constraint was the LED : due to sourcing issues, the array uses synchronous LEDs, whereas the #WizYasep  can drive asynchronous LEDs only (so far). A custom solution was required...

I came up with a pretty crazy system that was reduced to its essential components, using ideas I explored with Cheap bulk removable storage. The principle is simple : the output of the serial SPI Flash chip is almost directly sent to the data pin of the synchronous LED, while a microcontroller toggles the clock and other control signals.

This is possible because very high density Flash chips have hit the market at great prices and a 128M bits BIOS chip can store many many frames :

1 frame = 8×3×24×24 = 13824 bits, or roughly 1/10000th of a 16MB Flash chip (9709 frames to be exact).

At 25 frames per second, the chip can store 388 seconds of uncompressed, raw video. That's more than 6 minutes.

At first I created a system with parallel reading of 8 SPI chips but I had to change the strategy in the middle of the project because the videos are short enough to fit into a single chip. I also changed the number of output channels, from 8 to 12 and the TTL/buffers needed a revamp...

  • The return of the PPS24

    Yann Guidon / YGDES09/06/2019 at 15:11 0 comments

    Yesterday I received one of the frames for a slight repair and for enhancement. Time for a debrief...

    It's large, quite heavy, but remarkably well preserved. And it's simple. There might be a loose joint somewhere or some oxide on a contact and I'll have to locate and repair it, while also figuring out how and why I made the circuit this way. In the hurry of the delivery, I had to cut some corners...

    Remaking the PCB Could be a solution after all.

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Dan Maloney wrote 09/06/2019 at 17:46 point

Reminds me a bit of the recent project by @bitluni. Looks good!

  Are you sure? yes | no

Yann Guidon / YGDES wrote 09/06/2019 at 18:59 point

Yes I've seen the videos :-)

My first RGB array/screen was built years before (2013, at the same time as #Rosace) and I have gained experience with their design (such as redundancy and partitioning, which @bitluni doesn't implement because it doesn't have to run non stop for months or years in demanding environments). For very large scale installations (such as #Mons2015 LED Screen ElectroSuper), I had to develop custom driver systems, such as #WizYasep ... Here this system had to use synchronous LEDs (for budget reasons) and I thus had to develop "yet another system"... I hope to document it here :-)

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davedarko wrote 09/06/2019 at 06:53 point

very clean! :)

  Are you sure? yes | no

Yann Guidon / YGDES wrote 09/06/2019 at 18:59 point

Is that a compliment ? ;-)

You haven't seen the other side of the PCB though, there is a Big Mess Of Wires that I have to unravel...

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davedarko wrote 09/07/2019 at 06:43 point

why are you hiding such a beautiful beast then? :D the WS2812 part definitely looks mesmerising and clean!

  Are you sure? yes | no

Yann Guidon / YGDES wrote 09/07/2019 at 10:51 point

Hiding ?

Anyway these are not WS2812, it's a 4-wires string, with synchronous protocole (APA102 maybe). Otherwise I would have used a #WizYasep  :-)

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