Controlling LEDs - understanding bit/byte/pixel

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derdaani

Controlling LEDs - understanding bit/byte/pixel

in Programming Questions • 2 years ago

Hi guys,

so I'm trying to control a LED indicator board via Processing. Opening a connection and sending text works perfectly. But the board is capable of displaying simple graphic data.
The data sheet (which can be found here: data sheet) explains on pages 13-14 how a message has to be constructed:

Each Graphic Block built by 4 8x8 dots units
Sequence of data is shown below.

Graphic data mapping
Graphic Pixels: D0,D1,D2,...D255, four Pixel is represented by 1 Byte.
Byte 1 = D0..D3
Byte 2 = D4..D7
Byte 3 = D8..D11
...
Byte 63 = D252..256
Structure of each Data : Each Pixel composite by 2 bit. MSB is the most Left Bit
e.g. the first dot is RED ,the second dot is GREEN, the third dot is yellow and the forth dot is black.

Data = 10 01 11 00
              |    |    |    |
              |    |    |    -Black
              |    |    ----- Yellow
              |    --------- GREEN
              ------------ RED

I got 3 questions of which I'm hoping someone can explain:

As I understand the explanations, there is a "layout helper thing" called Graphic Block. One Graphic Block consists of 4 8x8 dots units. Inside one of the 4 8x8 units the graphic pixels are denoted left to right and row by row (see image). Four of these graphic pixels are represented by one byte. Every single pixel consists of 2 bit ("colour management"). Did I get the theoretical background right so far? And could someone explain to me, what exactly one LED is in this whole description? A "dot"?
The data sheet obviously was made for two different products - single colour and multi-colour LED boards. Mine consists of red LEDs only. How is the data structure supposed to look like then? 10 10 10 10?
I have no idea how to transfer all this theoretical background into code. Can you help me getting started please?

Looking forward to your replies, any help and suggestions is highly appreciated!

Replies(5)

PhiLho

Re: Controlling LEDs - understanding bit/byte/pixel

2 years ago

1. Seems correct. The "dot" concept is indeed a bit nebulous...
I wonder if the doc is made by non native English speakers (like me): I have coworkers regularly forgetting the trailing s in plural (even though we have it in French!). Here, I see "our Pixel", "2 bit". Idem for "most Left bit" and such. But I digress (and I probably do similar mistakes...).
Gathering information given there and some low-level experience (acquired some 30 years ago and not much used since then...), I believe that what they call "dot" is a combination of (two) bits leading to a given color. You need to set such "dot", two bit combination, to set the color of the Led. If your Leds are red only, I guess you can use any non-zero combination to light it. If you are lucky, 11 might illuminate more than 10, it is worth trying.
2. See above...
3. It is a bit hard without knowing what you want to display.
Perhaps the simplest way is to make an API taking x, y, light on/off parameters and altering a bit field to send to the board.
This translation somehow depends on your physical set up: we don't know how many graphic blocks you have, or how they are organized.
BTW, do you know bit manipulations? Operators like & | ^ << >> >>> If not, I advise to start looking at them, and the theory behind it (it isn't that hard).

derdaani

Re: Controlling LEDs - understanding bit/byte/pixel

2 years ago

Hi phi.lho,

thank you very much (again) for your quick reply! At the moment my work progress feels a bit like "another day another question"...

Re the setup:
This is the board I'm working on: LED indicator board
It consists of 80x7 LEDs (sidenote - the product with colour LEDs has the same 80x7 resolution LED board colour).
I'm still not sure about the dot concept. If a dot represents one LED, the "Each Graphic Block built by 4 8x8 dots units" doesn't seem to fit, as there are only 7 rows of LEDs on the board.

Re what I want to display:
As I'm still trying to figure out how the single LEDs are being controlled, I'd be more than happy to be able to light up one LED as a first step.
As a second step, I'd like to try and do a simple pattern (e.g. alternating on and off LEDs).
After that I will hopefully have understood the concepts and I'll dive into generative controls.

My biggest problem at the moment is that I just don't get, how a bit of data might look like. What data type, what formatting, etc. Can I just input a string like

String data = "11111111";

Or hex denotation?

String data = "FF";

I assume I'll do Bit-Operations on the data type byte and cast the result to String afterwards? Or do I need another data type etc?

You see, I literally don't have a clue how to start... So any line of code that might get me started would be appreciated :-)

Btw thanks for the hint re bit manipulations. I'll have a decent read on this topic now.

PhiLho

Re: Controlling LEDs - understanding bit/byte/pixel

2 years ago

I don't know the protocol of your board, how data is sent to it (this belongs more to the I & H section). But at the end, the board expects bits set and unset, so it is really binary data, not strings.
Thinking more about it, I think the best is to make a PGraphics at the expected size, where we can draw the usual way with Processing tools (point, line, rect, etc.).
Then the API will convert the pixels[] array to the expected data.
As noted above, the send to board issue belongs to another thread...

I think the Graphic Block concept is an abstraction over the hardware, perhaps made to accommodate bigger boards, or even sets of boards, I don't know. So I suppose the last row of these blocks is ignored.

Making code to convert PGraphics to Graphic Block shouldn't be too hard, I will have a look later.

derdaani

Re: Controlling LEDs - understanding bit/byte/pixel

2 years ago

Thanks for the hint re PGraphics! I read the function reference and had a look at the PGraphics example scetch.

I couldn't quite figure out where to find the mentioned pixels[] array and how to convert the PGraphics into Graphic Blocks ie. readable data for the LED board.

Can you give me a hint on how to get started? Thank you!

fiveagon

Re: Re: Controlling LEDs - understanding bit/byte/pixel

2 years ago

http://www.processing.org/reference/pixels.html

I'm not sure, but I think getting the pixels[] from a PGraphics object would be something like:

PGraphics pg;
...
pg.loadPixels();
something = pg.pixels[0];

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